Video: “How Pauling Bent the Double Bond”
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Cliff Mead: Welcome everybody; I am very pleased to see such a large crowd for this latest iteration of the Pauling Resident Scholar Talk. The Resident Scholar Program sponsored by Oregon State University Library Special Collections and supported by the Peter and Judith Freeman Fund, awards stipends for up to $2,500 per month, for up to three months. Stipends are awarded to individuals researching topics related to the history of twentieth century science and technology. Historians, librarians, doctoral or post-doctoral students and independent scholars are welcome to apply. The scholars must work in residence at the OSU Valley Library during the work period and our current Pauling resident scholar, the 2010 scholar, is Julia Bursten. Ms. Bursten, a doctoral student at the Department of History and the Philosophy of Science at the University of Pittsburgh, will examine how Pauling's structural approach to bonding in the 1930s influenced his later arguments in favor of the bent equivalent banana bond structure of double bonds as opposed to the pi bond structure which continues to be prevalent today. Ms. Bursten got her bachelors degree from Rice University in 2008 in philosophy. Her thesis was "Selecting a Structure of Scientific Communication". Among the awards and honors she has obtained in 2008 she won the departmental honors for the Department of Philosophy at Rice University. She was an invited speaker in 2008 at the West Virginia University Undergraduate Philosophy Conference. In 2009, the University of Pittsburgh Arts and Science Graduate Research Expo Panel winner and most recently she was a 2010 nominee for the Elizabeth Baranger Graduate Teaching Award. Ms. Bursten's talk today is how Pauling bent the double bond. [2:16]
Julia Bursten: Thank you all for coming out today, that was quite the introduction and I hope I can live up to it for the next few minutes. I am in a department of history and philosophy of science and since it looks like we have a little bit of a diverse crowd here today, I want to start out and just say a few words about what history and philosophy of science is, what it does, and then tell you a couple a couple of stories about how I got to be here and what I've found since I've been here. To start out, history and philosophy of science, I see and the discipline is tending to see more and more as an integrated discipline. It started out that you had historians of science, philosophers of science, and there are still some people who tend toward one more than the other. I came from philosophy, as you heard, during my undergraduate career and when I got to graduate school, I thought I was going to be doing primarily philosophy. Since I've been here I've been doing primarily history and the way that I got here has really shown me that the two can't be extracted, one from the other. They tend to run together and so for me, when I got to graduate school, I thought that I was going to be doing philosophy and physics, no history whatsoever, I just wanted to look at questions like "what can quantum mechanics tell us about the way the world is?"; "what is this weird matter that we see at the very small level?"; "how do particles communicate with one another along long distances?" and I spent the first year of my graduate career studying those sorts of questions. And after a year, I realized that the field is growing very rapidly and there are a lot of diverse scholars coming in with different backgrounds, most of whom are coming from physics itself and working on very intricate detailed problems that have to do with various small portions of difficult quantum mechanics. I came from philosophy, I don't have any degrees in math or physics and it seemed to me that it was going to be very difficult to make headway in this particular area. [4:59]
So I got home from a conference on the philosophy of quantum mechanics last summer and I was starting to realize that if I wanted to philosophy of quantum mechanics for the rest of my life, I was going to have to either take time off from graduate school and go get myself a couple of masters degrees in math and physics or figure out something else to do. And so I did what any responsible, mature adult would do and I ran home to my dad and hid under the cupboards. So while I was at home visiting my dad last summer, I was wandering around the house one day and I saw this picture, and this picture has been in my family home for as long as I can remember and you're going to recognize the guy on the left there, that's Linus Pauling in 1983 at Ohio State University. The guy on the right is my dad and my dad was fortunate enough to meet Pauling before he passed away and since we got this brand new digital camera and this is going to be recorded for posterity on the internet, can everyone turn and say "Hi Dad" real quick?
So, I was looking at this picture of my dad and thinking "you know, I don't really know that much about Pauling". I knew he did some stuff with chemistry, did some stuff with vitamin C, was crucial in the development of the structure of DNA and I decided that I wasn't doing anything else that day so I go start reading more about what he did earlier in his career. And as I read on, I found out that most his early career was devoted to what I am going to call theoretical, structural chemistry. And Pauling happened to be living at a very exciting time in the development of theoretical, structural chemistry. I am going to tell you a little bit now about why that time was so exciting, but in order to do that I to explain a little bit more about what theoretical, structural chemistry is. What theoretical, structural chemists try to do is they try to explain why matter behaves the way it does, why salt dissolves in water, why fire burns, and the kinds of explanations they look for are kinds that deal with the shapes of molecules, the shapes of structures of individual molecules, of groups of molecules, of individual atoms. So they like to play with shapes and for an example, you can see, this is going to be the kind of explanation that a theoretical, structural chemist will tell you if you ask say "I know carbon makes up diamonds and I know carbon makes up graphite, but diamonds and graphite are two different things, they have different properties, one's really hard, one writes well, one makes girls really happy when you give them to them, and what's the difference?" And the theoretical structural chemist is going to come back to you and say "this is the difference". The difference is the different shapes the atoms make, the different arrangements they have. So this drawing on the left [diamond] I will say is actually from the archive, it's an original sketch from Rodger Hayward who illustrated a lot of Pauling's work and it's a view down the structure of a diamond crystal. On the right you see graphite which is arranged in sheets, which explains why it writes so well, cause the sheets will just rub off on the page. So that is a typical sort of example you see from theoretical structural chemistry.
Now one of the most important things the structural chemists try to explain is "what the chemical bond is, why it acts the way it does, why certain things bond and certain things don't" and most of you will probably be familiar with the traditional ball and stick model of the chemical bond. This is another Hayward illustration where you have atoms stuck together with sticks but we know that's not exactly how the bond actually works. You know that it's not just a matter of gluing balls and sticks together, its not a matter of hooking certain parts together, its something a little more complicated. And toward the beginning of the twentieth century, a lot of structural chemists were really interested in trying to say what it was, what this more complicated explanation was. And by the time Pauling entered the scene, a couple of things had happened. This guy named Gilbert Norton Lewis had, along with a number of others, come up with this idea that atoms form cubic shapes and that the important part in the chemical bonds was pairing electrons of the edges of the cube. You see here these are diagrams from Lewis' 1916 paper where he proposed that electrons would meet up, pair up, get together and that's what we were going to call a bond. So this was an idea that was floating around in the area of structural chemistry for quite some time, by the time Pauling arrived in the early 1920s. [10:40]
But at the same time, the chemists were saying "okay, the electron's important, we are going to represent the atom with these cubes and have electrons at the edges". The physicists were also interested in the electron at the time and you will see things like the development of the Bohr model of the atom, where electrons are floating around a central nucleus in orbits, as opposed to staying static at the corners of a cube. And there's a problem. Those aren't the same descriptions of the atom. So what the chemists realized is that they were going to have to reconcile their view of the atom with the physicists' and they were going to have to give the chemical bond a quantum make-over. So I was reading through all of this and thinking about it and realizing I've got a little bit of quantum mechanics, because I've got the quantum mechanics that's trying to describe the physicists' atoms and I know that and I know the chemistry, my dad, as you may have guessed, is a chemist and I grew up with a lot of chemistry in my life. I love it, I took a lot of chemistry in college, so I decided to move my research interests a little bit more toward history and philosophy of post-quantum revolution chemistry. Now I am not the first to have done this, Mary Joe Nye is one of the more prominent historians in this area and my interests weren't actually historical as much when I started, they were more philosophical. So obviously, eventually I got around to doing some history otherwise I wouldn't be here today but in order to see where the history came from, we're going to have to do a little bit of philosophy along the way, I promise it won't hurt.
So we're looking at bonds and we're looking at the two different descriptions of the electron that are coming to us, one from the physicists and one from the chemists. What happened was the physicists got a more precise mathematical description before the chemists did. They had very intricate mathematical equations to tell us things like, where the electron was likely to be, how much energy it was likely to have, to relate these different properties of electrons that we want to know. And what they gave us looked a little bit like this, this looks complicated, if you were a chemist sitting around in the late 1920s, early 1930s you would think it looked complicated so you are in good company if you are confused right now. And it's even more complicated in an era where we don't have computers to solve things like the differential equations you see there. So in order to start talking about the chemical bond in quantum terms, the way that the chemists wanted to do, they had to start approximating and you can see this is actually some of Pauling's early note-taking on approximations for the chemical bond, its also complicated, but it was a little bit easier to solve without the help of computers and it allowed chemists like Pauling and like his colleagues Hund and Mullikan and Slater, some of whom are physicists, some of whom are chemists, to solve more complicated equations, to give descriptions of more complicated systems because I should back up and say one of the big problems of the physicists' description of the electron was that it was only precise, you could only say exactly what was going to happen, for very, very simple systems. So the system we saw here was just for a hydrogen atom and you can't get much more complicated than that, you just run into too many variables period and what the chemists wanted is they wanted to describe chemical bonds for more complicated systems than just hydrogen because chemistry's looking at more stuff than just hydrogen. So they needed to start approximating. [15:22]
So I got interested in the way this approximation worked as a philosopher. One of the things that philosophers of science like to ask in their spare time is "what's the relation between the descriptions, the equations that are in our textbooks, that we are using in our labs and the real stuff that's out there, what's the relation between the scientific description of the world and reality?" and I think this question is an incredibly worthwhile one, especially today when we are looking at very complicated descriptions of physics, of the genetic structure, and we're trying to figure out what exactly are all these things telling us about the world. So what I started asking myself is "what are these approximations that the chemists are giving, telling us about the world and what aren't they telling us, because they are cutting out some of the math in order to make their equations solvable, so they've got to be leaving out some part of the description, some part of the reality and I wanted to know where we draw that line. So I started researching, I started researching what people had said in the past about the relationship between these approximations and the real chemical bonds that are out there in the world and it led me back to Pauling yet again. I started looking at Pauling's textbooks from his latest edition of his famous textbook, The Nature of The Chemical Bond, its from 1960 and I started looking at what he had to say and what others had to say about the relationship between these chemical bond descriptions that we are getting and the reality that they are trying to describe.
So I was reading Pauling's textbook and I came across this passage which has to do with the double bond and Pauling tends to be very careful in his language, in his use of "the double bond is" or "the bond is" versus "this equation describes some situation" to try and distinguish between the reality that science is, hopefully, trying to give us and the equations that we have to work with to get there. So as I was reading I kept coming back to this passive of Pauling's on the double bond and it interested me for a couple of reasons. It interested me partly because he has a very nice description for some competing approximations for the double bond, he talks about the way his particular set of mathematics describes the double bond and he talks about the way another set of mathematics describes the double bond and the ways that they are different. And obviously, we got two competing approximations, neither of them can be, well they can't both perfectly be describing reality because they are telling us different things. So Pauling is really interested in his textbook, in his 1960 edition, in saying "I've got this particular way of describing the double bond, it's the best way" and he actually gets pretty defensive in the textbook, he goes through a series of five, six, seven different, separate lines of argument saying "this is why my description is the better one" and that's not something you find in textbooks most of the time. You don't see people arguing for a fairly contentious point in the middle of a text where they're trying to go through and explain the basics to people. So it seemed very curious to me that his appeared in the 1960 edition of The Nature of the Chemical Bond textbook and that's when I wrote to Cliff and that's when I came here. [19:56]
I wrote to Cliff and I said "you know I am really curious about why this particular argument shows up in this textbook and I want to figure out where it came from." So I was planning to come to the archive for two or three days on my own money because that's what you do when you are a grad student, you use the money that the department gives you and you crash on people's couches and you hope to find what you can in the forty-eight hours you have at the archive and then you go home. Now thanks to the generosity of Oregon State and the Freemans, thank you, I was able to stay a bit longer. I've been here since the beginning of July and the rest of what I want to talk to you today about is what I found since I came back to Pauling and the double bond. First of all, let's talk about what's at stake, we have these two competing descriptions of the way the double bonds work and I'm not going to get into the math because as you saw its messy and hard and difficult but I will say a little bit about what's going on in the atoms themselves. So we know that bonds are made up of relationships between electrons and when we have a single bond, we have a relationship between two electrons and two different atoms and when we have a double bond we have a relationship between four electrons and two atoms. Four is more than two and the more variables you add in your math, the more difficult it gets. So the descriptions that we have in mathematical terms for double bonds is more complicated than the ones we have for single bonds and they've been that way for as long as we have been trying to give these quantum mechanical mathematical descriptions for double bonds. So instead of talking about the math for the rest of the time that we have, I want to talk a little bit about pictures. So what we get from our quantum mechanical descriptions of chemical bonds is a bunch of pictures so the picture on the left is a sort of pixilated probability distribution that says "if I have a certain set of parameters, I expect to find an electron in one of those places" as you can see one of the way chemists like to represent this sort of mathematics in pictures is to fill out the picture and smooth it up and give it a nice, comfy, happy, bubbly shape because it's less intimidating that way. So we get these pictures and these are the sorts of pictures that we are going to deal with for the rest of the time here.
So in Pauling's 1960 book he has a couple of these pictures as well. He gives us these two and these are the two pictures of the double bond that we have to work with. On the top is Pauling's description of the double bond which he calls the bent equivalent bond, some people have also called it the banana bond because as you can see it looks like two bananas stuck between the molecules. On the bottom is the sigma-pi description of the double bond or the molecular orbital description of the double bond and they're different, that should be obvious, that's the point we are trying to convey here and that's the reason that Pauling has an argument to make at all. So when I started out, I was interested in where this top picture came from, I was interested in why Pauling was so eager to defend it in 1960 and why he would believe it was the right picture in the first place. So it turns, out I think, that the more interesting part of the story is not "where did this picture come from?" but "why is Pauling so adamant to defend it?" and we won't get into that until the end of the talk but I wanted to say a few words briefly about where the top picture came from in the first place, since that is what I set out to do when I came here. [24:49]
So early in Pauling's career he comes up with this idea, this picture of the double bond in terms of two bent equivalent bonds and it seems there are two big reasons, two big things we can cite as causing him to believe that this is the right kind of picture. The first is the fact that Pauling worked with crystal structures a lot when he was a graduate student and even when he was and undergrad and when he was working with crystal structures, he played with shapes as a good structural chemist does. He played with shapes like tetrahedra, like cubes, like octahedra. But you guys are all familiar with these platonic solids where you've got a bunch of different shapes stuck together. So in order to determine crystal structures, one of the things structural chemist will do is that they will play with shapes and see what shapes match well together, what shapes don't, and they'll try to model the structure of crystals, large molecules, by matching up the right kind of shapes together. And so Pauling was already pretty used to matching up shapes in order to get answers as structural chemist. So that combined with his work early on the nature of the chemical bond in the late 1920s and early 1930s and what happened to Pauling in the late 1920s is that he was reading Lewis and seeing these cubical atoms and thinking "there is something that is just not right there" and instead of a cubical atom he came up with the idea of an atom in the shape of a tetrahedron, four triangles stuck together in a pyramid. He came up with this idea as a product of the quantum mechanics, he wasn't the first to propose that atoms could take this shape, in fact Lewis did propose that in one of his earlier papers but he was the first to say, "I can point to the math and say this is why we have a tetrahedron" and he said that in one of his 1928 papers, one of his earliest, published papers but he didn't actually show any of the math and he didn't have much to back it up and he even said "I don't have the calculations yet" and it took him another three years of working with the math to get to the point where he could comfortably publish another paper saying "this is where the tetrahedral shape comes from."
He had this idea that when an atom enters into a bond, if it's got a certain number of available bonding sites, a certain number of other atoms that it can connect to, a certain number of electrons that it has available to it, that its going to take the shape of a tetrahedron and as I said I am not going to go into the math to argue for why he thought this, so you're just going to have to trust me that he's probably right. So he comes up with this idea for a tetrahedral atom and he was working with carbon a lot because carbon was one of the earliest atoms that takes this shape, earliest in the periodic table. And what he said is "we can represent a single bond the way we've been representing crystal structures by putting shapes together and we're going to put together the two corners of the tetrahedron and we're going to call that a single bond and we're going to figure that there is one electron at the edge of each corner of the tetrahedron, similar to the way Lewis did with the cubes, and we're going to smush them together and call that a bond." And he was writing this in 1931 and he mentions sort of as an aside "well we can do that with a single bond but we can also represent the double bond this way." [29:12]
This is the tetrahedron that we are talking about and so what he's saying is that there's an electron at each end here and we need two electrons for a bond so we are going to smush two ends together but for a double bond we need four electrons so instead of taking one corner from each tetrahedron, we are going to take two corners and connect them at an edge. This is actually the first picture we see of the bent equivalent model of the double bond, this is from Pauling's 1931 paper. You'll see that it doesn't look much like bananas but like you see later on, its just a drawing of the tetrahedra pressed together, probably because Pauling was still thinking about things in the way that someone whose working on crystal structure is thinking about them. So Pauling is writing this, its not a big part of his paper, its actually more of an aside, and it seems like he thinks that's probably the end of the story for a while, so he moved on to other things. He did publish the first edition of his textbook in the late 1930s and in it he mentions double bonds very briefly and he says "no general discussion of the orbitals involved in multiple-bond formation analogous to that just described for single bonds has been given and it seems probable that the orbitals involved in double-bond formation by a carbon atom in a molecule such as ethylene are of the bent-bond type." Doesn't say much about it beyond that and doesn't say much about it for another two decades almost. Pauling's a busy man, he doing a lot of other things, I am sure that there are many people here who could tell me in even more detail what else he was doing with his time. But he pretty much left the question, left the defense at that point until the late 1950s.
So at this point we sort of see where the structure of the double bond, where the bent equivalent structure came from, and what we saw in the meantime is Pauling's model, Pauling's mathematical approximation of the double bond in terms of what are called valence bond orbitals, was fairly dominant for a while in the field. It gave more accurate predictions for small molecules. And then something happened, probably starting in the 1930s but really taking root in the late 1940s and early 1950s, the Molecular Orbital model which was a competing approximation that gave different predictions started to take root and it started to take root again because math is hard and it made the math simpler for larger molecules among other things. So in the 1930s when Pauling was coming up with his idea of the double bond, he was in champion's corner, he didn't really feel the need to defend every little part of his theory and then a little bit of a David and Goliath switch happened and the molecular orbital model, the molecular orbital way of approximating the double bond started to get more attention, started to become more prominent and Pauling found his model being attacked on a number of different fronts. The most interesting politically and historically is probably the one that came from the USSR during the Cold War, there was actually a letter that went out which was later published in an English journal banning Russian scientists from using Pauling's particular way of approximating the double bond because it was Bourgeoisie and capitalist and because it was it didn't respect the communist ideals of the Soviet Union. There is actually a great website within the special collections website that can tell you more about that if you are interested. But for now, it's enough to say Pauling was finding himself attacked and there were three big factors that lead him to actively defend his model of the double bond beyond this general attack on valence bond theory and his resonance theory of the chemical bond. So we'll take them chronologically starting with this general attack and then going into. [34:31]
There was a paper published in the early 1950s as was part of the attack on the valence bond theory or as part of an advocacy for the molecular orbital theory is probably more accurate and there were a group of chemists at Cambridge University who published a series of about fifteen different papers advocating the molecular orbital theory of the double bond. Hall and Lennard-Jones were two of them and they put out a paper in 1951 that argued that the molecular orbital model and the valence bond model were simply equivalent for the double bond, that they give the same predictions, the math was all the same no matter how you want to draw the pictures so given that the molecular orbital math was a little bit easier to get through, if it had the same result in the end, we should use that one. And in 1951 Pauling was working on other things and who knows if he didn't see this paper or just wasn't interested in responding to it, he didn't actually come back to this until 1958 and when he did it formed the basis of his defense of the bent bond model of the double bond. So what you see here are some of the calculations that he gives, you can actually see where he says "the Hall-Lennard-Jones theorem states that there is no difference between bent bond and sigma pi, that's our other description of the bond, if the same orbitals are used. I suggest that different orbitals would be involved in concentrating bonds making the two different. So then he goes through a number of calculations, this is from one of his research notebooks, where he demonstrates that the two are in fact different and the demonstration happens because of this phenomenon of concentrating bond orbitals.
And in order to talk about concentrating bond orbitals, I know that's probably useless jargon at this point, but what happens is, we saw the big bubbly pictures of the tetrahedral carbon atom and when we start adding more terms to the mathematical description, higher order terms, the bubbles get skinny. They are going to be longer and skinnier and more directed toward an edge of the tetrahedron. And that's what he means when he is talking about concentrating bond orbitals. So he sees this Hall and Lennard-Jones paper and he decides to respond to it. I argue though in my work that he wouldn't have seen this Hall and Lennard-Jones paper or he wouldn't be interested in responding to it if another circumstance hadn't been in place at the same time or a little bit earlier that year. So Pauling wasn't terribly interested in structural chemistry in the middle of the 1950s and the late 1950s and he wasn't publishing many papers, he wasn't doing much work, he was working more on protein structure and on his peace work. But one of his former students, E. Bright Wilson, who he had a lot of contact with throughout his career was interested in theoretical, structural chemistry, he was publishing papers. And Pauling saw one of Wilson's papers in the end of 1957 and decided to write to Wilson and say "this is a really interesting idea you have, I want to talk about it too" and Wilson brought Pauling's attention back to theoretical, structural chemistry at least for a little while and Wilson's paper dealt a little bit with the concentration of bond orbitals, the idea of getting the balloons longer and skinnier, but it had more to do with this phenomenon of restricted rotation around single bonds and I am not going to go into what that is because we only have a limited time here. Suffice to say that Pauling read this paper and he had another paper in response that he wanted to write. He wanted to talk about the restricted rotation around single bonds in terms of the concentration of bond orbitals. So he started thinking about the concentration of bond orbitals a little more toward the beginning of 1958 and you can actually see here he carried on a long correspondence with Wilson about this subject throughout the late part of 1957 and the early part of 1958. And toward the end of one of his letters he says "I'm planning to publish some more papers on the phenomenon of the concentration of bond orbitals", this probably should have been done long ago. [39:54]
So this is something he has been thinking about for a while and just hasn't been publishing on and then he starts to get interested in it again. Wilson's brought it back to his attention and then he starts using it in his calculations, the calculations that we saw here against Hall and Lennard-Jones... So he takes this idea of concentration of bond orbitals and he uses it to make an argument saying that his model of the double bond, the bent equivalent model, and the molecular orbital model are not the same. And here this is just the research notebook and its from the middle of August in 1958, it could have been that nothing happened. He proved to himself that they weren't the same and was satisfied and moved onto other things, but luckily for me, because I'm trying to write a history paper on this, its not what happened. He actually went through and started taking this particular argument to a number of conferences. Starting in August 29th of 1958 he gave a series of three talks that argued specifically for his version of the bent bond of the double bond. And these are some notes that I found in the archive that indicate that his arguments at one of his conferences. This is for the speech he gave at a conference in honor of Kekule and you can see in his notes that he is saying "I'm going to make this argument that bent bonds are better than sigma pi bonds" and one of the reasons he gives there has to do with concentration of bond orbitals where he specifically mentions Wilson. So this is the historical path that I've been following, looking at how Wilson brought this back to his attention and how he started making arguments in favor of the bent equivalent model of bonds in response to this, in response to Hall and Lennard-Jones' argument that they were equivalent. So he went on and he gave a series of three different talks and I am going to butcher the name of this town in Sweden, I think its Valedin but if anyone is good on their Swedish town pronunciation please let me know, and then this one at the Kekule Conference in Whales, and another one, that was just a talk to the chemistry department at the University of Michigan in early 1959. And then in 1960, the third edition of The Nature of the Chemical Bond textbook comes out. And we saw what he was saying about double bonds in the first two editions, where he said "you know there's a good likelihood that my version is better but no one's really gone through and done any systematic treatment" and what he does in the third edition is he goes through and he gives that systematic treatment. He didn't publish any articles specifically on this before publishing it in the textbook, which is sort of interesting if you're an academic, its not really the normal way going through and making your argument in academia.
He did publish a paper that went along from the Kekule Conference that basically was a transcript of his speech. But the real, first published appearance we see of his argument in favor of the double bond are at that 1960 textbook and it looks like I got just a couple more minutes. I am going to ahead and tell you very briefly how those arguments unfold in the textbook because we've come full circle and gotten to the end of our story about why Pauling favored the bent equivalent model. So he gives in the textbook the idea that these two pictures are different, that's were we first see the three banded picture for the sigma pi bond and the two banded picture for the bent equivalent bond, it was on one of the slides earlier. And he says that "if we have two regions, our electrons are separated better from each other", that's sort of the intuitive reason that he gives for why we might favor the bent bond model and then he goes through and gives the argument about concentration of bond orbitals in response to Hall and Jones. And then he has a number of other empirical justifications for preferring the bent bond model, he says "look, I can explain certain calculations or the certain data that we've seen in our labs better with my bent bond model". So one of the things that I found most interesting from the series of empirical arguments is the idea that if you take a straight carbon single bond and you bend it at the angle that his calculations say you should bend it, that resulting arch is going to be the same length as the single bond and so he accounts for that and says the sigma pi model, the molecular orbital model has no way of accounting for that and as far as I can tell, that's right. I'm getting a little bit of a headshake back there, so I am glad that we're going to have some discussion during the question session. He says there are some other things that we can account for that are the bent bond model, things like the angles between double bonds and restricted rotation. And that's the argument that we see in 1960 and that's the argument that Pauling seems to stick to for the rest of his life. He was still writing grants and publishing articles using this model and also defending this model. During the 1970s and 1980s, you can look through and see what his NSF proposals where he wants to develop this model and actually see reviewers rejecting these proposal because its not the dominant model because people think that there is a better way of describing double bonds. And that's the end of Pauling's journey to defend the bent equivalent model but it's not the end of my journey with this story and it's really just the beginning of my philosophical project on approximations and their relation to reality in structural chemistry. So I think that's about where I stand right now and I just want to close by thanking everyone who has helped to make this possible. Thank you all for listening. [47:37]
Julia Bursten: Any questions? Can I answer any questions?
Cliff Mead: Hi. So, very interesting talk. And its very provocative too. I hope we will get some good questions back here. So anybody have any comments on the talk? Any questions... yes sir.
Audience Member: Double bonds don't exist. It's a fiction of our imaginations, it just helps us view things. You can't measure a double bond, there's no measurable quantity, what we can do is...
Julia Bursten: You can measure inter-nuclear distance.
Audience Member: Exactly, that's the only thing we can measure, and we can measure bond angles, and but bonds don't, that's just a... myth, so it could be nine bonds, who cares. Its bond lengths, angles that are important. I don't do molecular quantum mechanics, I teach some of it but I don't do all the things, so I am semi on the outside. The way you do molecular orbital theory now is you just, what you are going to do is minimize the total energy of the system... and you throw in all these orbitals that you want and you let quantum mechanics pick the right shape and the numbers come out spectacularly well. And there is not a lot of evidence that exists that supports the banana bond concept but Pauling was such a charismatic and...
Julia Bursten: He was very passionate.
Audience Member: Oh, he was passionate and he was smart and beating Pauling at an argument would be impossible for us little people. But this is a path that no one is taking anymore, it's a dead end in science.
Julia Bursten: And that's right and I am sorry if I didn't make that clear during my talk. This is not a model that is used much today, I'm not advocating that we use it today. To clarify the point about bond lengths that I was mentioning earlier, the idea is that what we can measure is not anything about the bond lengths themselves, we can measure the distance between the two nuclei in an atom and there is a certain difference, our double bond has a shorter inter-nuclear distance than our single bond. What Pauling says is "if we think about the difference in the distance being due to bending, something like an actual bond, then we can account for that difference as the arch that occurs from a single bond which is not to say that he thinks its, you know, a stick that sticks between the two nuclei. [50:22]
Audience Member: One other comment. You described sp3 hybridization. I thought Pauling was the author of sp2 and sp1 as well. So why didn't he use sp2 hybridization that we teach our freshmen. We teach our freshmen that, the model for the double bond as sp2 hybridization.
Julia Bursten: Why didn't he use the sp2 hybrid?
Audience Member: He was the father of that...
Julia Bursten: So the reason that I've seen during my research is that he was very in the tetrahedron as the basis, especially for everything related to carbon. We can look at graphite and see a trigonal planar hybridization but for the most part, when it's carbon, it's a tetrahedron, and that's one of the big bases of his 1931 paper especially. He talks about carbon triple bonding, double bonding, single bonding all in this tetrahedral form. Yeah you get the triple bond if you get the tetrahedron and you lay the two faces together. So he wants to build up a system where we have a few known principles and we can use those to build a large number of conclusions and if we start having more and more caveats about where we hybridize one way and where we hybridize another way, then the system isn't as simple as, isn't as elegant. When he was writing in 1931 anyway, they didn't have as terribly precise measurements to back up one versus the other. There weren't as many arguments in favor of the sigma pi model. I will say there were a couple of people who were working on that model during the...1929 might have been the first publication. So a guy named Erich Hückel, in particular, in Germany was arguing in favor of it. He wrote a bad paper, that doesn't mean that the model's bad but he wrote a bad paper and that was one of the reasons that model of the double bond didn't gain as much traction early on and it wasn't until later, after we had Pauling's idea of hybridization that we started representing the sigma pi bond in terms of that hybridization, where you get the two pi bonds that stick up like balloons on either side of the sigma bond. So that visualization of the sigma pi model is probably in large part due to Pauling, you know I am not going to stick my feet all the way out on a limb there but in the early formulations of the molecular orbital model of the double bond, it was all mathematics, no visualization whatsoever and some of the math wasn't good.
Cliff Mead: Peter, you had kind of another question?
Peter Freeman: Well I could have one, I suppose. The orbital hybridization picture doesn't do very well for methane, now that's kind of a disappointment. That's sort of a starting point, you can't explain the fact the top two sets, the three degenerate orbitals, the one right below that, there's another orbital slightly more stable and you don't get that picture if you use orbital hybridization. I think that's right. I think some of the importance and present day usage is just to help people kind of understand things and it seems more reasonable if you're thinking in terms of orbital hybridization to understand some things, especially the cyclopropane ring where the sort of banana bonds that are outside the actual triangle seem to be easier to understand and to visualize.
Mary Jo Nye: I just had a question about Pauling's wording and the first edition of The Nature of the Chemical Bond about because the slide you have there has the bent bond reference in square brackets and I wondered if you did that for emphasis or if he used other words in the book.
Julia Bursten: No he didn't use other words, he said in the following way and then he went on to describe "what is the bent bond model". I apologize, I might not have... I'd be happy to pull up the book for you.
Mary Jo Nye: No, that's not, I just wondered, so I mean he didn't use the words that were in the brackets.
Julia Bursten: The words that were in the brackets, right. He did not use the bent bond terminology there, he did use it in the 1931 paper, he didn't call it the bent bond model but he talked about bonds bending and in the first and second edition of The Nature of the Chemical Bond, he talks about the bonds bending but in the quote that I put up, he just says "if we think about the double bond in the following way" and then he goes on to describe and he talks about the bending in his description and I didn't want to reproduce the four paragraphs on one Powerpoint slide. So thank you for clarifying that.
Steve Lawson: It's kind of an annoying question I suppose, but I know that Pauling liked to visualize by build all kinds of models, his famous CPK, these ball and stick models and so forth. In doing this research did ever you come upon any statement about what he favored in terms of models, which model he felt best depicted the way he visualized the bonding in atoms?
Julia Bursten: That is a very interesting question, I didn't come across anything where he overtly says one way or the other, but if you look especially at this early work, the 1931 paper, it's all these sorts of drawings of tetrahedra that I showed you. [I'm going to give up trying to find those slides right now, but you see, he'll describe the tetrahedra that he draws in terms of favored bond directions]. He doesn't ever call them bonds themselves, he just says, this is a depiction of the directions of a bond, the way that a bond would occur without ever calling it a bond. So that's another part of the careful language he has.
Steve Lawson: I got the impression that because he used so many different materials and weighed them so many different weighs that maybe he didn't achieve this optimum visualization. I was just curious as to whether he really had ever picked one as a favorite depiction.
Julia Bursten: No, I mean I can't answer that question definitively but I'd say you're probably right, he never picked a definitive one and I did scour the racks of models that the archive has for us in the next room over and didn't ever find one that was a good depiction of just "this is the structure of the double bond". The closest we can come is the two pictures in the 1960 edition of The Nature of the Chemical Bond where we have a very basic probability distribution of the two different approximations.
Cliff Mead: Any other last questions or comments? Yes?
Audience Member: You said that Hall and Lennard-Jones, I believe, came from Cambridge. Wasn't that also where Watson and Crick were?
Julia Bursten: I believe so, I am not a biologist.
Audience Member: And Pauling was proved wrong on the structure of DNA by Watson and Crick. I'm just wondering does Cambridge University become a particular thorn in Pauling's side?
Julia Bursten: It's certainly possible, that's a good point I hadn't made that connection.
Cliff Mead: Mary Jo, do you want to chime in on this?
Mary Jo Nye: Well he sent his own son there.
Cliff Mead: And look how that turned out. Yes, Chris?
Chris Petersen: What do you think explains Pauling's continued support or defense of this model in his later years when he's having his grants denied, is it just pure stubbornness or does he have some other rationale behind it?
Julia Bursten: I think he really believes it. It's not just a matter of going down with the ship. He makes some comments, not in the textbook, but in correspondence during the publication of that third edition, where he says "I think the molecular orbital theory is going to pass like a ship in the night, it will be gone in five years, no one will ever have heard of it" and I think he was still waiting for the ship to pass by the time he stopped doing research. I got a chance to look a little bit at some of his notes before the proposed fourth edition of The Nature of the Chemical Bond textbook and found some stuff in the late 60s and early 70s where he is making even more elaborate arguments for the bent bond model of the double bond where he wants to talk about stress and strain energies and treat the bond almost as a piece of rebar. So its something that he stuck with because he thought it was the right thing to stick with.
Cliff Mead: Even ten years later, he's getting even more defensive. Any other comments? Questions? Thank you all for coming, we appreciate this and we hope you will be here for our next Pauling Resident Scholar speech. Thank you very much. [1:00:59]
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