1 00:00:00,040 --> 00:00:02,360 Welcome to the Deep Dive. Our mission here is to take 2 00:00:02,360 --> 00:00:06,760 stacks of complex scientific sources and really distill the 3 00:00:06,800 --> 00:00:10,760 absolute most critical, most memorable Nuggets of knowledge. 4 00:00:10,760 --> 00:00:13,400 And today we are, you know, putting out our protective 5 00:00:13,400 --> 00:00:15,600 glasses. We are, we're diving deep into a 6 00:00:15,600 --> 00:00:19,040 topic that is, I mean, it's the absolute bedrock of chemistry, 7 00:00:19,040 --> 00:00:23,200 material science and modern pharmacology, structural methods 8 00:00:23,200 --> 00:00:25,720 in inorganic chemistry. We're basically trying to 9 00:00:25,720 --> 00:00:27,960 unravel the invisible. If you're building a new 10 00:00:27,960 --> 00:00:30,960 compound, it doesn't matter if it's a better battery material 11 00:00:30,960 --> 00:00:33,200 and new catalyst or a life saving drug. 12 00:00:34,040 --> 00:00:38,280 The 1st and most fundamental hurdle is understanding exactly 13 00:00:38,280 --> 00:00:41,440 how its atoms are connected. So our mission today is to 14 00:00:41,480 --> 00:00:43,240 explore the tools and the concepts. 15 00:00:43,600 --> 00:00:46,480 The move is beyond just, you know, educated guesswork to 16 00:00:46,480 --> 00:00:49,680 definitive atomic level proof. And the motivation here, I mean, 17 00:00:49,680 --> 00:00:52,440 it couldn't be more profound. It's not enough to just draw a 18 00:00:52,440 --> 00:00:55,120 plausible sketch of a molecule on a piece of paper. 19 00:00:55,120 --> 00:00:55,880 Anymore. No. 20 00:00:56,000 --> 00:00:58,800 We need to move past that, past mere speculation, to truly 21 00:00:58,800 --> 00:01:02,280 understand materials and that understanding, it has to start 22 00:01:02,280 --> 00:01:03,920 with hard data. Exactly. 23 00:01:03,920 --> 00:01:07,400 You have to be able to ask and then answer these really 24 00:01:07,400 --> 00:01:09,240 sophisticated questions about structure. 25 00:01:09,520 --> 00:01:13,440 And if you can do that, it lets you predict properties, design 26 00:01:13,440 --> 00:01:17,040 completely new compounds, and just, you know, excel at 27 00:01:17,040 --> 00:01:19,440 interpreting all that structural information you're getting. 28 00:01:19,440 --> 00:01:22,640 And when we use the word structure in modern chemistry, 29 00:01:22,960 --> 00:01:25,160 we have to be clear, we're defining it very, very broadly. 30 00:01:25,520 --> 00:01:28,040 It's not just a picture. Structure is the complete 31 00:01:28,040 --> 00:01:33,400 information that connects atoms, electrons and and motion. 32 00:01:33,400 --> 00:01:34,240 Motion. OK, it's. 33 00:01:34,280 --> 00:01:36,720 Really. A five part definition you could 34 00:01:36,720 --> 00:01:38,720 say. OK, let's unpack those 5 core 35 00:01:38,720 --> 00:01:41,920 pieces of information because I think they take us way beyond 36 00:01:42,240 --> 00:01:44,960 the simple ball and stick models we all remember from school. 37 00:01:44,960 --> 00:01:47,520 Oh, absolutely. So the first and the most basic 38 00:01:47,600 --> 00:01:50,560 is connectivity. OK, just which atoms are linked 39 00:01:50,560 --> 00:01:52,000 together. It's the topology of the 40 00:01:52,000 --> 00:01:53,360 molecule. Responded to whom? 41 00:01:53,360 --> 00:01:56,640 Exactly. 2nd is geometry. Now this is quantitative. 42 00:01:57,080 --> 00:01:59,840 It covers the precise bond lengths, the bond angles, and 43 00:01:59,840 --> 00:02:02,840 the overall 3 dimensional shape. You know, is it tetrahedral? 44 00:02:02,840 --> 00:02:05,520 Is it octahedral? So connectivity is who is 45 00:02:05,520 --> 00:02:09,440 connected, geometry is how far apart and at what angle they all 46 00:02:09,440 --> 00:02:11,600 sit. Precisely. 3rd, we have 47 00:02:11,600 --> 00:02:13,920 symmetry. This is the arrangement of atoms 48 00:02:13,920 --> 00:02:17,760 in space relative to some central point or axis, and that 49 00:02:17,760 --> 00:02:20,920 defines its spatial classification. 4th we look at 50 00:02:20,920 --> 00:02:23,600 electron density, and this one is crucial. 51 00:02:23,840 --> 00:02:26,160 It tells us where the charge is distributed in the molecule, 52 00:02:26,160 --> 00:02:29,080 which dictates everything from reactivity to polarity. 53 00:02:29,200 --> 00:02:32,520 And the fifth one. And finally #5 is dynamics, 54 00:02:32,880 --> 00:02:36,040 which means motion. And this is, in many ways the 55 00:02:36,040 --> 00:02:39,480 ultimate goal, understanding how the atoms and the electrons are 56 00:02:39,480 --> 00:02:44,000 moving overtime from, you know, simple rotation to the vibration 57 00:02:44,000 --> 00:02:46,160 of a bond. It's really astonishing that we 58 00:02:46,160 --> 00:02:49,080 have tools sophisticated enough to measure all five of those 59 00:02:49,080 --> 00:02:52,560 things, especially dynamics at, you know, impossibly short time 60 00:02:52,560 --> 00:02:54,240 scales. It is, but what the source 61 00:02:54,240 --> 00:02:57,080 material really highlights. What I found most surprising is 62 00:02:57,080 --> 00:03:00,960 that for decades, chemists had this incredibly strong intuitive 63 00:03:00,960 --> 00:03:04,160 grasp of structure long before they could confirm any of it 64 00:03:04,160 --> 00:03:07,640 with an instrument. Structure as a concept came way 65 00:03:07,640 --> 00:03:11,200 before structural determination. That historical context is so 66 00:03:11,200 --> 00:03:13,840 important to understand. I mean, if you look back to the 67 00:03:13,840 --> 00:03:16,800 mid 19th century, chemical intuition was pretty much the 68 00:03:16,800 --> 00:03:20,560 only tool they had, and it was rooted in this emerging idea of 69 00:03:20,560 --> 00:03:24,080 valence. So in 1857, you have August von 70 00:03:24,080 --> 00:03:27,960 Kekule who formalizes this idea. He introduces the concept of 71 00:03:27,960 --> 00:03:31,400 valency, that carbon is to travel and oxygen is divalent. 72 00:03:31,680 --> 00:03:34,640 And he starts assigning connectivity based purely on the 73 00:03:34,640 --> 00:03:37,440 consistent ratios he's seeing in chemical reactions. 74 00:03:37,640 --> 00:03:41,120 And you have to think about what a massive conceptual leap that 75 00:03:41,120 --> 00:03:43,720 was before Kekule. I mean, a lot of scientists 76 00:03:43,720 --> 00:03:48,120 really viewed compounds as these kind of amorphous clusters of 77 00:03:48,120 --> 00:03:50,800 atoms, maybe not even following simple rules exactly. 78 00:03:50,880 --> 00:03:53,200 And Kekule comes along and provides this architectural 79 00:03:53,200 --> 00:03:56,280 framework congesting that atoms adhere to these fixed laws of 80 00:03:56,280 --> 00:03:59,000 combination. He managed to represent 81 00:03:59,000 --> 00:04:02,480 something as complex as acetic acid, ethanoic acid, with 82 00:04:02,480 --> 00:04:06,000 remarkable accuracy just by following the rules 4 bonds for 83 00:04:06,000 --> 00:04:08,880 carbon, 2 for oxygen. And that immediately led to a 84 00:04:08,880 --> 00:04:12,480 visual revolution. The very next year, 1858, you 85 00:04:12,480 --> 00:04:16,040 have Archibald Scott Cooper, who takes Cakeley's ideas and makes 86 00:04:16,040 --> 00:04:18,839 that connectivity explicit. He makes it visual by 87 00:04:18,839 --> 00:04:21,600 introducing lines to represent chemical bonds. 88 00:04:21,800 --> 00:04:24,800 Which seems so obvious to us now, but it was a huge deal. 89 00:04:24,840 --> 00:04:25,600 Huge. Deal. 90 00:04:25,600 --> 00:04:28,720 It wasn't just aesthetics. It was this profound 91 00:04:28,800 --> 00:04:32,800 intellectual move that separated the atom from its linkage and 92 00:04:32,800 --> 00:04:35,680 that directly paved the way for the functional group concept. 93 00:04:36,200 --> 00:04:39,120 Once you can see a cluster of atoms, like a hydroxyl group or 94 00:04:39,120 --> 00:04:41,680 a carbonyl group, you can start to predict how the whole 95 00:04:41,680 --> 00:04:43,080 molecule is going to behave and the. 96 00:04:43,080 --> 00:04:47,600 Momentum just picked up so fast. By 1861, Alexander M Butler, 97 00:04:47,600 --> 00:04:50,600 OFF, who was building on these drawings, was the first chemist 98 00:04:50,600 --> 00:04:52,800 to explicitly use the word structure. 99 00:04:52,800 --> 00:04:55,120 Yes, he was the one who recognized that compounds 100 00:04:55,120 --> 00:04:58,120 weren't just collections of elements, but regular, organized 101 00:04:58,120 --> 00:05:00,720 arrangements that conform strictly to those valency laws 102 00:05:00,720 --> 00:05:02,240 that. Feels like the moment chemistry 103 00:05:02,240 --> 00:05:05,480 shifted from being a descriptive science of you know what happens 104 00:05:05,480 --> 00:05:08,600 when I mix A&B to a predictive science of architecture. 105 00:05:08,880 --> 00:05:11,200 Absolutely. And the diagrams themselves? 106 00:05:11,200 --> 00:05:13,680 They rapidly evolved toward what we use today. 107 00:05:14,280 --> 00:05:18,160 In 1864, Alexander Crum Brown developed a diagrammatic formula 108 00:05:18,160 --> 00:05:22,000 that looks remarkably familiar. Circles for atoms, lines for 109 00:05:22,000 --> 00:05:23,440 bonds. And he even drew the double 110 00:05:23,440 --> 00:05:24,720 bonds right. He did. 111 00:05:24,920 --> 00:05:28,880 He recognized and explicitly represented the C double bond O 112 00:05:28,880 --> 00:05:32,400 carbonyl group in ethanoic acid using these diagrams. 113 00:05:32,400 --> 00:05:35,520 And the intuition was so strong that Crum Brown didn't even 114 00:05:35,520 --> 00:05:38,760 limit himself to 2D paper. He physically built one of the 115 00:05:38,760 --> 00:05:42,120 very first 3D structural models, right? 116 00:05:42,400 --> 00:05:45,400 Using wooden balls for atoms and connectors for bonds. 117 00:05:45,400 --> 00:05:48,320 He did, which demonstrated this explicit recognition of 118 00:05:48,320 --> 00:05:50,120 stereochemistry in crystal arrangement. 119 00:05:50,200 --> 00:05:53,000 It made simple lattices like the rock salt structure of sodium 120 00:05:53,000 --> 00:05:55,320 chloride intuitively understandable, long before 121 00:05:55,320 --> 00:05:57,600 anyone had an instrument they could actually measure the 122 00:05:57,600 --> 00:05:59,760 distance between a sodium and a chloride ion. 123 00:06:00,120 --> 00:06:02,800 So by the end of the 19th century, chemist basically had 124 00:06:02,800 --> 00:06:04,920 the picture. They understood connectivity 125 00:06:04,920 --> 00:06:08,240 valency and they had a very strong suspicion about 3D shapes 126 00:06:08,240 --> 00:06:10,560 in crystal order. But they lacked the physical 127 00:06:10,560 --> 00:06:14,200 evidence, the hard data, the precise bond lengths, the 128 00:06:14,200 --> 00:06:17,600 angles, the confirmation of where the atoms actually were. 129 00:06:17,920 --> 00:06:22,400 They had very few experimental tools beyond basic visible 130 00:06:22,400 --> 00:06:25,040 spectroscopy, which mostly just gave them information about 131 00:06:25,040 --> 00:06:27,560 Atomic Energy levels, not molecular geometry. 132 00:06:27,640 --> 00:06:31,240 And this gap, this huge gap between strong intuition and 133 00:06:31,240 --> 00:06:34,600 hard physical proof, is what set the stage for the true 134 00:06:34,600 --> 00:06:36,840 scientific revolution in the early 20th century. 135 00:06:36,880 --> 00:06:39,960 A revolution driven entirely by the invention of new physical 136 00:06:39,960 --> 00:06:40,720 methods. Yeah. 137 00:06:40,840 --> 00:06:44,240 And if you just chart the Nobel milestones, the impact of X-rays 138 00:06:44,240 --> 00:06:48,240 is well, it's unparalleled. When W Rungen discovered X-rays 139 00:06:48,240 --> 00:06:52,160 in 1895, it was a completely new form of high energy 140 00:06:52,160 --> 00:06:55,000 electromagnetic radiation. And the key was the wavelength. 141 00:06:55,080 --> 00:06:57,400 The key was the wavelength. It possessed wavelengths that 142 00:06:57,400 --> 00:07:00,240 were comparable to the distances between atoms in a crystal 143 00:07:00,240 --> 00:07:02,400 lattice, which made it the perfect tool for probing 144 00:07:02,400 --> 00:07:04,560 structure. And that link was exploited 145 00:07:04,560 --> 00:07:07,040 almost immediately, I think it was 1914. 146 00:07:07,200 --> 00:07:11,320 M von Lau demonstrated X-ray diffraction interference in 147 00:07:11,320 --> 00:07:14,160 crystals and proved experimentally for the very 148 00:07:14,160 --> 00:07:17,400 first time that crystals are made of these regular repeating 149 00:07:17,680 --> 00:07:19,880 3 dimensional arrangements of atoms. 150 00:07:19,960 --> 00:07:22,800 It was the ultimate confirmation of the structural intuition that 151 00:07:22,800 --> 00:07:25,480 chemists had held for, you know, for decades, right? 152 00:07:25,880 --> 00:07:29,760 And then the very next year, 1915, cemented the foundation of 153 00:07:29,760 --> 00:07:33,840 the whole field. WH and WL Bragg, a father and 154 00:07:33,840 --> 00:07:37,920 son team, developed crystal structure analysis using what we 155 00:07:37,920 --> 00:07:40,560 now know as Bragg's Law. And that's the mathematical 156 00:07:40,560 --> 00:07:42,600 relationship, right? The one that links the X-ray 157 00:07:42,600 --> 00:07:45,480 wavelength, the angle it hits the crystal, and the distance 158 00:07:45,480 --> 00:07:47,560 between the atomic planes. Exactly. 159 00:07:47,920 --> 00:07:50,560 And that's where the power lies. Bragg's law didn't just prove 160 00:07:50,560 --> 00:07:53,760 that atoms were ordered, it gave us the ability to calculate the 161 00:07:53,760 --> 00:07:56,960 exact quantitative positions of those atoms. 162 00:07:56,960 --> 00:08:00,560 And critically, the precise bond lengths and angles within the 163 00:08:00,560 --> 00:08:02,960 unit cell. Yes, it moves structure from a 164 00:08:02,960 --> 00:08:05,520 qualitative drawing to a quantitative measurement. 165 00:08:05,800 --> 00:08:08,760 And the X-ray story didn't stop there, PW. 166 00:08:08,760 --> 00:08:12,320 In 1936, he started applying it not just to solids, but the 167 00:08:12,320 --> 00:08:14,240 gases too. Right, combining it with the 168 00:08:14,240 --> 00:08:18,120 concept of molecular dye poles. But then came the the big 169 00:08:18,120 --> 00:08:20,760 mathematical breakthroughs that were needed to solve structures 170 00:08:20,760 --> 00:08:23,000 that weren't just simple high symmetry crystals. 171 00:08:23,200 --> 00:08:26,240 This is where the sources highlight the 1985 Nobel to 172 00:08:26,240 --> 00:08:29,000 Halptamen and Carl for developing direct methods. 173 00:08:29,440 --> 00:08:32,200 Now for our listeners who aren't crystallographers, why was this 174 00:08:32,200 --> 00:08:33,960 so monumental? What was the problem they 175 00:08:33,960 --> 00:08:36,159 solved? It was a massive mathematical 176 00:08:36,159 --> 00:08:38,200 hurdle. It's called the phase problem. 177 00:08:38,400 --> 00:08:41,360 When X-rays diffract, they produce a pattern, and that 178 00:08:41,360 --> 00:08:44,000 pattern depends on both the intensity and the phase of the 179 00:08:44,000 --> 00:08:46,760 scattered waves. Initially, chemists could only 180 00:08:46,760 --> 00:08:50,000 measure the intensity, the phase information which is 181 00:08:50,000 --> 00:08:52,800 mathematically essential to reconstruct the electron density 182 00:08:52,800 --> 00:08:55,040 map, and from that the atomic positions. 183 00:08:55,880 --> 00:08:58,600 That information was lost. So you had half the puzzle. 184 00:08:58,720 --> 00:09:01,920 You had half the puzzle. Direct methods provided a 185 00:09:01,920 --> 00:09:05,880 systematic, mathematical way to infer that lost phase 186 00:09:05,880 --> 00:09:09,720 information and that transformed structure determination from 187 00:09:09,720 --> 00:09:14,280 this long, often intractable process into a routine, solvable 188 00:09:14,280 --> 00:09:15,840 problem. Especially for complex 189 00:09:15,840 --> 00:09:18,360 biological molecules. It was a game changer. 190 00:09:18,440 --> 00:09:21,840 And we can't forget Dee Scheckman's 2011 Nobel for quasi 191 00:09:21,840 --> 00:09:23,720 crystals. Oh that was fantastic. 192 00:09:23,960 --> 00:09:26,280 It demonstrated a form of structural order that was non 193 00:09:26,280 --> 00:09:28,000 repeating but still highly ordered. 194 00:09:28,320 --> 00:09:32,520 It challenged 100 years of crystallize ha ha definitions of 195 00:09:32,520 --> 00:09:34,520 what a crystal even had to be. Right. 196 00:09:34,560 --> 00:09:37,000 And at the same time all this was happening with X-rays, 197 00:09:37,120 --> 00:09:39,680 spectroscopy was offering these complementary views. 198 00:09:39,680 --> 00:09:43,880 Exactly in In 1930, CV Ramen discovered the ramen effect, 199 00:09:44,080 --> 00:09:46,880 which exploits the inelastic scattering of light to probe 200 00:09:47,000 --> 00:09:49,800 molecular vibrations. That gave chemists a powerful 201 00:09:49,800 --> 00:09:52,120 new tool to identify functional groups in symmetry. 202 00:09:52,200 --> 00:09:54,560 And then came the revolution of magnetic resonance. 203 00:09:54,680 --> 00:09:58,000 In 1952, Block and Purcell were recognized for developing 204 00:09:58,000 --> 00:10:01,560 nuclear magnetic resonance and Mr. And for chemists, 205 00:10:01,640 --> 00:10:05,200 particularly in organic and inorganic synthesis, NMR has 206 00:10:05,200 --> 00:10:07,960 become well, you could argue it's the single most important 207 00:10:07,960 --> 00:10:10,200 tool in the lab. Why is it so powerful? 208 00:10:10,200 --> 00:10:12,880 Its importance lies in the fact that it's not just an 209 00:10:12,880 --> 00:10:16,480 identification tool like a fingerprint, it is a structure 210 00:10:16,480 --> 00:10:20,000 elucidation tool. NMR probes the behavior of 211 00:10:20,000 --> 00:10:25,120 specific atomic nuclei like hydrogen, carbon 13, fluorine 19 212 00:10:25,280 --> 00:10:28,040 in a magnetic field. And the signal you get tells you 213 00:10:28,040 --> 00:10:30,360 about its neighbors. Exquisitely so. 214 00:10:30,880 --> 00:10:33,640 The signal you get for a nucleus is so sensitive to its local 215 00:10:33,640 --> 00:10:37,080 electronic environment and through spin spin coupling to 216 00:10:37,080 --> 00:10:39,560 the connectivity of its neighbors, it essentially lets 217 00:10:39,560 --> 00:10:42,040 you map out who is connected to whom in the molecule. 218 00:10:42,800 --> 00:10:44,760 Beautiful. We also saw the development of 219 00:10:44,760 --> 00:10:48,240 Mussbauer spectroscopy, which Rudolf Mussbauer was recognized 220 00:10:48,240 --> 00:10:50,200 for in 1961. It's a more specialized 221 00:10:50,200 --> 00:10:51,720 technique. Very specialized, Yeah. 222 00:10:51,760 --> 00:10:55,280 It uses recoil free gamma ray absorption to probe the nuclear 223 00:10:55,280 --> 00:10:57,000 environment. It gives you this incredibly 224 00:10:57,000 --> 00:11:00,640 deep electronic detail, but only for specific isotopes like iron 225 00:11:00,640 --> 00:11:02,080 or tin. Gives you a very localized 226 00:11:02,080 --> 00:11:04,720 structural view. And the power out of NMR just 227 00:11:04,720 --> 00:11:07,880 kept evolving, didn't it? Culminating in Richard Ernst's 228 00:11:07,880 --> 00:11:11,720 1991 Nobel for his work on multi dimensional NMR. 229 00:11:11,760 --> 00:11:14,960 Yes. So if 1D NMR shows you direct 230 00:11:14,960 --> 00:11:19,560 connectivity, then 2D3D and even 40 NMR allow us to trace 231 00:11:19,560 --> 00:11:22,560 connections across multiple bonds, even through solvent 232 00:11:22,560 --> 00:11:24,600 molecules. And that's what made the 233 00:11:24,600 --> 00:11:27,040 structure and determination of massive biomolecules like 234 00:11:27,040 --> 00:11:30,360 proteins or huge inorganic clusters even possible. 235 00:11:30,520 --> 00:11:33,480 So by the late 1920's, the theoretical groundwork of 236 00:11:33,480 --> 00:11:36,360 quantum mechanics was established, and chemists were 237 00:11:36,360 --> 00:11:38,760 already learning to exploit pretty much the entire 238 00:11:38,760 --> 00:11:41,400 electromagnetic spectrum. The foundations were all there. 239 00:11:41,440 --> 00:11:44,440 They were, but the real acceleration is the sources note 240 00:11:44,440 --> 00:11:47,040 happened after 1980. The core methods were 241 00:11:47,040 --> 00:11:49,960 established, yes, but the electronics revolution that 242 00:11:49,960 --> 00:11:53,200 delivered sensitive detectors, massive magnets and fast 243 00:11:53,200 --> 00:11:54,880 computers. And that let chemists capture 244 00:11:54,880 --> 00:11:57,920 data exponentially faster. Faster and more importantly, it 245 00:11:57,920 --> 00:12:00,760 allowed them to access incredibly short time scales to 246 00:12:00,760 --> 00:12:03,240 study dynamics. We finally moved beyond just 247 00:12:03,240 --> 00:12:07,720 static images of molecules. It's exactly the 1971 Nobel to 248 00:12:07,720 --> 00:12:10,840 Gerhard Herzberg recognized the study of short lived 249 00:12:10,840 --> 00:12:14,160 intermediates and radicals, molecules that might only exist 250 00:12:14,160 --> 00:12:17,680 for fractions of a second but the ultimate resolution of 251 00:12:17,680 --> 00:12:20,320 motion. That came in 1999 with Ahmed 252 00:12:20,320 --> 00:12:23,920 Ziwal's Nobel for femtosecond reaction dynamics. 253 00:12:24,120 --> 00:12:26,960 A femtosecond is 10 to the -15 seconds. 254 00:12:27,160 --> 00:12:29,000 It's an impossible number to really imagine. 255 00:12:29,280 --> 00:12:32,320 It is, but to put it in perspective, the process of a 256 00:12:32,320 --> 00:12:36,000 chemical bond stretching, vibrating or ultimately breaking 257 00:12:36,360 --> 00:12:38,200 that takes hundreds of femtoseconds. 258 00:12:38,760 --> 00:12:41,760 Ziwal's technique essentially allowed scientists to create 259 00:12:41,760 --> 00:12:44,080 molecular movies. You could watch a reaction 260 00:12:44,080 --> 00:12:45,680 happen. You could watch the transition 261 00:12:45,680 --> 00:12:48,680 state of a reaction the exact moment a bond changes state in 262 00:12:48,680 --> 00:12:50,720 real time. It's the ultimate measure of 263 00:12:50,720 --> 00:12:52,200 molecular dynamics. So if you. 264 00:12:52,200 --> 00:12:54,560 Put all these milestones together, the core insight is 265 00:12:54,560 --> 00:12:57,080 just astonishing. In one century, structure went 266 00:12:57,080 --> 00:12:59,480 from being simply drawn based on chemical intuition. 267 00:12:59,480 --> 00:13:01,600 To being precisely measured via diffraction. 268 00:13:01,600 --> 00:13:04,240 To being resolved in time with femtosecond spectroscopy. 269 00:13:04,240 --> 00:13:06,640 And finally, with the computer revolution, it became reliably 270 00:13:06,640 --> 00:13:09,120 computed. That computational aspect is the 271 00:13:09,120 --> 00:13:11,840 necessary complement to all this experimental work. 272 00:13:12,440 --> 00:13:16,080 The 1998 Nobel to Popple and Cone recognize the development 273 00:13:16,080 --> 00:13:19,240 of computational methods, specifically AB initio 274 00:13:19,240 --> 00:13:22,640 calculations and density functional theory, or DfT. 275 00:13:22,760 --> 00:13:24,520 Right? Let's just clarify DfT for a 276 00:13:24,520 --> 00:13:26,440 second, since it's such a modern cornerstone. 277 00:13:26,920 --> 00:13:30,280 Why is DfT so powerful for structural chemists? 278 00:13:30,800 --> 00:13:33,200 Well, classical quantum methods. They struggle with the 279 00:13:33,320 --> 00:13:36,080 exponentially increasing complexity of calculating the 280 00:13:36,080 --> 00:13:39,560 wave function for every single electron in a big molecule. 281 00:13:40,240 --> 00:13:42,280 DfT simplifies the problem. How so? 282 00:13:42,520 --> 00:13:45,760 Instead of solving for all these complex wave functions, DfT 283 00:13:45,760 --> 00:13:49,440 focuses on calculating the total electron density, the charge 284 00:13:49,440 --> 00:13:51,600 distribution. And since the total energy of 285 00:13:51,600 --> 00:13:54,000 the system is a unique functional of this electron 286 00:13:54,000 --> 00:13:57,040 density, calculating the density field becomes much, much more 287 00:13:57,040 --> 00:13:59,440 computationally feasible. And that allows chemists to 288 00:13:59,440 --> 00:14:02,160 predict structures. Predict structures, bond 289 00:14:02,160 --> 00:14:05,000 energies, transition states, electronic properties, all for 290 00:14:05,000 --> 00:14:08,040 large, complex inorganic and biological systems with 291 00:14:08,040 --> 00:14:10,920 remarkable accuracy. It's often used now to confirm 292 00:14:10,920 --> 00:14:13,160 or even guide expensive experimental work. 293 00:14:13,240 --> 00:14:17,560 That's a huge shift in workflow. So today structural chemistry is 294 00:14:17,560 --> 00:14:20,600 generating these massive high dimensional data sets from all 295 00:14:20,600 --> 00:14:22,800 these different tools. And we're seeing machine 296 00:14:22,800 --> 00:14:26,680 learning and AI being used more and more to accelerate structure 297 00:14:26,680 --> 00:14:30,640 solution, to interpret complex Spectra and to guide the next 298 00:14:30,640 --> 00:14:32,960 generation of experiments and simulations. 299 00:14:32,960 --> 00:14:35,760 It's the. Cycle of structure from Kekules 300 00:14:35,760 --> 00:14:39,200 guests to AI's prediction. It's all about understanding 301 00:14:39,200 --> 00:14:42,880 structure, because our success as chemists relies entirely on 302 00:14:42,880 --> 00:14:45,040 relating known a chemical structures to measurable 303 00:14:45,040 --> 00:14:48,760 properties so we can then design new compounds with the functions 304 00:14:48,760 --> 00:14:49,920 we want. Exactly. 305 00:14:50,120 --> 00:14:53,120 So now that we have this incredible arsenal of tools, how 306 00:14:53,120 --> 00:14:55,760 does a modern chemist approach a brand new material? 307 00:14:55,760 --> 00:14:57,400 You can't just throw every machine at it. 308 00:14:57,560 --> 00:14:59,520 There has to be a systematic process, right? 309 00:14:59,520 --> 00:15:01,000 A hierarchy of questions. Oh. 310 00:15:01,120 --> 00:15:03,280 Absolutely. It's a systematic triage. 311 00:15:03,720 --> 00:15:06,560 You move from simple identity toward the deepest electronic 312 00:15:06,560 --> 00:15:10,040 and geometric detail. You can't, you know, study bond 313 00:15:10,040 --> 00:15:12,280 dynamics if you don't even know what you synthesized in the 314 00:15:12,280 --> 00:15:13,360 first place. OK. 315 00:15:13,360 --> 00:15:17,080 So the first step is always initial triage, purity and 316 00:15:17,080 --> 00:15:19,520 identity. Question one, does the material 317 00:15:19,520 --> 00:15:23,760 consist of or contain any known compound that we can identify? 318 00:15:23,760 --> 00:15:25,560 And this is where we rely on fingerprinting. 319 00:15:26,120 --> 00:15:29,040 Every molecule generates a unique spectrum or diffraction 320 00:15:29,040 --> 00:15:31,680 pattern, and that acts as a molecular fingerprint. 321 00:15:32,360 --> 00:15:35,360 If a database contains the structure of a known compound, a 322 00:15:35,360 --> 00:15:37,680 simple pattern match can confirm its identity. 323 00:15:37,760 --> 00:15:39,880 And the quality of that fingerprint, I assume, depends 324 00:15:39,880 --> 00:15:42,640 on the tool you use. Which ones give us the sharpest, 325 00:15:42,640 --> 00:15:45,520 most reliable fingerprints? You want to use high resolution 326 00:15:45,520 --> 00:15:49,440 methods for solids. Powder X-ray diffraction or PXRD 327 00:15:49,440 --> 00:15:52,320 is fantastic. The pattern of diffraction lines 328 00:15:52,320 --> 00:15:54,440 is unique to the crystal structure and phase. 329 00:15:54,600 --> 00:15:57,800 And for solutions. For solutions, NMR spectroscopy 330 00:15:57,800 --> 00:16:00,040 and mass spectrometry are excellent because they yield 331 00:16:00,040 --> 00:16:02,760 lots of sharp, distinct lines that correspond to different 332 00:16:02,760 --> 00:16:04,480 nuclei or fragments of the molecule. 333 00:16:05,080 --> 00:16:07,760 Low resolution techniques like UV VS. 334 00:16:08,000 --> 00:16:10,360 They give much broader, less specific information. 335 00:16:10,400 --> 00:16:11,520 OK. So that's identity. 336 00:16:12,040 --> 00:16:15,520 Then Question 2. Is it a pure single compound or 337 00:16:15,520 --> 00:16:17,720 is it a mixture? Equally crucial, especially in 338 00:16:17,720 --> 00:16:19,640 synthesis. And again you're using 339 00:16:19,640 --> 00:16:22,400 techniques that give you clear distinct peaks. 340 00:16:22,440 --> 00:16:27,000 NMR vibrational spectroscopy like IR and ramen Mass Spec and 341 00:16:27,000 --> 00:16:30,720 PXRD. If you see signals that you 342 00:16:30,720 --> 00:16:34,000 can't account for from your product, you have impurities. 343 00:16:34,000 --> 00:16:36,160 Now, here's a critical caveat that we picked up from the 344 00:16:36,160 --> 00:16:38,160 sources, and it really needs emphasizing. 345 00:16:38,240 --> 00:16:42,240 Never rely on just one method for purity assessment. 346 00:16:42,240 --> 00:16:44,680 Absolutely critical. It's a classic mistake to rely 347 00:16:44,680 --> 00:16:48,440 on only one technique. For instance, sometimes weak or 348 00:16:48,440 --> 00:16:50,840 broadened signals in the spectrum can indicate not just 349 00:16:50,840 --> 00:16:53,800 impurities, but maybe issues like fluxional behavior or 350 00:16:53,800 --> 00:16:55,600 dynamic disorder in your molecule. 351 00:16:55,600 --> 00:16:58,240 Or worse. Or worse, sometimes the signals 352 00:16:58,240 --> 00:17:01,360 from an impurity can overlap or completely mask the signals of 353 00:17:01,360 --> 00:17:04,520 your actual product, making a mixture appear pure in that one 354 00:17:04,520 --> 00:17:07,520 specific spectrum. A high quality synthesis demands 355 00:17:07,520 --> 00:17:10,200 complementary checks using several distinct methods. 356 00:17:10,200 --> 00:17:13,040 You want a data set with no observable features other than 357 00:17:13,040 --> 00:17:14,560 those of the product you think you made. 358 00:17:14,640 --> 00:17:16,839 That's cool. So once purity is confirmed, we 359 00:17:16,839 --> 00:17:19,160 move to molecular composition. Question three. 360 00:17:19,680 --> 00:17:22,560 What is its molecular weight and elemental composition? 361 00:17:22,800 --> 00:17:25,800 This is traditionally the realm of elemental analysis, which 362 00:17:25,800 --> 00:17:28,560 gives you the percentage of carbon, hydrogen, nitrogen and 363 00:17:28,560 --> 00:17:31,880 so on, combined with high resolution mass spectrometry. 364 00:17:32,440 --> 00:17:35,640 Mass spec gives you the precise molecular weight which lets you 365 00:17:35,640 --> 00:17:39,720 confirm the molecular formula. And question 4, what functional 366 00:17:39,720 --> 00:17:42,000 groups does it contain? Now we're starting to get 367 00:17:42,000 --> 00:17:44,640 specific about the chemistry. This is the domain of 368 00:17:44,640 --> 00:17:48,560 vibrational spectroscopy. Infrared IR and Reman 369 00:17:48,560 --> 00:17:51,640 spectroscopy are the workhorses here because they quickly show 370 00:17:51,640 --> 00:17:55,960 the presence of characteristic bonds, say a strong OH stretch 371 00:17:56,160 --> 00:17:59,800 or carbon nitrogen triple bond, by measuring the specific energy 372 00:17:59,800 --> 00:18:01,480 needed to make those bonds vibrate. 373 00:18:01,560 --> 00:18:04,440 Then you could supplement that. Oh yeah, if the compound has 374 00:18:04,440 --> 00:18:08,720 suitable nuclei or maybe unpaired electrons, then NMR or 375 00:18:08,720 --> 00:18:12,240 electron paramagnetic resonance EPR are added to the functional 376 00:18:12,240 --> 00:18:14,440 group analysis. OK, now we get to the core of 377 00:18:14,440 --> 00:18:16,040 structure, connectivity and shape. 378 00:18:16,280 --> 00:18:18,800 Question 5 How are the atoms linked together? 379 00:18:18,800 --> 00:18:21,960 And Question 6 What is its three-dimensional shape? 380 00:18:22,160 --> 00:18:24,280 And this is where things can get really complicated. 381 00:18:24,440 --> 00:18:28,360 Even for simple systems we have to distinguish between 382 00:18:28,520 --> 00:18:31,840 connectivity isomers where the atoms are LinkedIn a different 383 00:18:31,840 --> 00:18:35,200 order and geometric isomers. Where the atoms are LinkedIn the 384 00:18:35,200 --> 00:18:39,240 same order but just occupy different positions in 3D space. 385 00:18:39,240 --> 00:18:41,040 Exactly. Let's use the example from the 386 00:18:41,040 --> 00:18:44,680 sources F4SO1 fluorine, 1 sulfur, one oxygen. 387 00:18:44,680 --> 00:18:47,520 It's a small molecule, but it highlights all of this 388 00:18:47,520 --> 00:18:50,120 complexity really well. It's a great example. 389 00:18:50,480 --> 00:18:52,520 A chemist would first establish the connectivity. 390 00:18:53,160 --> 00:18:56,600 The sources suggest 2 basic connectivity isomers are 391 00:18:56,600 --> 00:18:59,000 possible. We can call them two RI and two 392 00:18:59,000 --> 00:19:02,480 out two where the atoms are bonded in distinct sequences. 393 00:19:02,560 --> 00:19:05,080 And telling those apart should be pretty straightforward. 394 00:19:05,360 --> 00:19:09,040 Usually yes, using NMR or vibrational methods, because the 395 00:19:09,040 --> 00:19:11,720 local environment and the number of signals will be fundamentally 396 00:19:11,720 --> 00:19:13,800 different based on which atoms are bonded together. 397 00:19:13,840 --> 00:19:16,960 But let's assume we confirm the connectivity of two point I. 398 00:19:17,200 --> 00:19:19,040 The next challenge is the geometry. 399 00:19:19,320 --> 00:19:21,360 Exactly. The simplest geometry for a 400 00:19:21,360 --> 00:19:25,280 compound like 2 I, which has one sulfur atom bonded to 4 401 00:19:25,280 --> 00:19:29,120 fluorines and one oxygen plus a lone pair, is often a trigonal 402 00:19:29,120 --> 00:19:32,400 bipyramid arrangement of the six electron domains around that 403 00:19:32,400 --> 00:19:35,240 central sulfur. OK, a trigonal bipyramid and in 404 00:19:35,240 --> 00:19:37,480 that shape there are two distinct types of positions, 405 00:19:37,480 --> 00:19:39,680 right? The axial positions top and 406 00:19:39,680 --> 00:19:42,720 bottom, and the three Equatorial positions around the waist. 407 00:19:43,280 --> 00:19:45,440 Correct. And since the atoms attached to 408 00:19:45,440 --> 00:19:49,080 the sulfur 4 fluorines in one oxygen are not identical, where 409 00:19:49,080 --> 00:19:52,640 that oxygen sits matters hugely. It creates geometric isomers. 410 00:19:52,640 --> 00:19:56,080 It creates geometric isomers like 2.3 and 2.4 V in the source 411 00:19:56,080 --> 00:19:59,400 diagrams, structure 2-3, maybe the isomer with a single oxygen 412 00:19:59,400 --> 00:20:01,640 atom sits in one of the three Equatorial positions. 413 00:20:01,800 --> 00:20:05,320 Structure 2.4, however, would have that oxygen occupying one 414 00:20:05,320 --> 00:20:06,960 of the two axial positions instead. 415 00:20:07,080 --> 00:20:10,440 And the sources show that the other connectivity isomer 2.3 416 00:20:10,440 --> 00:20:12,720 can also have its own geometric isomers. 417 00:20:12,840 --> 00:20:15,960 So the key take away here is that for one simple formula at 418 00:20:15,960 --> 00:20:18,040 4:00. So we generated two different 419 00:20:18,040 --> 00:20:21,760 connectivity isomers and within them multiple geometric isomers 420 00:20:21,760 --> 00:20:24,520 just based on whether the oxygen is axial or Equatorial. 421 00:20:24,720 --> 00:20:27,360 And that's why geometric analysis is non negotiable. 422 00:20:27,960 --> 00:20:30,680 These different geometric isomers will have profoundly 423 00:20:30,680 --> 00:20:33,680 different properties because they have different bond angles, 424 00:20:33,880 --> 00:20:36,880 different dipole moments, and crucially, different molecular 425 00:20:36,880 --> 00:20:38,960 symmetries. So you could tell them apart 426 00:20:38,960 --> 00:20:41,480 with something like fluorine 19 NMR. 427 00:20:41,600 --> 00:20:43,680 You could. You'd expect to see completely 428 00:20:43,680 --> 00:20:46,520 distinct patterns based on how many chemically equivalent 429 00:20:46,520 --> 00:20:48,920 fluorine environments exist in each isomer. 430 00:20:49,200 --> 00:20:51,320 That difference in symmetry is the chemist's clue. 431 00:20:51,600 --> 00:20:54,120 And this really highlights why geometry is so important. 432 00:20:54,520 --> 00:20:56,880 It dictates reactivity and stability. 433 00:20:57,480 --> 00:21:01,120 A longer bond almost always translates to a weaker bond. 434 00:21:01,160 --> 00:21:03,720 Lower dissociation energy more prone to breaking. 435 00:21:03,760 --> 00:21:07,280 And the specific 3D shape controls how molecules interact 436 00:21:07,280 --> 00:21:10,880 with everything, solvents, catalysts, receptor sites in a 437 00:21:10,880 --> 00:21:13,000 drug. So once those macro level 438 00:21:13,000 --> 00:21:16,000 questions, purity, connectivity, geometry are settled, then we 439 00:21:16,000 --> 00:21:19,000 drill down to the deepest atomic and electronic detail. 440 00:21:19,320 --> 00:21:21,400 This is the final four questions in the hierarchy. 441 00:21:21,480 --> 00:21:24,480 Question 7. What is its molecular symmetry, 442 00:21:24,800 --> 00:21:27,680 which, as you said, is crucial for predicting polarity and the 443 00:21:27,680 --> 00:21:29,600 possibility of chirality? Eight. 444 00:21:30,200 --> 00:21:32,120 What is its geometrical structure? 445 00:21:32,600 --> 00:21:36,200 This is the ultimate precise quantitative measurement of bond 446 00:21:36,200 --> 00:21:39,280 lengths and angles, the absolute nuclear positions. 447 00:21:39,560 --> 00:21:41,560 This is where the X-ray gold standards come in. 448 00:21:42,040 --> 00:21:44,400 Nine, what is its electronic structure? 449 00:21:44,400 --> 00:21:47,680 So now we're asking about orbital occupation energy 450 00:21:47,680 --> 00:21:50,600 distribution, which controls the chemical bonding itself. 451 00:21:50,800 --> 00:21:55,840 And finally, 10 how is the electron density, the charge 452 00:21:55,840 --> 00:21:58,720 distributed in space? This is often the hardest to 453 00:21:58,720 --> 00:22:01,640 measure and requires the most specialized techniques, but it's 454 00:22:01,640 --> 00:22:04,800 the key to understanding bond polarity and reaction 455 00:22:04,800 --> 00:22:06,760 mechanisms. OK, so for the most precise 456 00:22:06,760 --> 00:22:09,480 geometrical measurements, the definitive bond lengths and 457 00:22:09,480 --> 00:22:12,200 angles, we go back to the gold standards of diffraction. 458 00:22:12,280 --> 00:22:15,440 The primary technique is single crystal X-ray diffraction, or 459 00:22:15,440 --> 00:22:17,720 SCXRD. For most atoms, this provides 460 00:22:17,720 --> 00:22:20,040 the most precise nuclear positions you can get. 461 00:22:20,160 --> 00:22:22,080 But there are times when X-rays struggle. 462 00:22:22,240 --> 00:22:25,600 They do, particularly with very light atoms like hydrogen, which 463 00:22:25,600 --> 00:22:27,800 scatter X-rays very weakly because they only they have one 464 00:22:27,800 --> 00:22:28,840 electron. And that's where neutron 465 00:22:28,840 --> 00:22:30,480 diffraction comes in. Exactly. 466 00:22:30,760 --> 00:22:32,560 Neutron diffraction is often complementary. 467 00:22:32,880 --> 00:22:35,640 Neutrons scatter off the nucleus, not the electron cloud. 468 00:22:36,160 --> 00:22:39,600 And crucially, hydrogen and deuterium have high neutron 469 00:22:39,600 --> 00:22:42,800 scattering cross sections. So when you're studying systems 470 00:22:42,800 --> 00:22:45,640 where the hydrogen position is vital, like hydrogen bonding in 471 00:22:45,640 --> 00:22:48,680 biology or materials for hydrogen storage, neutron 472 00:22:48,680 --> 00:22:52,760 diffraction provides detail that SCXRD just can't match. 473 00:22:52,960 --> 00:22:55,800 We also noted some specialized methods depending on the phase 474 00:22:55,800 --> 00:22:58,920 of the material, right. Yes, if your material is a gas, 475 00:22:59,120 --> 00:23:01,240 you might use electron diffraction or microwave 476 00:23:01,240 --> 00:23:03,480 spectroscopy to determine geometry. 477 00:23:04,160 --> 00:23:07,600 If you're studying a liquid or an amorphous solid, techniques 478 00:23:07,600 --> 00:23:12,000 like XAFS or specialized NMR are needed since SEXRD requires a 479 00:23:12,000 --> 00:23:13,960 crystal. And for electronic structure, 480 00:23:13,960 --> 00:23:16,320 how do we actually probe the fundamental orbitals? 481 00:23:16,520 --> 00:23:18,800 We use methods that involve high energy excitation. 482 00:23:19,320 --> 00:23:22,000 Most power spectroscopy, as we said, can directly probe the 483 00:23:22,000 --> 00:23:23,960 orbital, like on density around the nucleus. 484 00:23:24,120 --> 00:23:27,320 For specific isotopes more generally, though, we rely on 485 00:23:27,320 --> 00:23:28,840 methods that probe energy levels. 486 00:23:28,920 --> 00:23:32,160 Uvvs for valence electrons. Right, but for deeper detail, we 487 00:23:32,160 --> 00:23:35,040 use techniques that involve ionizing the core electrons, the 488 00:23:35,040 --> 00:23:37,440 ones deep inside the atom. Like XPS and Xas? 489 00:23:37,480 --> 00:23:41,240 Let's just clarify those. So XPS or X-ray photoelectron 490 00:23:41,240 --> 00:23:44,480 spectroscopy measures the kinetic energy of electrons that 491 00:23:44,480 --> 00:23:48,000 get ejected by X-rays. This kinetic energy tells us the 492 00:23:48,000 --> 00:23:51,600 binding energy of those core electrons, which is exquisitely 493 00:23:51,600 --> 00:23:54,560 sensitive to the atoms oxidation state and its local chemical 494 00:23:54,560 --> 00:23:56,040 environment. And XAS. 495 00:23:56,160 --> 00:23:59,080 XAS, or X-ray absorption spectroscopy probes the 496 00:23:59,080 --> 00:24:01,960 transitions of electrons from those core shells into 497 00:24:01,960 --> 00:24:05,600 unoccupied molecular orbitals. Together, these methods give you 498 00:24:05,600 --> 00:24:08,480 quantitative information on the relative energies of your 499 00:24:08,480 --> 00:24:11,480 occupied and vacant orbitals and the atoms charge state. 500 00:24:11,720 --> 00:24:14,680 And again, these experimental data points, orbital energies, 501 00:24:14,680 --> 00:24:17,880 charge distribution, they're often correlated and confirmed 502 00:24:17,880 --> 00:24:19,840 by computational methods like DfT. 503 00:24:19,840 --> 00:24:22,160 It's the definition of a complementary toolkit. 504 00:24:22,560 --> 00:24:25,720 No single method provides all 5 pieces of structural 505 00:24:25,720 --> 00:24:28,080 information. A skilled chemist has to design 506 00:24:28,080 --> 00:24:31,120 a minimum set of complementary experiments and Mr. for 507 00:24:31,120 --> 00:24:33,680 connectivity. SEXRD for geometry. 508 00:24:33,920 --> 00:24:36,360 XP. PS for electronic state to build 509 00:24:36,360 --> 00:24:38,600 a robust comprehensive structural model. 510 00:24:38,800 --> 00:24:42,040 OK, let's shift our focus now to the underlying physics. 511 00:24:42,520 --> 00:24:45,760 Whether we're probing geometry with diffraction or electronic 512 00:24:45,760 --> 00:24:49,280 structure with spectroscopy, most of these techniques rely on 513 00:24:49,280 --> 00:24:52,800 one of two fundamental processes, spectroscopy or 514 00:24:52,800 --> 00:24:55,520 diffraction. What is the core physical 515 00:24:55,520 --> 00:24:58,120 distinction between them? The distinction really lies in 516 00:24:58,120 --> 00:25:01,120 the interaction Energy. Ectroscopy measures the 517 00:25:01,120 --> 00:25:04,560 absorption or emission of electromagnetic radiation. 518 00:25:05,200 --> 00:25:07,840 This means the molecule has to undergo a transition between 519 00:25:07,840 --> 00:25:10,920 quantized energy states. O there's an energy change. 520 00:25:11,080 --> 00:25:14,400 Critically, yes, it involves an inelastic interaction. 521 00:25:14,560 --> 00:25:18,080 There is a net change in energy in the sample as the molecule is 522 00:25:18,080 --> 00:25:20,880 excited to a higher energy level like higher vibrational or 523 00:25:20,880 --> 00:25:23,080 electronic state. And diffraction is different. 524 00:25:23,320 --> 00:25:25,800 Diffraction involves the scattering and subsequent 525 00:25:25,800 --> 00:25:29,240 interference of waves or particles, X-rays, neutrons, 526 00:25:29,240 --> 00:25:31,840 electrons by the atoms or their electron clouds. 527 00:25:32,320 --> 00:25:35,000 The resulting interference pattern reveals periodicities 528 00:25:35,000 --> 00:25:38,080 like inner atomic distances. The key difference is that 529 00:25:38,080 --> 00:25:40,520 diffraction is typically an elastic interaction. 530 00:25:40,520 --> 00:25:44,080 No net energy change. No net change in the energy 531 00:25:44,080 --> 00:25:47,360 state of the sample molecule. The atoms remain in their ground 532 00:25:47,360 --> 00:25:49,800 state. That distinction between elastic 533 00:25:49,800 --> 00:25:54,240 no energy change and inelastic and energy change is really the 534 00:25:54,240 --> 00:25:58,520 key to choosing the right tool. And since spectroscopy relies on 535 00:25:58,520 --> 00:26:01,920 radiation, we have to revisit the electromagnetic spectrum. 536 00:26:02,240 --> 00:26:04,640 It's the entire source of information for these 537 00:26:04,640 --> 00:26:06,880 techniques. The spectrum is fundamentally 538 00:26:06,880 --> 00:26:09,000 linked to the transitions we can observe. 539 00:26:09,560 --> 00:26:13,040 The relationship between energy, which we call E frequency and 540 00:26:13,040 --> 00:26:17,080 wavelength, is governed by those two fundamental equations, the 541 00:26:17,080 --> 00:26:21,240 speed of light C equals me and Planck's relationship E equals. 542 00:26:21,320 --> 00:26:24,200 High, so as the energy goes up, the frequency goes up and the 543 00:26:24,200 --> 00:26:25,760 wavelength gets shorter. Exactly. 544 00:26:26,160 --> 00:26:28,800 And by exploiting different regions of the spectrum, we're 545 00:26:28,800 --> 00:26:31,280 physically hitting the molecule with the perfect amount of 546 00:26:31,280 --> 00:26:34,160 energy to probe a specific fundamental transition. 547 00:26:34,160 --> 00:26:36,640 OK, so let's walk up the spectrum, starting at the lowest 548 00:26:36,640 --> 00:26:40,760 energy, the radio wave region. Here we find NMR radio waves 549 00:26:40,760 --> 00:26:43,200 have just enough energy to induce excitation between 550 00:26:43,200 --> 00:26:46,360 nuclear spin states, but only when the sample is placed in a 551 00:26:46,360 --> 00:26:50,200 very strong magnetic field. These transitions are incredibly 552 00:26:50,200 --> 00:26:53,360 subtle energetically, but they're highly sensitive to the 553 00:26:53,360 --> 00:26:54,800 surrounding electronic environment. 554 00:26:54,800 --> 00:26:58,240 Which is why NMR is so great for determining connectivity. 555 00:26:58,240 --> 00:27:00,120 That's right. Now moving up slightly to the 556 00:27:00,120 --> 00:27:03,400 microwave region. This is the energy realm of EPR, 557 00:27:03,400 --> 00:27:06,840 electron paramagnetic resonance and also pure rotational 558 00:27:06,840 --> 00:27:09,600 spectroscopy. So what can microwaves excite? 559 00:27:10,080 --> 00:27:12,960 Microwaves are energetic enough to excite uncared electrons 560 00:27:12,960 --> 00:27:16,000 between their quantum states, which is what EPR measures. 561 00:27:16,600 --> 00:27:20,280 This is vital for studying free radicals or many transition 562 00:27:20,280 --> 00:27:23,160 metal complexes. They can also excite molecules 563 00:27:23,160 --> 00:27:26,560 between rotational states, which can give you extremely precise 564 00:27:26,560 --> 00:27:30,400 bond lengths and angles for small gas phase molecules. 565 00:27:30,600 --> 00:27:33,200 Then we arrive at the infrared, the IR region. 566 00:27:33,280 --> 00:27:36,000 IR radiation hits the sweet spot for molecular motion. 567 00:27:36,640 --> 00:27:38,800 This energy is sufficient to excite transitions between 568 00:27:38,800 --> 00:27:40,960 vibrational states. The stretching, the bending, the 569 00:27:40,960 --> 00:27:44,000 rocking of bonds. This is why IR and Rahman are 570 00:27:44,000 --> 00:27:46,360 the workhorses for identifying functional groups. 571 00:27:46,360 --> 00:27:49,640 And for determining molecular symmetry, because the number and 572 00:27:49,640 --> 00:27:52,280 type of vibrations you can observe are dictated by the 573 00:27:52,280 --> 00:27:55,440 molecule symmetry. Higher still, we enter the UV 574 00:27:55,440 --> 00:27:58,400 and visible light regions. This energy is sufficient to 575 00:27:58,400 --> 00:28:01,040 promote valence electrons between electronic states, 576 00:28:01,440 --> 00:28:03,800 typically from an occupied orbital to a vacant 1. 577 00:28:04,480 --> 00:28:07,640 This is the basis for electronic spectroscopy, and it's often the 578 00:28:07,640 --> 00:28:10,840 source of the beautiful colors we see in transition metal 579 00:28:10,840 --> 00:28:13,640 complexes. And at the very high end of UV 580 00:28:13,640 --> 00:28:15,560 you can actually start knocking electrons out. 581 00:28:15,680 --> 00:28:17,280 Exactly. You sort of have enough energy 582 00:28:17,280 --> 00:28:20,440 to ionize the molecule, ejecting electrons from the valence 583 00:28:20,440 --> 00:28:22,000 orbitals. And finally, we get to the 584 00:28:22,000 --> 00:28:25,240 highest energy radiation, X-rays and gamma rays. 585 00:28:25,360 --> 00:28:28,680 X-rays carry enough energy to induce photo excitation in the 586 00:28:28,680 --> 00:28:31,640 molecules core electrons, the ones deep inside the atom. 587 00:28:32,120 --> 00:28:35,880 This is the mechanism for XPS and Xas, providing that deep 588 00:28:35,880 --> 00:28:38,880 detail on oxidation States and binding energies. 589 00:28:39,000 --> 00:28:41,920 And gamma rays. Gamma rays, the absolute highest 590 00:28:41,920 --> 00:28:45,680 energy, are used in most Bower spectroscopy, the induced 591 00:28:45,680 --> 00:28:48,200 transitions between energy levels within the nucleus, 592 00:28:48,200 --> 00:28:51,440 itself a unique probe of the nuclear environment. 593 00:28:51,480 --> 00:28:54,120 It's a remarkable systematic exploitation. 594 00:28:54,360 --> 00:28:57,320 We move from these low energy probes for nuclear spin all the 595 00:28:57,320 --> 00:29:00,280 way up to high energy probes that RIP electrons out of core 596 00:29:00,280 --> 00:29:03,800 shells and excite the nucleus itself, all to get a specific 597 00:29:03,800 --> 00:29:06,240 piece of structural information. Exactly. 598 00:29:06,240 --> 00:29:09,480 And this systematic approach leads us directly to our final 599 00:29:09,680 --> 00:29:13,280 critical conceptual tool, molecular symmetry. 600 00:29:13,400 --> 00:29:17,080 OK, so why is defining symmetry so precisely important for a 601 00:29:17,080 --> 00:29:18,880 chemist? It feels like we're leaving the 602 00:29:18,880 --> 00:29:22,200 lab instruments behind and entering the world of pure math 603 00:29:22,280 --> 00:29:25,400 with group theory. We are, in a way, but the 604 00:29:25,400 --> 00:29:27,280 physics of the molecule obeys the math. 605 00:29:27,800 --> 00:29:30,840 Symmetry and bonding are well, they're inextricably linked. 606 00:29:31,360 --> 00:29:33,720 Defining symmetry precisely allows for molecular 607 00:29:33,720 --> 00:29:36,480 classification, but more practically, it directly 608 00:29:36,480 --> 00:29:39,240 dictates properties. A molecule cannot be polar 609 00:29:39,240 --> 00:29:41,120 unless it lacks certain symmetry elements. 610 00:29:41,720 --> 00:29:44,320 A molecule cannot be chiral unless it lacks improper 611 00:29:44,320 --> 00:29:46,920 rotation. And beyond that, symmetry is 612 00:29:46,920 --> 00:29:49,640 essential for predicting and interpreting the results from 613 00:29:49,640 --> 00:29:51,320 spectroscopy. Absolutely. 614 00:29:51,760 --> 00:29:54,960 If you know the symmetry, you can use group theory to simplify 615 00:29:54,960 --> 00:29:57,560 the complex calculations you need to build molecular 616 00:29:57,560 --> 00:30:01,040 orbitals, and you can predict which vibrational modes will be 617 00:30:01,040 --> 00:30:04,560 active, meaning observable in your IR and rematch Spectra. 618 00:30:05,400 --> 00:30:08,760 The language we use to describe this precise classification is 619 00:30:08,760 --> 00:30:12,000 group theory, and it all relies on identifying the symmetry 620 00:30:12,000 --> 00:30:15,160 elements a molecule possesses. OK, let's just nail down the 621 00:30:15,160 --> 00:30:18,520 necessary distinction again, the difference between an operation 622 00:30:18,520 --> 00:30:19,720 and an element. Right. 623 00:30:19,880 --> 00:30:22,800 A symmetry operation is the physical process or motion that 624 00:30:22,800 --> 00:30:25,840 generates a configuration that's indistinguishable from where you 625 00:30:25,840 --> 00:30:28,520 started, so a rotation or reflection. 626 00:30:28,640 --> 00:30:30,280 And the element. The symmetry element is the 627 00:30:30,280 --> 00:30:32,560 geometrical property that defines that operation. 628 00:30:32,920 --> 00:30:36,200 It's the point, the line, or the plane that the molecule must 629 00:30:36,200 --> 00:30:37,920 possess for the operation to work. 630 00:30:38,000 --> 00:30:40,600 OK, let's go through the five fundamental operations and their 631 00:30:40,600 --> 00:30:43,280 elements. The simplest is the identity. 632 00:30:43,760 --> 00:30:46,360 The identity operation is just doing nothing. 633 00:30:46,880 --> 00:30:49,720 It sounds trivial, but it's mathematically necessary to 634 00:30:49,720 --> 00:30:51,880 ensure that every molecule belonged to a group. 635 00:30:52,560 --> 00:30:55,280 Every single molecule possesses the identity element E. 636 00:30:55,480 --> 00:30:57,520 Fair enough. Next is rotation. 637 00:30:57,520 --> 00:30:59,800 CN, the rotation. Operation is rotating the 638 00:30:59,800 --> 00:31:03,160 molecule by 360 / N degrees about an axis. 639 00:31:03,640 --> 00:31:05,760 If the resulting view is identical to the starting view, 640 00:31:05,760 --> 00:31:09,000 it's an Nfold rotation. The element is the Nfold 641 00:31:09,000 --> 00:31:11,200 rotation axis, which is a line we call CN. 642 00:31:11,320 --> 00:31:13,760 And a key concet here is the rincial axis. 643 00:31:13,840 --> 00:31:16,040 Rincial axis is the one with the highest value of North. 644 00:31:16,800 --> 00:31:20,080 By convention, we always align this axis with the Z axis in our 645 00:31:20,080 --> 00:31:22,920 coordinate system. Let's use xenon tetrafluoride 646 00:31:22,960 --> 00:31:27,800 XCF 4, which is square planar. What does that primary CN axis 647 00:31:27,800 --> 00:31:31,040 tell us? SC4 has a primary C4 axis that's 648 00:31:31,040 --> 00:31:32,880 perpendicular to the plane of the molecule. 649 00:31:33,160 --> 00:31:36,400 This means you can rotate it by 90° and it looks identical. 650 00:31:36,840 --> 00:31:39,520 The existence of this C4 axis immediately tells you you're 651 00:31:39,520 --> 00:31:42,080 dealing with a molecule of high symmetry, which simplifies your 652 00:31:42,080 --> 00:31:44,080 analysis. And it has other axis too. 653 00:31:44,280 --> 00:31:47,240 It does. It also has four secondary C2 654 00:31:47,240 --> 00:31:50,400 axis that are perpendicular to that C4 axis. 655 00:31:50,800 --> 00:31:53,560 Two of them pass through opposite fluorine atoms and the 656 00:31:53,560 --> 00:31:56,040 other two bisect the FXEF bond angles. 657 00:31:56,120 --> 00:31:57,640 OK. Moving to the third operation, 658 00:31:57,720 --> 00:32:00,920 Reflection Sigma. Reflection is reflecting all the 659 00:32:00,920 --> 00:32:04,160 atoms through a mirror plane. The element is the mirror plane, 660 00:32:04,880 --> 00:32:08,000 and we use subscripts to relate the plane to the principle axis. 661 00:32:08,000 --> 00:32:11,240 So the horizontal mirror lane is perpendicular to the principle 662 00:32:11,240 --> 00:32:13,240 axis. And in RSF for example, the 663 00:32:13,240 --> 00:32:15,480 plane containing all the atoms is the stuff. 664 00:32:15,840 --> 00:32:17,600 What about the other types of planes? 665 00:32:17,760 --> 00:32:19,240 Then you have vertical mirror planes. 666 00:32:19,760 --> 00:32:21,560 These are planes that contain the principal axis. 667 00:32:21,680 --> 00:32:26,720 A molecule like water H2O which has AC2 axis has two vertical 668 00:32:26,720 --> 00:32:29,200 mirror planes that both contain that C2 axis. 669 00:32:29,200 --> 00:32:31,320 And the final type, the dihedral plane. 670 00:32:31,440 --> 00:32:34,200 The dasheed stands for dihedral. These are also vertical planes 671 00:32:34,200 --> 00:32:37,320 that contain the principal axis, but they specifically bisect the 672 00:32:37,320 --> 00:32:40,360 angle between two of those perpendicular C2 axis. 673 00:32:40,480 --> 00:32:43,920 This distinction often passing through atoms passing between 674 00:32:43,920 --> 00:32:45,880 them. It's subtle, but it's essential 675 00:32:45,880 --> 00:32:48,720 for classifying the high symmetry groups like D4H, which 676 00:32:48,720 --> 00:32:52,680 is what CF4 belongs to. The 4th operation is inversion. 677 00:32:53,120 --> 00:32:56,960 I inversion involves projecting every single atom through a 678 00:32:56,960 --> 00:33:00,160 single central point, the center of inversion I, to an equal 679 00:33:00,160 --> 00:33:03,360 distance on the opposite side. So if an atom has coordinates 680 00:33:03,360 --> 00:33:07,160 XYZ, the inversion operation sends it to XYZ. 681 00:33:07,600 --> 00:33:10,040 If the molecule looks identical after you do that to every atom, 682 00:33:10,080 --> 00:33:11,600 it possesses the center of inversion. 683 00:33:11,680 --> 00:33:13,560 And does that center of inversion always have to be 684 00:33:13,560 --> 00:33:16,240 occupied by an atom? No, and this is a common point 685 00:33:16,240 --> 00:33:19,200 of confusion. For octahedral SF6, the center 686 00:33:19,200 --> 00:33:22,400 is at the sulfur atom. For linear CO2, it's at the 687 00:33:22,400 --> 00:33:26,040 carbon atom, but if you consider a simple diatomic molecule like 688 00:33:26,040 --> 00:33:30,200 N2 or H2, the center of inversion lies precisely midway 689 00:33:30,200 --> 00:33:32,800 between the two nuclei, where there's no out of it all. 690 00:33:33,000 --> 00:33:36,200 And the final operation, improper rotation SCN. 691 00:33:36,320 --> 00:33:38,800 This one combines 2 of the simpler operation. 692 00:33:38,800 --> 00:33:40,400 It's the rotation reflection operation. 693 00:33:40,720 --> 00:33:43,840 You first perform an N fold rotation CN about an axis, and 694 00:33:43,840 --> 00:33:46,360 then you immediately follow it up with a reflection in a plane 695 00:33:46,360 --> 00:33:48,640 that's perpendicular to that axis, the S. 696 00:33:48,880 --> 00:33:51,920 The element is the improper rotation axis SN. 697 00:33:51,920 --> 00:33:53,960 And there are a couple of equivalences here we need to 698 00:33:53,960 --> 00:33:55,280 remember. Absolutely. 699 00:33:55,480 --> 00:33:58,560 And S1 axis is equivalent to just single reflection. 700 00:33:58,920 --> 00:34:03,280 And an S2 axis which is a 180° rotation followed by a 701 00:34:03,280 --> 00:34:06,200 perpendicular reflection is equivalent to the inversion 702 00:34:06,200 --> 00:34:09,159 operation I mean. So if we find an S1 or S2, we 703 00:34:09,159 --> 00:34:13,239 just use the simpler symbols or I to keep the notation clean 704 00:34:13,320 --> 00:34:15,000 Exactly. And by systematically 705 00:34:15,000 --> 00:34:17,199 identifying all of these symmetry elements that a 706 00:34:17,199 --> 00:34:20,199 molecule possesses, we can then assign it to a point group. 707 00:34:20,400 --> 00:34:22,960 The point group is simply the collection of all the symmetry 708 00:34:22,960 --> 00:34:25,159 operations that can be performed on that molecule. 709 00:34:25,840 --> 00:34:28,880 Chemists use a standardized decision tree to identify the 710 00:34:28,880 --> 00:34:31,040 principal axis and all the accompanying planes and 711 00:34:31,040 --> 00:34:34,000 inversion centers to assign the correct point group notation 712 00:34:34,199 --> 00:34:37,920 like C2VD3H or TD. Let's run through the examples 713 00:34:37,920 --> 00:34:40,800 we provided for our listener just to solidify these concepts, 714 00:34:40,920 --> 00:34:43,320 starting with H2O, the water molecule. 715 00:34:43,800 --> 00:34:45,920 Water is bent, which is relatively low symmetry. 716 00:34:45,920 --> 00:34:48,880 That belongs to the C2V point group and that the name tells 717 00:34:48,880 --> 00:34:51,080 you everything. It is AC2 principal rotation 718 00:34:51,080 --> 00:34:55,719 axis, A 180° rotation and two vertical mirror planes that both 719 00:34:55,719 --> 00:34:57,800 contain that axis. And because it lacks A 720 00:34:57,800 --> 00:35:00,520 horizontal plane, a SUS, it's polar. 721 00:35:00,760 --> 00:35:02,520 Exactly. The dipoles don't cancel. 722 00:35:02,560 --> 00:35:05,960 Our previous example Xef 4 the square planar molecule. 723 00:35:06,120 --> 00:35:08,520 Xef 4 belongs to the D4 edge point group. 724 00:35:08,920 --> 00:35:12,200 The D tells you there's a principal C4 axis plus 4 725 00:35:12,200 --> 00:35:15,080 perpendicular C2 axis. The H tells you there's a 726 00:35:15,080 --> 00:35:17,800 horizontal mirror plane. This high symmetry means the 727 00:35:17,800 --> 00:35:21,280 molecule is non polar regardless of the individual Xef bond 728 00:35:21,280 --> 00:35:24,240 polarities. How about BF3 boron trifluoride 729 00:35:24,240 --> 00:35:28,480 which is trigonal planar? BF3 belongs to D3HA principal C3 730 00:35:28,480 --> 00:35:32,760 axis 3 perpendicular C2 axis and a horizontal mirror plane like 731 00:35:32,760 --> 00:35:35,040 XEF 4. This is a high symmetry group, 732 00:35:35,360 --> 00:35:38,440 meaning it's non polar overall because the Bondi poles cancel 733 00:35:38,440 --> 00:35:41,520 out perfectly. And finally the sulfate ion SO42 734 00:35:41,520 --> 00:35:44,560 minus which is tetrahedral. Sulfate belongs to one of the 735 00:35:44,560 --> 00:35:47,680 highest symmetry point groups P8 for tetrahedral. 736 00:35:47,920 --> 00:35:50,240 These groups are defined by multiple high order axis, 737 00:35:50,240 --> 00:35:55,160 specifically for C3 axis axis, 3C2 axis and 6S4 improper 738 00:35:55,160 --> 00:35:57,720 rotation axis. It's a highly symmetrical system 739 00:35:57,720 --> 00:36:00,280 which leads to very unique spectroscopic properties. 740 00:36:00,440 --> 00:36:03,880 So this precise language of symmetry is clearly essential. 741 00:36:03,960 --> 00:36:07,640 It moves structure from just a nice drawing to a predictive 742 00:36:07,640 --> 00:36:11,160 tool that dictates whether a molecule will be polar, whether 743 00:36:11,160 --> 00:36:14,280 it can be chiral, and precisely how many distinct peaks we 744 00:36:14,280 --> 00:36:17,240 should expect to see in a technique like NMR or IR 745 00:36:17,240 --> 00:36:18,840 spectroscopy. Exactly. 746 00:36:19,240 --> 00:36:22,040 Symmetry is the mathematical framework that ties geometry to 747 00:36:22,040 --> 00:36:24,400 property. Without it, interpreting the 748 00:36:24,400 --> 00:36:27,240 complex data sets that modern instruments generate would be, 749 00:36:27,960 --> 00:36:30,040 well, almost impossible. So what does this all mean? 750 00:36:30,040 --> 00:36:32,840 In the final synthesis? We've traced this journey of 751 00:36:32,840 --> 00:36:36,920 structure from 19th century intuition all the way to 21st 752 00:36:36,920 --> 00:36:40,440 century femtosecond resolution. The central theme is continuity 753 00:36:40,440 --> 00:36:42,520 and acceleration. Structural chemistry is the 754 00:36:42,520 --> 00:36:45,400 engine of chemistry because understanding structure allows 755 00:36:45,400 --> 00:36:47,080 us to predict and design function. 756 00:36:47,640 --> 00:36:49,840 In just over a century we've developed this incredible array 757 00:36:49,840 --> 00:36:53,120 of tools, spectroscopy, diffraction computation, to 758 00:36:53,120 --> 00:36:56,200 probe every aspect of structure from basic purity and 759 00:36:56,200 --> 00:36:58,960 connectivity right up to the dynamic electronic distribution 760 00:36:58,960 --> 00:37:01,480 of charge. And the journey is of course 761 00:37:01,480 --> 00:37:03,640 continuing. We noted the rise of 762 00:37:03,640 --> 00:37:07,000 computational methods like DfT and now the integration of 763 00:37:07,000 --> 00:37:09,920 machine learning and AI to handle the immense high 764 00:37:09,920 --> 00:37:11,920 dimensional data sets these tools produce. 765 00:37:12,200 --> 00:37:15,960 AI is accelerating structure solution, interpreting complex 766 00:37:15,960 --> 00:37:18,880 patterns, and guiding experiments that were previously 767 00:37:18,880 --> 00:37:20,880 just too time consuming for humans to do alone. 768 00:37:21,040 --> 00:37:23,760 But those advanced systems are built entirely upon the 769 00:37:23,760 --> 00:37:25,960 fundamentals that we've discussed today, the 770 00:37:25,960 --> 00:37:29,120 quantitative relationship between radiation, energy and 771 00:37:29,120 --> 00:37:32,520 molecular transitions, the physics of elastic versus 772 00:37:32,520 --> 00:37:35,640 inelastic scattering, and the rigorous mathematical 773 00:37:35,640 --> 00:37:38,200 constraints imposed by molecular symmetry. 774 00:37:38,360 --> 00:37:39,640 You have to know the fundamentals. 775 00:37:39,640 --> 00:37:42,480 So here's a final provocative thought to take with you, tying 776 00:37:42,480 --> 00:37:44,560 back to our simple example of F4. 777 00:37:44,560 --> 00:37:48,400 SO we saw that even with only five atoms bonded to a central 778 00:37:48,400 --> 00:37:52,120 sulfur, the decision of whether the oxygen atom was axial or 779 00:37:52,120 --> 00:37:55,360 Equatorial dramatically changed the molecular symmetry and 780 00:37:55,360 --> 00:37:57,320 consequently all the expected properties. 781 00:37:57,720 --> 00:38:00,360 The difficulty of assigning the correct structure for a molecule 782 00:38:00,360 --> 00:38:03,840 that small really highlights why precise structural analysis is 783 00:38:03,840 --> 00:38:07,440 non negotiable in chemistry. And the implication for the vast 784 00:38:07,440 --> 00:38:09,760 complex multi component materials we use today, 785 00:38:10,240 --> 00:38:13,960 catalysts, solar cells, superconductors, is profound. 786 00:38:14,320 --> 00:38:17,760 You have to ask what crucial undiscovered property, the 787 00:38:17,800 --> 00:38:20,720 unique catalytic pathway, a dramatically improved magnetic 788 00:38:20,720 --> 00:38:24,120 behavior, or a stability profile that changes everything is 789 00:38:24,120 --> 00:38:26,840 currently hidden in a conflict material simply because its 790 00:38:26,840 --> 00:38:29,000 exact bond lengths or angles remain unknown. 791 00:38:29,080 --> 00:38:32,240 The unseen architecture holds the key to the next generation 792 00:38:32,240 --> 00:38:35,160 of material science. That was the deep dive into 793 00:38:35,160 --> 00:38:37,280 structural methods and inorganic chemistry. 794 00:38:37,280 --> 00:38:38,160 Thank you for joining us.