1 00:00:00,040 --> 00:00:03,560 I want you to picture a ballroom, a massive, gilded 2 00:00:03,560 --> 00:00:05,680 ballroom. And it's filled with, I don't 3 00:00:05,680 --> 00:00:08,760 know, a billion statues. And these aren't just any 4 00:00:08,760 --> 00:00:11,240 statues. They're all perfectly paired 5 00:00:11,240 --> 00:00:14,240 couples, you know, frozen in an embrace. 6 00:00:14,680 --> 00:00:16,720 They're totally, still, totally silent. 7 00:00:17,400 --> 00:00:21,040 Now imagine that somewhere in this endless sea of stone 8 00:00:21,040 --> 00:00:23,960 figures, there is one single dancer. 9 00:00:24,040 --> 00:00:25,320 Just one. Just one. 10 00:00:25,440 --> 00:00:28,280 And this dancer is alive. They're spinning, they're 11 00:00:28,280 --> 00:00:30,920 moving, they have this, this frenetic energy. 12 00:00:31,080 --> 00:00:33,840 That is a very evocative image to start with. 13 00:00:33,840 --> 00:00:35,760 I'd do my best. Now, if you were just standing 14 00:00:35,760 --> 00:00:38,400 at the door of this ballroom looking in, you probably 15 00:00:38,400 --> 00:00:39,640 wouldn't even see that one dancer. 16 00:00:39,640 --> 00:00:41,120 They'd be completely lost in the crowd. 17 00:00:41,480 --> 00:00:44,160 You'd need some kind of special way to find them, a way to make 18 00:00:44,160 --> 00:00:46,760 all the statues invisible and make that one dancer glow like a 19 00:00:46,760 --> 00:00:48,760 flare. And that, in a nutshell, is 20 00:00:48,760 --> 00:00:50,000 essentially what we're talking about today. 21 00:00:50,000 --> 00:00:51,840 Exactly. We are talking about a technique 22 00:00:51,840 --> 00:00:54,800 that I have to say, sounds like absolute science fiction. 23 00:00:54,920 --> 00:00:58,320 It involves these massive Electro magnets blasting samples 24 00:00:58,320 --> 00:01:00,840 with microwaves and and detecting the fundamental 25 00:01:00,840 --> 00:01:04,160 magnetic personality of the electron itself. 26 00:01:04,400 --> 00:01:08,520 It's a tool that can ignore the billion paired up molecules in a 27 00:01:08,520 --> 00:01:13,280 sample and just zoom in with laser like precision on the one 28 00:01:13,960 --> 00:01:18,240 unpaired weirdo in the mix. That weirdo, as you call it, is 29 00:01:18,240 --> 00:01:20,520 actually the key to some of the most interesting and important 30 00:01:20,520 --> 00:01:23,320 chemistry in the universe. Well, today we are doing a deep 31 00:01:23,320 --> 00:01:26,880 dive into electron paramagnetic resonance or EPR. 32 00:01:27,480 --> 00:01:29,680 And I have to say, looking through the research for this, 33 00:01:30,080 --> 00:01:33,200 it really feels like the detective story of the Chemistry 34 00:01:33,200 --> 00:01:35,560 World. It's specialized, it's 35 00:01:35,560 --> 00:01:38,680 incredibly sensitive, and it requires some serious detective 36 00:01:38,680 --> 00:01:41,760 work to interpret the clues. It absolutely is a detective 37 00:01:41,760 --> 00:01:44,520 story. And just to clear the air right 38 00:01:44,520 --> 00:01:47,520 at the start, because the names can be a bit confusing, you 39 00:01:47,520 --> 00:01:51,600 might also hear this technique referred to as ESRESR electron 40 00:01:51,600 --> 00:01:52,840 spin resonance. OK. 41 00:01:52,840 --> 00:01:54,080 So are they two different machines? 42 00:01:54,080 --> 00:01:56,280 Is this like AVHS and Betamax situation? 43 00:01:56,280 --> 00:01:58,440 No, not at all. They are the same beast 44 00:01:58,440 --> 00:02:01,400 fundamentally. ESR electron spin resonance is I 45 00:02:01,400 --> 00:02:03,600 guess the older term. It really focuses on the fact 46 00:02:03,600 --> 00:02:05,480 that we're resonating the spin of the electron. 47 00:02:05,800 --> 00:02:09,639 EPR electron paramagnetic resonance is the broader, more 48 00:02:09,639 --> 00:02:12,120 modern term. It kind of acknowledges that the 49 00:02:12,120 --> 00:02:15,000 electron isn't just spinning, it has orbital motion to it's part 50 00:02:15,000 --> 00:02:18,680 of a whole paramagnetic system. But honestly, in the lab you'll 51 00:02:18,680 --> 00:02:20,920 hear people use the terms completely interchangeably. 52 00:02:20,920 --> 00:02:23,360 The physics underneath is exactly the same. 53 00:02:23,360 --> 00:02:25,920 OK, that's good to know. So let's unpack this. 54 00:02:26,280 --> 00:02:29,160 Our mission today is to understand how we can track and 55 00:02:29,160 --> 00:02:32,320 map these these unpaired electrons. 56 00:02:32,560 --> 00:02:35,760 But before we get to the how the magnets in the microwaves, we 57 00:02:35,760 --> 00:02:39,200 need to talk about The Who you mentioned the molecular dancer 58 00:02:39,200 --> 00:02:42,960 in my ballroom analogy. Who exactly are we looking for 59 00:02:42,960 --> 00:02:45,360 with this machine? That is the single most 60 00:02:45,360 --> 00:02:46,640 important question to start with. 61 00:02:46,920 --> 00:02:50,520 The primary requirement, the absolute gatekeeper for this 62 00:02:50,520 --> 00:02:53,720 entire technique, is that your sample must be paramagnetic. 63 00:02:53,720 --> 00:02:55,280 Paramagnetic. It's a great word. 64 00:02:55,280 --> 00:02:57,640 It sounds expensive, sounds like something you put on a resume. 65 00:02:58,040 --> 00:03:00,480 But can we get a, you know, a plain English definition for the 66 00:03:00,480 --> 00:03:03,000 listener who maybe isn't wearing a lab coat right now? 67 00:03:03,240 --> 00:03:07,160 Certainly, in the simplest possible terms, for a species to 68 00:03:07,160 --> 00:03:10,760 be paramagnetic, it has to have at least one unpaid electron 69 00:03:11,000 --> 00:03:12,960 unpaired. So a loner. 70 00:03:13,080 --> 00:03:14,120 A loner? Yes. 71 00:03:14,920 --> 00:03:17,920 In quantum mechanical terms, we would say the total electron 72 00:03:17,920 --> 00:03:21,600 spin, which we denote as S has to be greater than or equal to 73 00:03:21,600 --> 00:03:23,320 1/2. OK, so let's put that in 74 00:03:23,320 --> 00:03:25,800 context, because when I think of molecules, I usually think of 75 00:03:25,800 --> 00:03:29,080 stability, I think of bonds, you know, everything nice and paired 76 00:03:29,080 --> 00:03:30,680 up. And you absolutely should if you 77 00:03:30,680 --> 00:03:33,600 look at the vast, vast majority of stable molecules. 78 00:03:33,800 --> 00:03:36,600 The water in your glass, the plastic in your keyboard, the 79 00:03:36,600 --> 00:03:40,440 proteins making up your muscles. Almost all of their electrons 80 00:03:40,440 --> 00:03:45,040 are paired up. 1 Electron has a spin we call up and its partner 81 00:03:45,040 --> 00:03:48,320 has a spin we call down. They cancel each other out 82 00:03:48,320 --> 00:03:50,560 perfectly magnetically. They're silent. 83 00:03:50,600 --> 00:03:52,200 They're the statues in the ballroom. 84 00:03:52,280 --> 00:03:54,280 They are. We call those materials 85 00:03:54,280 --> 00:03:57,880 diamagnetic. And here is the absolute kicker 86 00:03:58,320 --> 00:04:01,680 to an EPR machine. Diamagnetic materials are 87 00:04:01,680 --> 00:04:05,080 completely invisible. See, that is fascinating, right? 88 00:04:05,080 --> 00:04:07,600 Because usually when we talk about analytical techniques 89 00:04:07,600 --> 00:04:10,760 like, I don't know, taking a photo or doing an X-ray, the 90 00:04:10,760 --> 00:04:12,920 goal is to see everything. You want a picture of the whole 91 00:04:12,920 --> 00:04:16,680 scene, but you're saying EPR is useful specifically because it 92 00:04:16,680 --> 00:04:19,000 doesn't see everything. Precisely. 93 00:04:19,399 --> 00:04:21,519 That is its superpower. Think about it. 94 00:04:21,519 --> 00:04:24,760 If you're a biologist and you're studying a complex enzyme 95 00:04:24,760 --> 00:04:28,720 reaction that's happening inside a cell, it's in a soup of 96 00:04:29,000 --> 00:04:34,000 proteins, water, salts, buffers, a ton of stuff, just a huge 97 00:04:34,000 --> 00:04:36,720 amount of stuff. If you used a technique that saw 98 00:04:36,720 --> 00:04:39,200 everything, your signal would be completely swamped. 99 00:04:39,200 --> 00:04:40,360 It would just be noise. You're. 100 00:04:40,360 --> 00:04:41,880 Trying to find a needle in a haystack. 101 00:04:42,280 --> 00:04:46,360 But with EPR, because the water and the proteins and the salts 102 00:04:46,360 --> 00:04:50,560 are all mostly diamagnetic, they create 0 background noise. 103 00:04:50,560 --> 00:04:53,080 Oh wow. So if you have just one tiny 104 00:04:53,080 --> 00:04:56,800 radical intermediate or one specific transition metal atom 105 00:04:56,800 --> 00:05:00,080 with an unpaired electron, the EPR will see only that. 106 00:05:00,320 --> 00:05:03,360 So the haystack becomes transparent and the needle glows 107 00:05:03,360 --> 00:05:05,280 neon red. That's a perfect way to 108 00:05:05,280 --> 00:05:06,840 visualize it. It gives you absolute 109 00:05:06,840 --> 00:05:09,640 specificity. If you see an EPR signal, you 110 00:05:09,640 --> 00:05:12,360 know with 100% certainty that you have unpaired electrons. 111 00:05:12,360 --> 00:05:14,560 There's no ambiguity. You're not accidentally looking 112 00:05:14,560 --> 00:05:16,320 at the solvent or the glass tube it's in. 113 00:05:16,520 --> 00:05:19,800 So we're really looking for the radicals, the triplet states, 114 00:05:20,160 --> 00:05:22,680 the transition metal compounds. That's the list. 115 00:05:22,680 --> 00:05:25,960 Those are our dancers. And this selectivity allows for 116 00:05:25,960 --> 00:05:29,120 something really cool, something called in situ tracking. 117 00:05:29,200 --> 00:05:31,440 In situ, so in the reaction itself. 118 00:05:31,600 --> 00:05:35,280 Exactly because radicals are often very short lived, they're 119 00:05:35,280 --> 00:05:39,240 the volatile teenagers of the chemical world, always reacting 120 00:05:39,240 --> 00:05:42,080 quickly. We can sometimes use EPR to 121 00:05:42,080 --> 00:05:44,880 track these unstable intermediates in a chemical 122 00:05:44,880 --> 00:05:47,520 reaction that would be totally invisible to other methods. 123 00:05:47,640 --> 00:05:50,800 So you're watching the fleeting moments of chemistry happen in 124 00:05:50,800 --> 00:05:52,840 real time. You're catching a glimpse of the 125 00:05:52,840 --> 00:05:55,680 dancer mid spin before they change their pose. 126 00:05:55,680 --> 00:05:57,680 I love that. It's like having night vision 127 00:05:57,680 --> 00:06:01,040 goggles that only see heat and it just ignores all the other 128 00:06:01,040 --> 00:06:03,840 visual clutter. Now Speaking of other methods, I 129 00:06:03,840 --> 00:06:08,560 feel like we have to address the elephant in the room or maybe 130 00:06:08,640 --> 00:06:10,480 the the cousin in the room. I think I know where you're 131 00:06:10,480 --> 00:06:12,520 going with this most. People who have taken a 132 00:06:12,520 --> 00:06:16,160 chemistry class or even just had an MRI at a hospital have heard 133 00:06:16,160 --> 00:06:18,400 of NMR Nuclear magnetic resonance ah. 134 00:06:19,080 --> 00:06:22,280 Yes, NMR, the famous sibling, the one that gets all the press. 135 00:06:22,520 --> 00:06:24,920 It really does. NMR is everywhere. 136 00:06:25,360 --> 00:06:28,240 So is EPR just NMR's weird little brother? 137 00:06:28,600 --> 00:06:31,000 Are they related? They are very, very closely 138 00:06:31,000 --> 00:06:33,080 related. In fact, the fundamental physics 139 00:06:33,080 --> 00:06:34,760 parallels each other almost perfectly. 140 00:06:35,280 --> 00:06:38,920 In NMR, we're using a magnetic field to split the spin states 141 00:06:38,920 --> 00:06:42,280 of the nucleus, usually protons or carbon 13. 142 00:06:42,280 --> 00:06:46,120 Right, the core of the atom. The core in EPR we're doing the 143 00:06:46,120 --> 00:06:48,760 exact same thing, but we're splitting the spin states of the 144 00:06:48,760 --> 00:06:50,840 electron, the cloud orbiting that core. 145 00:06:50,840 --> 00:06:53,520 OK, so same concept, different target. 1 looks at the center of 146 00:06:53,520 --> 00:06:55,400 the atom, the other looks at the orbiting cloud. 147 00:06:55,720 --> 00:06:57,960 But looking at the stats, it seems like the power level 148 00:06:57,960 --> 00:07:00,200 involved is, well, it's very different. 149 00:07:00,360 --> 00:07:02,320 Drastically different. Let's just look at the frequency 150 00:07:02,320 --> 00:07:05,520 of the energy we use. NMR typically uses radio waves. 151 00:07:05,520 --> 00:07:07,760 We're talking about the megahertz M Hertz range. 152 00:07:07,840 --> 00:07:09,840 Radio waves. That's pretty low energy. 153 00:07:09,840 --> 00:07:11,920 That's like listening to the Top 40 in your car. 154 00:07:12,080 --> 00:07:16,200 Very chill, very low impact. EPR on the other hand operates 155 00:07:16,200 --> 00:07:19,200 in the microwave range. We are talking gigahertz. 156 00:07:19,720 --> 00:07:22,280 G Hertz. So NMR is listening to the radio 157 00:07:22,400 --> 00:07:26,640 and EPR is cooking your dinner. In terms of the frequency, yes, 158 00:07:26,800 --> 00:07:30,440 we are pumping in a lot more energy and that all relates back 159 00:07:30,440 --> 00:07:33,120 to the magnetic moment of the particle we're looking at. 160 00:07:33,120 --> 00:07:36,120 The magnetic moment. How magnetic it is essentially. 161 00:07:36,760 --> 00:07:39,480 You have to remember the electron is much much lighter 162 00:07:39,480 --> 00:07:43,760 than a nucleus. A single proton is roughly 2000 163 00:07:43,760 --> 00:07:46,880 times heavier than an electron. So the electron is a 164 00:07:46,880 --> 00:07:49,320 lightweight. A total lightweight, but it has 165 00:07:49,320 --> 00:07:53,800 a massive magnetic personality. It's magnetic moment is vastly 166 00:07:53,800 --> 00:07:56,920 larger than that of a proton. So even though it's tiny, it's a 167 00:07:56,920 --> 00:07:59,600 much stronger magnet. A much, much stronger magnet. 168 00:07:59,600 --> 00:08:02,480 And because the electron is so much more magnetic, it responds 169 00:08:02,480 --> 00:08:05,240 much more vigorously to the external magnetic field we 170 00:08:05,240 --> 00:08:07,520 apply. This leads to a huge difference 171 00:08:07,520 --> 00:08:09,480 in sensitivity. I wanted to ask about that. 172 00:08:09,480 --> 00:08:11,280 I was reading through the background on this and I I saw a 173 00:08:11,280 --> 00:08:15,000 note that said EPR is approximately 3 times more 174 00:08:15,000 --> 00:08:19,160 sensitive than NMR and I sat there thinking only three times. 175 00:08:19,400 --> 00:08:21,160 That doesn't seem right if the electron is so much more 176 00:08:21,160 --> 00:08:23,520 magnetic. I see where that confusion 177 00:08:23,520 --> 00:08:26,400 probably comes from. That's likely a very specific 178 00:08:26,400 --> 00:08:29,120 comparison of signal to noise under, you know, some very 179 00:08:29,120 --> 00:08:32,080 specific conditions, or maybe just a typo and a summary 180 00:08:32,080 --> 00:08:35,960 somewhere. In practice, in the real world, 181 00:08:36,720 --> 00:08:39,200 the difference is orders of magnitude. 182 00:08:39,280 --> 00:08:40,760 OK, that makes a lot more sense to me. 183 00:08:40,840 --> 00:08:43,720 Let's put it in practical terms for a chemist in the lab. 184 00:08:44,000 --> 00:08:47,000 If I want to run a standard NMR spectrum, I usually need a 185 00:08:47,000 --> 00:08:50,840 sample concentration in the millimolar range, right? 186 00:08:51,040 --> 00:08:53,480 I need a decent amount of stuff dissolved in my tube. 187 00:08:53,760 --> 00:08:57,160 Which, if you spent six months of your PhD trying to synthesize 188 00:08:57,160 --> 00:08:59,840 a tiny pinch of white powder, is a lot to ask. 189 00:09:00,040 --> 00:09:02,720 It can be heartbreaking to dump your entire life's work into an 190 00:09:02,720 --> 00:09:07,920 NMR tube, yes, but for EPR we can often work with micromolar 191 00:09:07,920 --> 00:09:10,960 concentration. Micromolar, so 1000 times less 192 00:09:10,960 --> 00:09:12,440 concentrated. Or even less. 193 00:09:12,440 --> 00:09:16,160 We need a tiny, tiny fraction of the material to get a 194 00:09:16,160 --> 00:09:19,320 screamingly loud signal. So it's thousands of times more 195 00:09:19,320 --> 00:09:20,800 sensitive in practice. Correct. 196 00:09:20,800 --> 00:09:23,920 It's built to detect minute traces of these paramagnetic 197 00:09:23,920 --> 00:09:25,680 species. But of course, there's a 198 00:09:25,680 --> 00:09:27,720 tradeoff. Nature never gives you a free 199 00:09:27,720 --> 00:09:29,840 lunch. And this brings us to the time 200 00:09:29,840 --> 00:09:33,360 scale of the experiment. We call this relaxation time. 201 00:09:33,560 --> 00:09:35,440 This sounded like a spa treatment when I read it. 202 00:09:35,440 --> 00:09:38,640 Relaxation time, but I'm guessing the electrons aren't 203 00:09:38,640 --> 00:09:40,600 getting a massage and some cucumber water. 204 00:09:41,200 --> 00:09:45,400 Not quite, although in a way it is about them calming down. 205 00:09:45,680 --> 00:09:48,640 Relaxation is effectively how long it takes for the excited 206 00:09:48,640 --> 00:09:52,200 spin to return to its ground state to release that energy it 207 00:09:52,200 --> 00:09:54,120 just absorbed from the microwaves. 208 00:09:54,200 --> 00:09:58,240 OK, so how do the two techniques NMR and EPR compare on that 209 00:09:58,240 --> 00:10:00,680 front? Well, in NMR the nucleus is 210 00:10:00,680 --> 00:10:04,720 heavy and frankly a bit lazy. Relaxation takes seconds, 211 00:10:04,800 --> 00:10:07,160 sometimes many seconds. So it holds on to that energy 212 00:10:07,240 --> 00:10:08,080 for a while. It does. 213 00:10:08,080 --> 00:10:11,400 It's a slow, leisurely process. You can pulse it with energy, 214 00:10:11,400 --> 00:10:13,000 wait a second or two, pulse it again. 215 00:10:13,000 --> 00:10:14,960 It's very forgiving. But electrons. 216 00:10:15,120 --> 00:10:18,520 Electrons are jittery, hyperactive things in EPR. 217 00:10:18,600 --> 00:10:21,320 Relaxation happens in microseconds or even 218 00:10:21,320 --> 00:10:23,600 nanoseconds. It is lightning fast. 219 00:10:23,600 --> 00:10:25,920 Nanoseconds. So the electron absorbs the 220 00:10:25,920 --> 00:10:28,000 energy and spits it back out almost instantly. 221 00:10:28,120 --> 00:10:30,600 Almost instantly. And I want you to file that away 222 00:10:30,600 --> 00:10:33,360 in your mind because that speed difference, that hyperactive 223 00:10:33,360 --> 00:10:36,880 relaxation has huge, huge implications for how we 224 00:10:36,880 --> 00:10:38,240 actually, you have to take the measurement. 225 00:10:38,600 --> 00:10:41,800 It's the reason why if you walk into an EPR lab, you often see 226 00:10:41,800 --> 00:10:46,720 these giant steaming tanks of liquid nitrogen or liquid 227 00:10:46,720 --> 00:10:48,680 helium. We literally have to freeze 228 00:10:48,680 --> 00:10:50,840 things to slow them down enough to see them. 229 00:10:51,040 --> 00:10:52,880 But we'll get to that big chill section in a bit. 230 00:10:52,880 --> 00:10:54,800 I'm looking. Forward to that, but first I 231 00:10:54,800 --> 00:10:56,760 really want to visualize the actual physics. 232 00:10:57,120 --> 00:10:59,800 So we've got this machine. We've established it has a 233 00:10:59,800 --> 00:11:02,800 microwave source, a detector, and a massive electromagnet. 234 00:11:02,880 --> 00:11:04,960 We put our sample in this thing called the cavity. 235 00:11:05,560 --> 00:11:09,480 Now we turn the magnet on. What is physically happening to 236 00:11:09,480 --> 00:11:13,520 that lonely unpaired electron inside my sample tube? 237 00:11:13,560 --> 00:11:17,600 OK, so imagine the electron is a tiny bar magnet, like the kind 238 00:11:17,600 --> 00:11:18,920 you played with in elementary school. 239 00:11:19,160 --> 00:11:20,800 It has a North Pole and a South Pole. 240 00:11:21,160 --> 00:11:23,680 We call this it's magnetic moment. 241 00:11:23,680 --> 00:11:24,520 Got it. Tiny magnet. 242 00:11:24,640 --> 00:11:28,360 Now, when there's no external magnetic field, before we turn 243 00:11:28,360 --> 00:11:31,720 the machine on, that little electron magnet can point in any 244 00:11:31,720 --> 00:11:34,000 direction it wants. It's tumbling around, pointing 245 00:11:34,000 --> 00:11:36,960 up, down, left, right. It has no preference. 246 00:11:36,960 --> 00:11:38,840 All directions are equal in energy. 247 00:11:38,840 --> 00:11:39,560 It's. A free spirit. 248 00:11:39,720 --> 00:11:42,040 It is. But the moment you turn on that 249 00:11:42,040 --> 00:11:45,160 massive external magnet, which we call the field be dollars 250 00:11:45,160 --> 00:11:48,960 dollars, that freedom is gone. The electron is forced to align 251 00:11:49,640 --> 00:11:52,080 and quantum mechanics says it only has two choices. 252 00:11:52,240 --> 00:11:54,560 It's not like a compass needle that can swing around 253 00:11:54,560 --> 00:11:57,200 continuously. It can effectively align 254 00:11:57,200 --> 00:12:00,480 parallel to the field or anti parallel parallel to the field. 255 00:12:00,480 --> 00:12:03,040 So it can either go with the flow or it can swim upstream. 256 00:12:03,120 --> 00:12:06,680 That is a spot on analogy. Aligning with the field, going 257 00:12:06,680 --> 00:12:09,000 with the flow, that's the low energy state. 258 00:12:09,000 --> 00:12:11,520 That's where it wants to be. Aligning against the field. 259 00:12:11,520 --> 00:12:14,080 Swimming upstream is the high energy state. 260 00:12:14,080 --> 00:12:16,760 So we've created an energy gap. Before the magnet was on, 261 00:12:16,920 --> 00:12:19,400 everyone was on the same level. Now we have a floor and a 262 00:12:19,400 --> 00:12:20,320 ceiling. Exactly. 263 00:12:20,320 --> 00:12:23,760 We have split the electrons energy state into two distinct 264 00:12:23,760 --> 00:12:27,760 levels and this phenomenon has a name, the Zeeman effect. 265 00:12:27,760 --> 00:12:30,000 The Zeeman effect. And the stronger the magnet we 266 00:12:30,000 --> 00:12:33,200 use, the wider the gap becomes between that floor and that 267 00:12:33,200 --> 00:12:35,240 ceiling. This is where that equation I 268 00:12:35,240 --> 00:12:36,400 saw comes in. I'm going to try it. 269 00:12:36,920 --> 00:12:44,000 We EG, Mehmub B0, MSC. Don't let the letter scare you. 270 00:12:44,000 --> 00:12:46,120 It looks more intimidating than I'm. 271 00:12:46,160 --> 00:12:48,480 Trying not to, but let's break it down so we know what we're 272 00:12:48,480 --> 00:12:50,320 really looking at. It's actually quite 273 00:12:50,320 --> 00:12:52,800 straightforward. MLR's is just the energy of that 274 00:12:52,800 --> 00:12:55,520 level we're talking about. B dollars is just the strength 275 00:12:55,520 --> 00:12:58,960 of the big magnet you're using, and M lever dollars is just the 276 00:12:58,960 --> 00:13:01,240 spin state. Is it pointing with the field 277 00:13:01,240 --> 00:13:04,200 which we call plus 12 or against the field which is -12? 278 00:13:04,200 --> 00:13:07,200 And then we have the constants, the Greek letters. 279 00:13:07,360 --> 00:13:09,440 Right. And Miller is the Bohr magneton. 280 00:13:09,480 --> 00:13:11,640 That's just a fundamental constant of nature that 281 00:13:11,800 --> 00:13:14,160 basically describes how magnetic an electron is. 282 00:13:14,280 --> 00:13:17,440 You can think of it as the conversion rate between spin and 283 00:13:17,440 --> 00:13:20,200 magnetism. OK, so that's just a number, but 284 00:13:20,200 --> 00:13:22,960 then there's Joel Dollar. Ah yes, the Joel factor. 285 00:13:23,200 --> 00:13:26,080 Now, usually in physics, constants are just constants, 286 00:13:26,080 --> 00:13:29,320 like the speed of light or π the kind of boring numbers you just 287 00:13:29,320 --> 00:13:31,080 have to memorize. But I get the sense that the 288 00:13:31,080 --> 00:13:32,960 Jollers is actually the star of the show here. 289 00:13:33,200 --> 00:13:34,920 It absolutely is. In many ways. 290 00:13:35,240 --> 00:13:38,040 Daters are the whole reason we can use EPR to identify 291 00:13:38,040 --> 00:13:39,760 different chemicals. How so? 292 00:13:39,880 --> 00:13:43,000 Well, for a completely free electron, and by that I mean 293 00:13:43,000 --> 00:13:45,720 imagine an electron floating in the vacuum of deep space, 294 00:13:45,720 --> 00:13:49,880 totally alone, no atoms nearby. For that electron date all is a 295 00:13:49,880 --> 00:13:55,440 fundamental constant. It is exactly 2.000232. 2.0, 296 00:13:55,520 --> 00:14:01,480 0232. 2.00232 Extremely precise. Physics is nothing if not 297 00:14:01,480 --> 00:14:03,520 precise. However, as chemists, we don't 298 00:14:03,520 --> 00:14:06,120 study electrons in deep space. Right, we study them here on 299 00:14:06,120 --> 00:14:07,560 Earth. We study them trapped inside 300 00:14:07,560 --> 00:14:10,480 molecules or caught in the crystal lattice of a solid. 301 00:14:10,800 --> 00:14:14,000 And in those environments the electron isn't truly free, it's 302 00:14:14,000 --> 00:14:15,560 interacting with other electrons. 303 00:14:15,560 --> 00:14:20,040 It has orbital angular momentum. It's feeling the traffic of the 304 00:14:20,040 --> 00:14:22,000 molecule around. It so the local neighborhood 305 00:14:22,000 --> 00:14:24,720 actually changes the fundamental behavior of the electron. 306 00:14:24,760 --> 00:14:26,760 It changes its apparent magnetic personality. 307 00:14:26,760 --> 00:14:30,080 Yes, the local electronic environment shifts the data 308 00:14:30,080 --> 00:14:33,800 value away from that free electron value of 2.00232. 309 00:14:33,800 --> 00:14:36,520 And this is huge. This is everything this shift 310 00:14:36,520 --> 00:14:39,680 tells us about the structure. If I measure a sample and I 311 00:14:39,680 --> 00:14:45,400 calculate AG value of say 1.94, I know immediately I'm probably 312 00:14:45,400 --> 00:14:47,680 dealing with a specific type of transition. 313 00:14:47,680 --> 00:14:49,400 Metal comp complex, little interesting. 314 00:14:49,400 --> 00:14:53,320 If I see AG value of 2.0 O five, I might be looking at an organic 315 00:14:53,320 --> 00:14:56,840 radical on a carbon atom. The G value tells us what the 316 00:14:56,840 --> 00:14:59,800 electron is sitting on. It's like a fingerprint, or 317 00:14:59,800 --> 00:15:02,280 maybe an accent. The electron speaks with a 318 00:15:02,280 --> 00:15:04,440 slightly different accent depending on whether it lives on 319 00:15:04,440 --> 00:15:06,720 a carbon atom or a vanadium atom. 320 00:15:06,920 --> 00:15:10,600 That's a brilliant analogy. The accent reveals the origin 321 00:15:10,760 --> 00:15:14,040 and it gets even more complex and frankly more interesting. 322 00:15:14,520 --> 00:15:16,920 The Nates mentioned something about it being anisotropic. 323 00:15:17,120 --> 00:15:19,280 Anisotropic. Yeah, that's a $10 word that 324 00:15:19,280 --> 00:15:20,760 trips up a lot of students, I'm sure. 325 00:15:21,000 --> 00:15:23,120 It sounds like something that requires a cream from the 326 00:15:23,120 --> 00:15:25,080 dermatologist. It does, doesn't it? 327 00:15:25,360 --> 00:15:27,320 But all it means is direction dependent. 328 00:15:27,600 --> 00:15:30,080 Explain that. Imagine you have a molecule 329 00:15:30,080 --> 00:15:33,880 that's flat like a pancake, and it's trapped in a solid crystal 330 00:15:33,880 --> 00:15:37,080 so it can't move. The electron in that pancake 331 00:15:37,080 --> 00:15:40,200 feels a different magnetic environment depending on whether 332 00:15:40,200 --> 00:15:43,280 the big magnet is hitting the pancake from the top, the flat 333 00:15:43,280 --> 00:15:44,920 side, or from the edge. Oh, I see. 334 00:15:44,920 --> 00:15:47,360 So if the magnet is perpendicular to the pancake, 335 00:15:47,560 --> 00:15:50,480 the electrons are kind of running in circles one way. 336 00:15:50,720 --> 00:15:53,200 If it's parallel, they're running a different way. 337 00:15:53,240 --> 00:15:54,680 That's a great way to think about it. 338 00:15:54,920 --> 00:15:58,160 And as a result, the G value, which remember sets the energy 339 00:15:58,160 --> 00:16:01,840 gap, actually changes as you rotate the sample in the magnet. 340 00:16:02,000 --> 00:16:04,640 No way. Yes, if you measure a single 341 00:16:04,640 --> 00:16:08,840 crystal you you might see the GL value shift from say $2.00 all 342 00:16:08,840 --> 00:16:11,600 the way to $2.20 just by turning it 90°. 343 00:16:11,760 --> 00:16:14,040 That seems like it would make things incredibly complicated. 344 00:16:14,120 --> 00:16:16,600 It does, but it also makes it incredibly powerful. 345 00:16:16,720 --> 00:16:20,240 It means if we measure very carefully, we can map out the 3D 346 00:16:20,240 --> 00:16:22,200 electronic structure of the molecule. 347 00:16:22,440 --> 00:16:25,240 We can tell how the orbitals that hold that electron are 348 00:16:25,240 --> 00:16:27,720 oriented in space. That is wild. 349 00:16:28,240 --> 00:16:30,600 But what if the sample isn't a perfect crystal? 350 00:16:30,800 --> 00:16:32,960 What if it's just a liquid? Great question. 351 00:16:33,800 --> 00:16:36,600 In a liquid solution, the molecules are all tumbling 352 00:16:36,600 --> 00:16:39,560 around incredibly fast, they're spinning like crazy. 353 00:16:39,560 --> 00:16:43,520 So all those directional differences, top, side, front, 354 00:16:43,720 --> 00:16:46,240 they all get averaged out. So you just see one single 355 00:16:46,240 --> 00:16:48,200 number. You just see one single sharp 356 00:16:48,200 --> 00:16:51,880 number, the isotropic average. So liquids give you the average 357 00:16:51,880 --> 00:16:55,160 accent, but solids can give you the specific directional 358 00:16:55,160 --> 00:16:57,000 dialects. You can absolutely say that, 359 00:16:57,000 --> 00:16:59,280 yes. So the whole workflow is if you 360 00:16:59,280 --> 00:17:01,440 put the sample in the machine, hit it with microwaves, you 361 00:17:01,440 --> 00:17:04,560 measure the absorption, you calculate this $2.00 value, and 362 00:17:04,560 --> 00:17:06,920 then you play detective to figure out the structure. 363 00:17:07,000 --> 00:17:09,760 That is the basic workflow in a nutshell, yes. 364 00:17:09,760 --> 00:17:11,599 Now I want to talk about the data itself, right? 365 00:17:11,760 --> 00:17:15,400 Because if you are listener were to walk into a lab and look at a 366 00:17:15,400 --> 00:17:18,319 computer screen running an EPR experiment, you might be very 367 00:17:18,319 --> 00:17:20,680 confused. When I look at other chemistry 368 00:17:20,680 --> 00:17:25,000 graphs, like say a UV VS or an infrared spectrum, I see peaks, 369 00:17:25,280 --> 00:17:28,520 I see hills, the line goes up, it forms a mountain, and then it 370 00:17:28,520 --> 00:17:30,880 comes back, right? A classic absorption peak. 371 00:17:31,160 --> 00:17:34,880 But looking at the source images for EPR, these don't look like 372 00:17:34,880 --> 00:17:37,160 hills at all. They look like a squiggly line 373 00:17:37,160 --> 00:17:40,720 that goes up and then sharply dives down, crossing through the 374 00:17:40,720 --> 00:17:43,480 center line into the negative, and then it comes back up. 375 00:17:43,720 --> 00:17:47,160 It looks like a sine wave or maybe a heartbeat on an EKG. 376 00:17:47,280 --> 00:17:50,360 Yes, that is a classic confusing point for beginners. 377 00:17:51,040 --> 00:17:55,760 In standard continuous wave or CWEPR, we don't actually record 378 00:17:55,760 --> 00:17:59,400 the absorption curve directly. We record the first derivative 379 00:17:59,400 --> 00:18:01,880 of the absorption. Calculus strikes again. 380 00:18:02,080 --> 00:18:05,360 Why on earth would we do that? Is it just to be difficult? 381 00:18:06,280 --> 00:18:09,200 Sometimes it feels that way, but no, there's a very good reason. 382 00:18:09,200 --> 00:18:12,040 It's purely an engineering solution to dramatically improve 383 00:18:12,040 --> 00:18:14,360 the sensitivity. OK, remember we're looking for 384 00:18:14,360 --> 00:18:17,120 tiny, tiny signals. If we just tried to measure the 385 00:18:17,120 --> 00:18:20,680 absorption the hill, it might be so small and broad that it gets 386 00:18:20,680 --> 00:18:23,120 completely lost in the electronic noise of the machine. 387 00:18:23,120 --> 00:18:25,280 Like trying to see a small bump on a really bumpy Rd. 388 00:18:25,320 --> 00:18:27,680 Exactly. So we use a clever trick called 389 00:18:27,680 --> 00:18:30,760 magnetic field modulation. We wiggle the main magnetic 390 00:18:30,760 --> 00:18:34,120 field up and down just a tiny bit, very, very fast while we 391 00:18:34,120 --> 00:18:35,640 slowly sweep through the measurement. 392 00:18:36,000 --> 00:18:39,320 And that wiggling does what? It allows us to use something 393 00:18:39,320 --> 00:18:41,240 called Phase sensitive detection. 394 00:18:41,840 --> 00:18:45,560 It's an electronic filter that is tuned to look for a signal 395 00:18:45,560 --> 00:18:47,880 that is wiggling at that exact frequency. 396 00:18:47,880 --> 00:18:49,800 So it filters out all the random noise. 397 00:18:49,840 --> 00:18:52,840 It's an incredible filter, just kills the noise, but the 398 00:18:52,840 --> 00:18:56,720 mathematical output of that whole process just happens to be 399 00:18:56,720 --> 00:18:59,680 the slope of the line, the derivative, rather than the 400 00:18:59,680 --> 00:19:01,960 height of the line. So the point where the line 401 00:19:01,960 --> 00:19:05,440 crosses 0, that sharp dive in the middle of the signal, Yeah, 402 00:19:05,440 --> 00:19:07,720 that's actually the very top of the absorption hill. 403 00:19:07,880 --> 00:19:10,800 Exactly that zero crossing is the resonant field. 404 00:19:10,960 --> 00:19:13,760 It's the point where the slope goes from positive as you're 405 00:19:13,760 --> 00:19:16,200 going up the hill to negative as you're going down the other 406 00:19:16,200 --> 00:19:17,120 side. Got it. 407 00:19:17,240 --> 00:19:19,000 So we're looking for that zero crossing. 408 00:19:19,480 --> 00:19:22,320 Now let's circle back to that Big Chill topic we pinned 409 00:19:22,320 --> 00:19:24,040 earlier. The whole freezing thing. 410 00:19:24,080 --> 00:19:26,760 Right, the cryogenics. You said electrons relax. 411 00:19:26,760 --> 00:19:29,120 They lose their energy really really fast. 412 00:19:29,440 --> 00:19:32,200 Microseconds or less. Why is that a problem? 413 00:19:32,400 --> 00:19:35,400 Well, it's a problem of balance. It's a Goldilocks problem. 414 00:19:35,800 --> 00:19:39,040 We have two main types of relaxation we worry about T 415 00:19:39,040 --> 00:19:42,280 taller and T Audi $2.00 T dollar is called spin lattice 416 00:19:42,280 --> 00:19:45,000 relaxation. This is the process of the 417 00:19:45,000 --> 00:19:48,440 electron dumping it's extra energy back into the surrounding 418 00:19:48,440 --> 00:19:50,240 environment, the molecular lattice. 419 00:19:50,720 --> 00:19:54,720 Now, if this process happens too slowly, if the electron holds on 420 00:19:54,720 --> 00:19:57,680 to that energy for too long, we have a big problem. 421 00:19:58,360 --> 00:20:01,240 Remember, our experiment is trying to pump electrons from 422 00:20:01,240 --> 00:20:04,720 the floor to the ceiling. But if the ceiling gets full 423 00:20:04,720 --> 00:20:06,600 because the electrons won't come back down. 424 00:20:06,600 --> 00:20:09,080 The upper bunk is taken. You can't put anyone else up 425 00:20:09,080 --> 00:20:09,440 there. Right. 426 00:20:09,440 --> 00:20:11,160 You can't promote any more electrons. 427 00:20:11,160 --> 00:20:14,240 The signal saturates. The absorption just stops 428 00:20:14,240 --> 00:20:16,040 because there's nowhere for the electrons to go. 429 00:20:16,160 --> 00:20:17,880 The signal vanishes from your screen. 430 00:20:17,880 --> 00:20:21,360 OK, so too slow is bad, but you said earlier the electrons are 431 00:20:21,360 --> 00:20:23,320 usually really fast. They are. 432 00:20:23,320 --> 00:20:25,240 And that brings us to the opposite problem. 433 00:20:25,480 --> 00:20:28,680 If the relaxation is too fast, if the electron jumps up and 434 00:20:28,680 --> 00:20:31,880 falls back down almost instantly, we run head first 435 00:20:31,880 --> 00:20:34,840 into a fundamental law of the universe, the Heisenberg 436 00:20:34,840 --> 00:20:38,120 Uncertainty Principle. Oh boy, here we go. 437 00:20:38,320 --> 00:20:39,840 Quantum mechanics. Greatest hit. 438 00:20:40,200 --> 00:20:41,680 You can't know everything at once. 439 00:20:41,840 --> 00:20:45,360 That's essentially it. The principle in this context 440 00:20:45,360 --> 00:20:48,600 states that the uncertainty in the energy of a state times the 441 00:20:48,600 --> 00:20:51,720 uncertainty in the time it exists for is roughly a constant 442 00:20:52,200 --> 00:20:56,680 delta, EA, prox bar, a last bar. OK, let's translate that into 443 00:20:56,680 --> 00:21:01,000 something we can visualize. It means that if the time delta 444 00:21:01,000 --> 00:21:04,120 T or, which is the lifetime of our excited electron stay, is 445 00:21:04,120 --> 00:21:08,200 very, very short, then the uncertainty in the energy delta 446 00:21:08,200 --> 00:21:11,680 E must be very, very large to compensate. 447 00:21:11,800 --> 00:21:15,800 And since energy corresponds to the frequency or the position on 448 00:21:15,800 --> 00:21:18,120 our graph. A large uncertainty in energy 449 00:21:18,120 --> 00:21:21,520 means the line on our graph isn't a sharp, crisp spike, it 450 00:21:21,520 --> 00:21:24,600 becomes a broad, fuzzy smear. So if the electron relaxes too 451 00:21:24,600 --> 00:21:27,280 fast, the line gets so incredibly wide that it 452 00:21:27,280 --> 00:21:29,480 effectively just flattens out and looks like the baseline. 453 00:21:29,480 --> 00:21:31,240 Exactly. It blurs out of existence. 454 00:21:31,240 --> 00:21:33,440 So let me get this straight. If it's too slow, you see 455 00:21:33,440 --> 00:21:35,920 nothing because of saturation. If it's too fast, you see 456 00:21:35,920 --> 00:21:37,320 nothing because of Heisenberg blurring. 457 00:21:37,600 --> 00:21:39,920 We need a Goldilocks zone. We do. 458 00:21:39,960 --> 00:21:43,440 We need it to be just right. And for many interesting 459 00:21:43,440 --> 00:21:46,760 systems, especially transition metal complexes at room 460 00:21:46,760 --> 00:21:50,280 temperature, the relaxation is way, way too fast. 461 00:21:50,360 --> 00:21:52,240 Why? The thermal energy in the room, 462 00:21:52,240 --> 00:21:54,600 the vibrations of the molecule, it's all just knocking the 463 00:21:54,600 --> 00:21:57,760 electrons around too much. The lines are hopelessly broad. 464 00:21:57,760 --> 00:21:59,880 You see nothing. So how do we slow them down? 465 00:22:00,080 --> 00:22:02,800 We put them on ice. Serious ice. 466 00:22:03,120 --> 00:22:06,200 We cool the sample down with liquid nitrogen to get to 77 467 00:22:06,200 --> 00:22:10,040 Kelvin, or for the really jittery fast relaxing cases we 468 00:22:10,040 --> 00:22:13,520 have to use liquid helium to get down to 4 Kelvin. 4 Kelvin that 469 00:22:13,520 --> 00:22:17,840 is 4° above absolute zero. That is colder than deep space. 470 00:22:18,080 --> 00:22:21,120 It is at that temperature all that thermal motion effectively 471 00:22:21,120 --> 00:22:23,400 stops. The lattice quiets down. 472 00:22:23,640 --> 00:22:26,640 This dramatically lengthens the relaxation time the delta tuler. 473 00:22:26,640 --> 00:22:29,720 Which in turn shrinks the energy uncertainty, the delta E. 474 00:22:30,000 --> 00:22:34,040 And suddenly, that broad, invisible smear sharpens up into 475 00:22:34,040 --> 00:22:36,560 a beautiful, crisp, readable signal. 476 00:22:36,720 --> 00:22:40,080 So when we see a photo of an EPR lab and there are these giant 477 00:22:40,080 --> 00:22:43,120 stainless steel tanks of cryogens and white vapor curling 478 00:22:43,120 --> 00:22:45,440 along the floor, it's not just for show. 479 00:22:45,440 --> 00:22:47,800 It's not just to look like a man scientist's layer. 480 00:22:47,960 --> 00:22:50,720 No, though it does add to the ambience. 481 00:22:50,760 --> 00:22:54,000 It is absolutely fundamental to the measurement for many, many 482 00:22:54,000 --> 00:22:56,440 samples. Without the cold, the physics 483 00:22:56,440 --> 00:22:58,200 simply won't let you see the data. 484 00:22:58,440 --> 00:23:01,440 That's amazing. OK, so we've frozen our sample. 485 00:23:01,440 --> 00:23:03,680 We've tuned in our microwaves, we found the signal, we've 486 00:23:03,680 --> 00:23:06,720 calculated our G value. But usually we don't just see 487 00:23:06,720 --> 00:23:11,920 one single line crossing 0. We see a pattern comb of lines, 488 00:23:12,520 --> 00:23:15,560 a forest of lines. Sometimes this is where the 489 00:23:15,560 --> 00:23:19,160 detective work goes from identifying this suspect to 490 00:23:19,360 --> 00:23:22,000 interrogating them. This is hyperfine coupling. 491 00:23:22,000 --> 00:23:23,720 This is my favorite part. This is where you get the 492 00:23:23,720 --> 00:23:26,000 fingerprint of the molecule. This is where the molecule 493 00:23:26,000 --> 00:23:28,640 really tells you its life story. So the idea is that the electron 494 00:23:28,640 --> 00:23:32,000 isn't just floating in a void, it's usually orbiting A nucleus 495 00:23:32,760 --> 00:23:36,120 and many nuclei, like hydrogen or nitrogen or vanadium. 496 00:23:36,160 --> 00:23:38,800 They have their own spins. There are little tiny magnets 497 00:23:38,800 --> 00:23:39,440 too. Correct. 498 00:23:39,440 --> 00:23:42,520 So now you have a situation where the electron is feeling 499 00:23:42,520 --> 00:23:45,800 the massive magnetic field from the machine's electromagnet, but 500 00:23:45,800 --> 00:23:49,560 it's also feeling a tiny local magnetic field from the nucleus 501 00:23:49,560 --> 00:23:51,520 it's sitting right next to. It's like you're trying to 502 00:23:51,520 --> 00:23:54,480 listen to a loudspeaker at a concert, but someone is 503 00:23:54,480 --> 00:23:57,040 whispering right in your ear. That's a really good way to put 504 00:23:57,040 --> 00:23:58,840 it. That little whisper changes what 505 00:23:58,840 --> 00:24:01,800 you hear. Imagine the nucleus is another 506 00:24:01,800 --> 00:24:04,360 tiny bar magnet right next to our electron. 507 00:24:05,160 --> 00:24:07,600 That nuclear magnet can also point up or down. 508 00:24:08,120 --> 00:24:11,840 If it points up, it adds a little bit to the total magnetic 509 00:24:11,840 --> 00:24:15,600 field the electron feels. If it points down, it subtracts 510 00:24:15,600 --> 00:24:18,120 a little bit from the field. So the single energy level we 511 00:24:18,120 --> 00:24:20,400 had before now splits. It splits again. 512 00:24:20,400 --> 00:24:23,880 Instead of one line where the electron flips, you now have two 513 00:24:23,880 --> 00:24:25,960 slightly different fields where it can flip. 514 00:24:26,240 --> 00:24:29,520 One for the nucleus up case and one for the nucleus down case. 515 00:24:29,680 --> 00:24:32,640 And this creates the pattern. This creates the pattern and the 516 00:24:32,640 --> 00:24:35,720 notes distinguish between 2 main ways this interaction can 517 00:24:35,720 --> 00:24:38,680 happen. The 1st is dipole dipole 518 00:24:38,680 --> 00:24:40,320 coupling. Dipole dipole. 519 00:24:40,480 --> 00:24:42,880 That sounds like the magnets on my fridge interacting with each 520 00:24:42,880 --> 00:24:44,160 other. It is exactly that kind of 521 00:24:44,160 --> 00:24:45,640 interaction. It's an interaction through 522 00:24:45,640 --> 00:24:47,720 space. But just like we talked about 523 00:24:47,720 --> 00:24:51,160 with the anisotropy of the G value, this interaction is very 524 00:24:51,160 --> 00:24:53,400 direction dependent. So in a liquid. 525 00:24:53,520 --> 00:24:57,000 In a liquid solution where the molecules are tumbling rapidly, 526 00:24:57,240 --> 00:25:00,080 the dipole dipole interaction averages out to zero. 527 00:25:00,080 --> 00:25:02,120 It completely vanishes. OK. 528 00:25:02,120 --> 00:25:04,640 So if we're looking at a liquid sample, we don't see dipole 529 00:25:04,640 --> 00:25:07,680 dipole, but we definitely still see splitting. 530 00:25:07,680 --> 00:25:09,760 So that must be the other one, the Fermi contact. 531 00:25:09,840 --> 00:25:14,080 Yes, and the Fermi content interaction is, well, it's a bit 532 00:25:14,080 --> 00:25:16,320 spooky. It's purely a quantum mechanical 533 00:25:16,320 --> 00:25:18,640 effect. Also, classical physics says 2 534 00:25:18,640 --> 00:25:22,320 objects can't occupy the same space at the same time, but 535 00:25:22,320 --> 00:25:25,560 quantum mechanics says that an electron in a spherical's 536 00:25:25,560 --> 00:25:30,200 orbital has a non 0 probability of being found at the nucleus. 537 00:25:30,360 --> 00:25:32,840 Wait, you mean literally inside the nucleus? 538 00:25:32,920 --> 00:25:36,360 Like the electron cloud overlaps with the space where the protons 539 00:25:36,360 --> 00:25:38,600 and neutrons are. Literally occupying the same 540 00:25:38,600 --> 00:25:41,760 physical space, the electron's wave function has amplitude at 541 00:25:41,760 --> 00:25:44,000 the nucleus. This creates a direct, very 542 00:25:44,000 --> 00:25:47,600 strong magnetic interaction that doesn't depend on direction, so 543 00:25:47,600 --> 00:25:49,720 it doesn't average out when the molecule tumbles. 544 00:25:50,200 --> 00:25:52,520 This is what gives us those beautiful sharp splitting 545 00:25:52,520 --> 00:25:54,840 patterns we see in liquid phase EPR. 546 00:25:55,000 --> 00:25:57,480 That is mind bending. The electron is basically 547 00:25:57,480 --> 00:26:00,640 haunting the nucleus. OK, so let's try to predict some 548 00:26:00,640 --> 00:26:03,040 of these patterns because there's a simple rule for this, 549 00:26:03,040 --> 00:26:04,920 right? The two I plus one rule. 550 00:26:04,920 --> 00:26:08,640 Simple algebra I is the quantum number for the spin of the 551 00:26:08,640 --> 00:26:11,320 nucleus. The rule is simply $2.00 plus 552 00:26:11,440 --> 00:26:13,760 other one, and that tells you how many lines you get from 553 00:26:13,760 --> 00:26:16,520 coupling to that one nucleus. OK, pop quiz time. 554 00:26:16,800 --> 00:26:22,040 If I have a hydrogen atom, a proton, I know from NMR that 555 00:26:22,040 --> 00:26:24,640 hydrogen has a spin of 12. So you plug that into the 556 00:26:24,640 --> 00:26:29,320 formula $2.00 * 12 plus. That's 1 + 1 = 2, so I should 557 00:26:29,320 --> 00:26:31,800 see a doublet 2 lines. Correct A1 to 1 doublet. 558 00:26:31,960 --> 00:26:35,480 What about, say, nitrogen 14? The notes say that has a spin of 559 00:26:35,480 --> 00:26:39,160 1. So $2.00 * 1 + 1 = 3 three you 560 00:26:39,160 --> 00:26:41,840 get a triplet 3 lines of equal intensity. 561 00:26:42,080 --> 00:26:44,360 And the physical distance between those lines on the 562 00:26:44,360 --> 00:26:45,640 spectrum, what does that tell us? 563 00:26:45,960 --> 00:26:48,720 That distance is what we call the hyperfine coupling constant, 564 00:26:48,720 --> 00:26:50,160 and it's usually denoted as Earth. 565 00:26:50,160 --> 00:26:52,920 The magnitude tells you how intimate the relationship is 566 00:26:52,920 --> 00:26:54,640 between the electron and that nucleus. 567 00:26:54,640 --> 00:26:57,440 So a big value. If A is big, the lines are 568 00:26:57,440 --> 00:27:00,080 spread far apart. That means the electron is 569 00:27:00,080 --> 00:27:02,840 spending a lot of time very close to that nucleus. 570 00:27:03,240 --> 00:27:06,400 The spin density is high. If it's small, the electron is 571 00:27:06,400 --> 00:27:08,720 probably further away, barely visiting. 572 00:27:08,720 --> 00:27:10,760 So it literally maps the electron density. 573 00:27:10,800 --> 00:27:13,760 It's a way of saying the electron lives mostly here, but 574 00:27:13,760 --> 00:27:15,320 not so much there. You've got it. 575 00:27:15,760 --> 00:27:17,800 It's a map of the unpaired electrons home. 576 00:27:17,960 --> 00:27:20,880 This is the perfect time to walk through the case studies from 577 00:27:20,880 --> 00:27:23,680 our source material because this is where we take all this theory 578 00:27:23,680 --> 00:27:26,800 and alley it to actual chemical mysteries case study #1 579 00:27:27,240 --> 00:27:29,760 Vanadium. Specifically a complex called 580 00:27:29,760 --> 00:27:33,600 Vinay CAC Tutu. Yes, a classic EPR standard. 581 00:27:33,640 --> 00:27:35,840 A lot of people use this to calibrate their machines. 582 00:27:35,960 --> 00:27:37,960 So we have a vanadium atom at the center. 583 00:27:38,200 --> 00:27:42,320 The main isotope is vanadium 51, and it's almost 100% abundant in 584 00:27:42,320 --> 00:27:44,840 nature, which is convenient. It has a nuclear spin. 585 00:27:44,840 --> 00:27:47,440 I have a 72. 72 That's a hefty spin. 586 00:27:47,440 --> 00:27:49,640 That's a lot of magnetic options for the nucleus. 587 00:27:49,640 --> 00:27:52,840 So let's do the math $2.00 * 72 plus fullest dollar. 588 00:27:52,840 --> 00:27:56,040 That's $7.00 + 1 = 88. So you would predict that you 589 00:27:56,040 --> 00:28:00,080 should see 8 evenly spaced lines, and if you look at the 590 00:28:00,080 --> 00:28:04,040 spectrum, that's exactly what you see, 8 distinct jagged 591 00:28:04,040 --> 00:28:06,320 heartbeats spread across the screen. 592 00:28:06,640 --> 00:28:08,800 But the notes point out a subtle detail. 593 00:28:08,800 --> 00:28:10,920 The lines aren't all perfectly identical. 594 00:28:10,920 --> 00:28:13,680 It's not a perfect comb. The ones on the very outside the 595 00:28:13,680 --> 00:28:17,720 far left and far right lines, they look a little shorter and 596 00:28:17,720 --> 00:28:20,160 maybe broader. Yes, they are broader and 597 00:28:20,160 --> 00:28:22,480 shorter. This is a very cool, subtle 598 00:28:22,480 --> 00:28:24,840 effect that's caused by that tumbling we were talking about 599 00:28:24,840 --> 00:28:26,920 earlier. The molecule spinning around in 600 00:28:26,920 --> 00:28:27,800 the liquid. Right. 601 00:28:28,120 --> 00:28:31,040 If the molecule is very big or if the solvent is thick and 602 00:28:31,040 --> 00:28:35,040 viscous like syrup, the molecule might not be tumbling quite fast 603 00:28:35,040 --> 00:28:37,960 enough to perfectly and completely average out all that 604 00:28:37,960 --> 00:28:41,600 anisotropic stuff we discussed. We call this the slow tumbling 605 00:28:41,600 --> 00:28:44,040 regime. So just by looking at the line 606 00:28:44,040 --> 00:28:46,840 with, by seeing that the outer lines are fatter than the inner 607 00:28:46,960 --> 00:28:50,160 lines, we can actually get information about how fast the 608 00:28:50,160 --> 00:28:52,400 molecule is physically spinning in the solution. 609 00:28:52,720 --> 00:28:54,960 We can't. It gives us dynamic information. 610 00:28:55,200 --> 00:28:59,000 We can learn about the viscosity of the solvent or the 611 00:28:59,000 --> 00:29:01,160 hydrodynamic size of the molecule itself. 612 00:29:01,640 --> 00:29:04,520 We can tell if our molecule is maybe binding to something big 613 00:29:04,520 --> 00:29:07,240 like a protein, because that would slow it down and broaden 614 00:29:07,240 --> 00:29:09,240 those outer lines. That is incredibly cool. 615 00:29:09,240 --> 00:29:12,520 It's like having a tiny speedometer for molecules. 616 00:29:13,080 --> 00:29:16,160 OK, let's move on. Case study #2 The benzene 617 00:29:16,160 --> 00:29:20,560 radical anion which is 66 feet age 6-6 dollars. 618 00:29:20,840 --> 00:29:22,640 Right time for some organic chemistry. 619 00:29:22,760 --> 00:29:26,720 So benzene is a flat ring of 6 carbon atoms with six hydrogens 620 00:29:26,720 --> 00:29:30,040 attached around the outside, and we've just added one single 621 00:29:30,080 --> 00:29:32,920 extra electron to this whole ring system. 622 00:29:33,040 --> 00:29:35,160 And this is a thing of pure chemical beauty. 623 00:29:35,160 --> 00:29:37,880 In your classical intuition, you might think, OK, the electron 624 00:29:37,880 --> 00:29:39,600 just sits on one of the carbon atoms, right? 625 00:29:39,960 --> 00:29:43,160 But quantum mechanics says, no, that electron is delocalized. 626 00:29:43,160 --> 00:29:45,720 It is smeared out, running around that entire ring like 627 00:29:45,720 --> 00:29:48,440 it's a racetrack, visiting all six carbons and all six 628 00:29:48,440 --> 00:29:50,600 hydrogens equally. So it's coupling to all six 629 00:29:50,600 --> 00:29:53,760 hydrogens at the same time? Yes, and they are all equivalent 630 00:29:53,760 --> 00:29:56,200 to each other. So in this case we use a simpler 631 00:29:56,200 --> 00:29:58,280 rule. The number of lines is just one 632 00:29:58,280 --> 00:30:01,080 plus other dollars, where N is the number of equivalent nuclei. 633 00:30:01,080 --> 00:30:05,440 OK, so N is 6, so $6 + 1 = 7 lines and. 634 00:30:05,920 --> 00:30:09,480 You do, but here the lines definitely aren't all the same 635 00:30:09,480 --> 00:30:11,320 height. The notes show a graph where the 636 00:30:11,320 --> 00:30:14,880 middle line is huge and tall, and the outer lines are just 637 00:30:14,880 --> 00:30:17,160 tiny little blips. The whole pattern looks like a 638 00:30:17,160 --> 00:30:18,800 bell curve. Why is that? 639 00:30:18,800 --> 00:30:21,960 Why the different intensities? Right when you have multiple 640 00:30:21,960 --> 00:30:23,560 equivalent nuclear. I like this. 641 00:30:23,760 --> 00:30:26,800 The intensities of the lines follow Pascal's triangle. 642 00:30:26,800 --> 00:30:29,440 It's a binomial distribution. I need a refresher on that. 643 00:30:29,760 --> 00:30:32,880 Think about flipping 6 coins. The outermost line in the 644 00:30:32,880 --> 00:30:36,240 spectrum represents the state where all six of the hydrogen 645 00:30:36,240 --> 00:30:39,480 nuclear spins are pointing up. There's only one way to do that. 646 00:30:39,600 --> 00:30:41,960 Up, up, up, up, up. So it's a rare event. 647 00:30:42,040 --> 00:30:43,800 A very rare event, so it's a small. 648 00:30:43,800 --> 00:30:45,720 Short line, but the big line in the middle. 649 00:30:45,760 --> 00:30:48,320 The big line in the middle represents the state where three 650 00:30:48,320 --> 00:30:51,720 of the spins are up and three are down, and there are many 651 00:30:51,720 --> 00:30:54,120 different combinations that give you that exact result. 652 00:30:54,120 --> 00:30:55,800 Oh, I see. You could have up, up, up, down, 653 00:30:55,800 --> 00:30:58,240 down, down, or up, down, up, down, up, down, and so on. 654 00:30:58,440 --> 00:31:02,000 There are 20 different ways to get a three up 3 downstate, so 655 00:31:02,000 --> 00:31:04,640 that state is statistically much more likely to occur. 656 00:31:04,680 --> 00:31:06,560 So the line for that state is much taller. 657 00:31:06,600 --> 00:31:08,920 Much taller. The intensity ratio for six 658 00:31:08,920 --> 00:31:14,680 equivalent spin 12 nuclei is 1.6.15.15.6.1. 659 00:31:14,840 --> 00:31:18,080 It's mathematically perfect. It is, and when a chemist sees 660 00:31:18,080 --> 00:31:21,880 that beautiful symmetric pattern, they know for a fact my 661 00:31:21,880 --> 00:31:24,240 unpaired electron is symmetrically spread over that 662 00:31:24,240 --> 00:31:26,800 whole ring. If the ring were bent or broken, 663 00:31:26,920 --> 00:31:29,600 that perfect symmetry would be gone and the pattern would 664 00:31:29,600 --> 00:31:32,080 collapse into a mess. Nature following in the 665 00:31:32,080 --> 00:31:32,920 textbook. I love it. 666 00:31:33,280 --> 00:31:34,880 Now let's level up the complexity. 667 00:31:34,880 --> 00:31:38,920 Case study #3 The tetrazine anion, which is called BPATC. 668 00:31:38,920 --> 00:31:40,840 This is a molecule with a lot of nitrogen in it. 669 00:31:41,040 --> 00:31:44,520 Yes, it has a central ring with four equivalent nitrogen atoms, 670 00:31:44,520 --> 00:31:47,000 and we know that nitrogen 14 has a nuclear spin of 1. 671 00:31:47,000 --> 00:31:50,960 Okay, math time again. One nitrogen gives 3 lines, Two 672 00:31:50,960 --> 00:31:53,000 equivalent nitrogens gives 5 lines. 673 00:31:53,000 --> 00:31:55,080 What about four? It turns out that four 674 00:31:55,080 --> 00:31:59,080 equivalent spin one nuclei gives you 9 lines, and if you look at 675 00:31:59,080 --> 00:32:03,200 the spectrum, it's perfect 9 evenly spaced lines of varying 676 00:32:03,200 --> 00:32:05,720 intensity. This proves again that the 677 00:32:05,720 --> 00:32:09,400 electron is delocalized over all four of those nitrogen atoms in 678 00:32:09,400 --> 00:32:12,600 the ring. But then things get crazy. 679 00:32:12,880 --> 00:32:16,600 The notes show us a tungsten complex made with this molecule. 680 00:32:16,960 --> 00:32:21,640 The formula is MOBI Opco 42 Two. This thing looks like a monster. 681 00:32:21,640 --> 00:32:23,320 It is a more complex structure, yes. 682 00:32:23,320 --> 00:32:26,520 And the spectrum is described as a quintet of quintets. 683 00:32:26,920 --> 00:32:29,000 What on earth does that mean? This is where we see a 684 00:32:29,000 --> 00:32:31,800 superposition of couplings. This is where the detective has 685 00:32:31,800 --> 00:32:35,240 to feel back multiple layers. In this new big complex, the 686 00:32:35,240 --> 00:32:38,240 structure has lower symmetry. The electron isn't seeing all 687 00:32:38,240 --> 00:32:40,320 four of those nitrogen atoms equally anymore. 688 00:32:40,320 --> 00:32:42,440 Because the big tungsten metal atoms are attached to it. 689 00:32:42,440 --> 00:32:44,560 Exactly. The tungsten atoms distort the 690 00:32:44,560 --> 00:32:46,640 ring. Now the electron sees two of the 691 00:32:46,640 --> 00:32:49,440 nitrogen strongly and the other two nitrogens more weakly. 692 00:32:49,440 --> 00:32:51,120 So it's a hierarchy of interactions. 693 00:32:51,120 --> 00:32:53,360 Exactly. First, the electron couples to 694 00:32:53,360 --> 00:32:56,520 the two strong nitrogens. Two equivalent nitrogens with 695 00:32:56,520 --> 00:32:59,320 spin one gives a quintet 5 lines. 696 00:32:59,440 --> 00:33:02,520 The intensity ratio is 1.2.3.2.1. 697 00:33:02,640 --> 00:33:07,360 OK so in my mind I'm picturing 5 big broad lines. 698 00:33:07,720 --> 00:33:12,360 But then each one of those five big lines is split again by the 699 00:33:12,360 --> 00:33:15,000 much weaker interaction with the other two nitrogens. 700 00:33:15,000 --> 00:33:17,920 The weak ones. Right, so each big line splits 701 00:33:17,920 --> 00:33:21,440 into its own smaller quintet. A quintet of quintet. 702 00:33:21,440 --> 00:33:25,960 So you'd see 25 lines in total, but they'd be organized in these 703 00:33:25,960 --> 00:33:29,080 5 distinct clusters. That's exactly what you see, and 704 00:33:29,080 --> 00:33:32,040 by analyzing which coupling is big, that's the wide spacing 705 00:33:32,040 --> 00:33:35,680 between the clusters, and which coupling is small, that's a fine 706 00:33:35,680 --> 00:33:39,280 spacing within each cluster, we can map exactly how the electron 707 00:33:39,280 --> 00:33:42,600 sits on that distorted molecule. You can actually say it spends 708 00:33:42,600 --> 00:33:45,240 more time on the north side of the ring than on the South side. 709 00:33:45,440 --> 00:33:47,600 It's an incredible level of resolution. 710 00:33:47,600 --> 00:33:50,640 It really is the ultimate spy tool for mapping out electronic 711 00:33:50,640 --> 00:33:52,320 structure. I want to talk about one last 712 00:33:52,320 --> 00:33:55,280 case study because it solves a mystery using isotopes, which I 713 00:33:55,280 --> 00:33:57,520 thought was so clever. It's the satellite mystery. 714 00:33:57,560 --> 00:34:01,200 Ah yes, the zirconium case. This is one of my favorite 715 00:34:01,200 --> 00:34:03,520 examples of how important it is to look at the noise. 716 00:34:03,680 --> 00:34:07,560 So we have a spectrum for a zirconium species, and at first 717 00:34:07,560 --> 00:34:09,760 glance if you're rushing it just looks like a big doublet in the 718 00:34:09,760 --> 00:34:11,400 middle of the screen. 2 strong lines. 719 00:34:11,440 --> 00:34:13,760 Right. A simple doublet and a doublet 720 00:34:13,760 --> 00:34:16,719 usually means coupling to one spin 12 nucleus. 721 00:34:16,760 --> 00:34:20,360 In this particular molecule, there was a single hydrogen atom 722 00:34:20,360 --> 00:34:22,719 nearby. So mystery solved, right? 723 00:34:23,400 --> 00:34:25,280 The electron is coupled to the hydrogen. 724 00:34:25,520 --> 00:34:28,679 Not quite, because the notes say that if you zoom in and you have 725 00:34:28,679 --> 00:34:31,840 to look really, really close, way down near the baseline 726 00:34:31,840 --> 00:34:34,480 through these tiny little blips on either side of the main 727 00:34:34,480 --> 00:34:36,040 signal, they call them satellites. 728 00:34:36,159 --> 00:34:37,880 This is the isotope detective work. 729 00:34:38,040 --> 00:34:41,159 Zirconium is a metal, and in nature it's a mix of different 730 00:34:41,159 --> 00:34:43,440 isotopes. Most zirconium isotopes have 731 00:34:43,440 --> 00:34:45,920 even mass numbers, and they have no nuclear spin. 732 00:34:45,920 --> 00:34:48,880 They have I-0. So for most of the atoms in the 733 00:34:48,880 --> 00:34:52,000 sample, the vast majority the zirconium is magnetically 734 00:34:52,000 --> 00:34:53,719 invisible. The electron just seize the 735 00:34:53,719 --> 00:34:55,960 hydrogen, hence the big doublet in the middle. 736 00:34:55,960 --> 00:34:58,840 Correct, but there's also zirconium 91. 737 00:34:58,920 --> 00:35:01,720 The rare one. The rare $191.00 has a nuclear 738 00:35:01,720 --> 00:35:05,520 spin of I dollar and 522, but it's not very common. 739 00:35:05,520 --> 00:35:09,600 Only about 11.2% of all natural zirconium is this one isotope. 740 00:35:09,800 --> 00:35:13,000 So in about 89% of the molecules there's no zirconium splitting, 741 00:35:13,440 --> 00:35:15,720 but in 11% of the molecules there is splitting from the 742 00:35:15,720 --> 00:35:18,480 metal. In in a nucleus with spin 52 743 00:35:18,480 --> 00:35:24,000 gives $252 + 1 equus 66 lines. So flanking that big central 744 00:35:24,000 --> 00:35:27,200 doublet, you see these faint tiny sets of 6 lines. 745 00:35:27,320 --> 00:35:30,120 Those are from the 11% of molecules that contain a 746 00:35:30,120 --> 00:35:33,240 magnetic zirconium nucleus. And the fact that we can see 747 00:35:33,240 --> 00:35:34,840 them at all, what does that prove? 748 00:35:34,840 --> 00:35:37,160 Why does the chemist care about these tiny little blips? 749 00:35:37,160 --> 00:35:41,160 It proves unequivocally that the unpaired electron is physically 750 00:35:41,160 --> 00:35:45,120 located on the zirconium atom. If the electron were on a ligand 751 00:35:45,120 --> 00:35:49,320 far away, say on a carbon chain just dangling off the side, it 752 00:35:49,320 --> 00:35:52,520 would be too far away to feel the zirconium nucleus is tiny 753 00:35:52,600 --> 00:35:54,880 magnetic field. The coupling constant would be 754 00:35:54,880 --> 00:35:56,960 0. The satellites wouldn't exist. 755 00:35:56,960 --> 00:35:59,320 So the blips of the smoking gun. They say the electron is right 756 00:35:59,320 --> 00:36:00,880 here sitting on the metal center. 757 00:36:01,080 --> 00:36:04,440 Exactly, it confirms the oxidation state and the precise 758 00:36:04,440 --> 00:36:07,480 location of the spin density. And you know, if you were 759 00:36:07,480 --> 00:36:10,960 incredibly rich, you could buy isotopically enriched zirconium 760 00:36:10,960 --> 00:36:13,440 91100% pure. If you have the budget for. 761 00:36:13,560 --> 00:36:16,760 It if you did that and you made your molecule with it, the big 762 00:36:16,760 --> 00:36:19,080 central doublet would vanish completely. 763 00:36:19,240 --> 00:36:21,120 The satellites would become the whole spectrum. 764 00:36:21,120 --> 00:36:25,200 You'd see a massive complex 6 line pattern dominating the 765 00:36:25,200 --> 00:36:27,160 screen. It's just amazing how much 766 00:36:27,160 --> 00:36:30,320 information can be hidden in what looks like noise if you 767 00:36:30,320 --> 00:36:33,360 just know where to look. It's not noise, it's the data. 768 00:36:33,840 --> 00:36:37,240 Fairpoint, before we wrap this up, I do have to complain on 769 00:36:37,240 --> 00:36:40,640 behalf of all the students who might be listening the units. 770 00:36:41,040 --> 00:36:43,880 Why are the units such a headache in this field? 771 00:36:43,880 --> 00:36:46,080 I was trying to follow the math and the notes and it felt like I 772 00:36:46,080 --> 00:36:48,280 needed a conversion chart from three different centuries. 773 00:36:48,400 --> 00:36:52,240 Oh, it is an absolute mess. We we apologize on behalf of the 774 00:36:52,240 --> 00:36:55,120 entire scientific community. It's a historical accident, 775 00:36:55,120 --> 00:36:57,520 really. So we have Gauss and we have 776 00:36:57,520 --> 00:36:59,560 Tesla. Right, the magnetic field, the 777 00:36:59,560 --> 00:37:02,920 dollar field is measured in Gauss G by the old school 778 00:37:02,920 --> 00:37:08,360 physicist or in Milla Tesla Mt by modern SI compliant folks. 779 00:37:08,680 --> 00:37:11,800 For the record, 10,000 Gauss is equal to 1 Tesla. 780 00:37:11,960 --> 00:37:14,920 SO1 Milla Tesla is 10 Gauss. OK, that's manageable. 781 00:37:15,040 --> 00:37:17,600 But then the interaction energy, that coupling constant that's 782 00:37:17,600 --> 00:37:20,600 sometimes reported in Gauss. Yes, sometimes we just measure 783 00:37:20,600 --> 00:37:23,560 the splitting distance on the screen in field units, so Gauss 784 00:37:23,560 --> 00:37:26,720 or Milli Tesla, but physically it represents an energy 785 00:37:26,720 --> 00:37:28,840 interaction. So other times it's reported in 786 00:37:28,840 --> 00:37:31,400 frequency units. Megahertz, Emma. 787 00:37:31,400 --> 00:37:32,640 Hertz. And then there's the one that 788 00:37:32,640 --> 00:37:35,200 really hurt my brain. Inverse centimeters. 789 00:37:35,800 --> 00:37:40,000 Inverse centimeters, my favorite non intuitive unit. 790 00:37:40,320 --> 00:37:43,040 It's a legacy term from optical spectroscopy. 791 00:37:43,040 --> 00:37:45,640 It relates to the wave number of light that would have that 792 00:37:45,640 --> 00:37:47,560 amount of energy. It's yeah, it's. 793 00:37:47,560 --> 00:37:49,520 Confusing. So if I'm a student trying to 794 00:37:49,520 --> 00:37:53,240 solve a problem, I might be given a magnetic field in Tesla 795 00:37:53,600 --> 00:37:57,200 and instrument frequency in gear Hertz and a coupling constant in 796 00:37:57,200 --> 00:37:59,360 inverse centimeters and I have to make them all talk to each 797 00:37:59,360 --> 00:38:01,240 other. It happens all the time, and the 798 00:38:01,240 --> 00:38:04,440 reality is to make sense of a really complex spectrum with 799 00:38:04,440 --> 00:38:07,160 mixed units and overlapping lines, like that tungsten 800 00:38:07,160 --> 00:38:10,080 quintet of quintets. You rarely do it by hand with a 801 00:38:10,080 --> 00:38:11,800 ruler anymore. You use the computer. 802 00:38:11,960 --> 00:38:15,320 You use simulation software. You feed all the parameters, 803 00:38:15,320 --> 00:38:18,560 your G values, your coupling constants, your line widths into 804 00:38:18,560 --> 00:38:21,480 the software and it generates A theoretical spectrum. 805 00:38:21,640 --> 00:38:24,480 Then you just tweak the numbers until your simulation perfectly 806 00:38:24,480 --> 00:38:27,520 overlays your experimental data. The computer is the final 807 00:38:27,520 --> 00:38:29,520 arbiter. Indeed, and it handles all the 808 00:38:29,520 --> 00:38:31,280 unit conversions for you. Thank goodness. 809 00:38:31,360 --> 00:38:34,000 Thank goodness for that. So let's bring this all back 810 00:38:34,000 --> 00:38:35,840 home. To summarize our deep dive 811 00:38:35,840 --> 00:38:38,680 today. EPR is the magnetic sister of 812 00:38:38,680 --> 00:38:42,040 NMR. It uses microwaves to flip the 813 00:38:42,040 --> 00:38:46,160 spins of unpaired electrons. It is incredibly sensitive and 814 00:38:46,160 --> 00:38:49,360 it's incredibly specific because it ignores all the paired up 815 00:38:49,360 --> 00:38:53,000 statues in the ballroom. And through the magic of these G 816 00:38:53,000 --> 00:38:56,120 factors and the hyperfine coupling patterns, it allows us 817 00:38:56,120 --> 00:38:59,600 to map the precise location and the chemical environment of an 818 00:38:59,600 --> 00:39:02,080 electron in a molecule. And not just its static 819 00:39:02,080 --> 00:39:04,200 structure, as we saw with the line widths and the slow 820 00:39:04,200 --> 00:39:05,920 tumbling. It also tells us about motion, 821 00:39:05,920 --> 00:39:09,520 about tumbling, about dynamics. It really is a window into the 822 00:39:09,520 --> 00:39:13,200 electronic structure of these oddball molecules that no other 823 00:39:13,200 --> 00:39:16,040 method can provide. It sees the radicals of the 824 00:39:16,040 --> 00:39:18,280 molecular world. I love that it's the tool for 825 00:39:18,280 --> 00:39:19,040 the rebels. Yeah. 826 00:39:19,880 --> 00:39:22,440 Now, as we finish up, I want to leave you, our listener, with a 827 00:39:22,440 --> 00:39:24,440 thought. We talked a lot today about 828 00:39:24,440 --> 00:39:27,480 freezing samples liquid helium for Kelvin. 829 00:39:27,640 --> 00:39:30,720 We do this to stop the motion to beat the Heisenberg uncertainty 830 00:39:30,720 --> 00:39:32,680 principle to actually see a signal. 831 00:39:32,680 --> 00:39:35,520 To lengthen the relaxation time, yes, that's the key. 832 00:39:35,600 --> 00:39:41,040 But biology happens at 37°C. Life is wet, it's warm, and it's 833 00:39:41,040 --> 00:39:44,640 constantly moving. The enzymes in our bodies that 834 00:39:44,640 --> 00:39:48,080 use radical mechanisms to digest our food or repair our DNA, 835 00:39:48,360 --> 00:39:49,800 they're working at body temperature. 836 00:39:50,200 --> 00:39:54,000 So if we can only see them when they're frozen solid in a block 837 00:39:54,000 --> 00:39:57,200 of helium, are we actually seeing the truth? 838 00:39:57,360 --> 00:39:59,120 That is the big question in the field. 839 00:39:59,200 --> 00:40:02,520 Are we just seeing the dancer frozen mid leap or if we lock 840 00:40:02,520 --> 00:40:04,920 them into a pose that they never actually hold in real life 841 00:40:04,920 --> 00:40:07,800 during the actual bands, how much of the reality of the 842 00:40:07,800 --> 00:40:10,280 reaction do we lose when we freeze the frame? 843 00:40:10,440 --> 00:40:13,400 That's a profound and very real problem. 844 00:40:13,520 --> 00:40:17,680 We are capturing a state, a snapshot, but are we capturing 845 00:40:17,680 --> 00:40:20,400 the function? It's something every structural 846 00:40:20,400 --> 00:40:23,160 biologist has to wrestle with. We'd build these beautiful 847 00:40:23,160 --> 00:40:26,320 models based on the Frozen snapshots, but we must always 848 00:40:26,320 --> 00:40:28,600 remember that the map is not the territory. 849 00:40:29,120 --> 00:40:31,880 Something to Mull over next time you look at a static picture of 850 00:40:31,880 --> 00:40:33,760 a molecule. It's probably dancing when 851 00:40:33,760 --> 00:40:36,240 you're not looking. Thanks for joining us on the 852 00:40:36,240 --> 00:40:37,960 Deep Dive. It was a pleasure, as always. 853 00:40:37,960 --> 00:40:38,600 See. You next time.