The Development of the Concept of the Chemical Bond

[00:00:00] INTRODUCER:
It’s a pleasure to welcome you to this Hitchcock Lecture. Any chemist would consider it an enormous privilege to introduce Linus Pauling, but I’m in great difficulty at the moment. On the one hand, I want to do justice to Dr. Pauling’s many contributions to science and to humanity, And on the other hand, I outlined some of the things I was going to say to my wife, who said, “Keep it brief.”

And I figure there’s no way one can outline all the great accomplishments of Dr. Pauling and keep it brief. So I’m torn between justice and preserving my marriage.

(laughter)

And I have fastened on a compromise which I’m afraid will do neither. In the first place, simply reading the honors of Dr. Pauling would take a good part of this hour. He has received practically every medal of the American Chemical Society, foreign societies that I could think of.

So many honorary degrees and lectureships that, uh, I could not go on and list them all. And I will simply handle this entire category by saying that he is now a s-level above people who receive medals. He has medals named after him, such as the Linus Pauling Award of the American Chemical Society.

Even if I l- do not give the list of medals and honors, I could try to list his accomplishments in science, and that would take too long. So I have told a few things because some of you, I think all of you know Dr. Pauling, there are a few other people in the world who are a legend in his own time. But I– you might, know, like to know a little of his history.

He got his bachelor’s degree at the University of or-Oregon State University, then went to Caltech, where he majored in chemistry, but saw the advantage of applying physical methods to chemistry and went, after getting his degree, to Europe, where he studied physics in the laboratories of Sommerfeld, Bohr and Schrödinger. And then returning to this country to apply chemistry, where he was a joint– had joint lectureships between Caltech and Berkeley, uh, he started his classic work on X-ray crystallography using two types of physical approaches to chemistry, one X-ray crystallography at the experimental level, where he solved the structures of mica, topaz, silicas, the silicates, and many sulfide compounds just to start. These are some of the most complex structures one can imagine.

And then used theoretical quantum mechanics, introduced the theory of hybrid orbitals, electronegativity, valence bond theory, and the theory of resonance into our organic chemistry. And at that time he wrote this book, which I am holding lovingly because I know many chemists in this audience of my generation. It was the Bible, The Nature of the Chemical Bond.

I think it is still, Dr. Pauling tells me, it’s still in print, going through hundreds of thousands of copies. It was written as a research book on the frontier of science at that time, but so beautifully written that it was a textbook at that time and still remains a textbook today. He became interested in biological molecules and applied X-ray crystallography to these and also theory.

And among his findings, of course, was the alpha helix, the first sort of true order in the structure of proteins, for which he was– received his first Nobel Prize. In my opinion, it was simply an excuse of the Nobel Committee to pick the alpha helix. They could have picked any of a large number of other discoveries of Dr. Pauling.

I think as a result of these studies and his interest in hemoglobin at Caltech. He also realized the importance of a minor perturbation in protein structure changing the function of the protein and introduced the term molecular disease, which was a perfect description of sickle cell anemia and did classic work in this study for which I told you I wouldn’t list all of his honors. He received honors from practically every medical society in the world and the Martin Luther King Achievement Award for that particular contribution.

Uh, he studied s– uh, nucleic acids, and I think those of you who’ve read The Double Helix realize that he was very close to solving that structure, and perhaps if he had had Rosalind Franklin’s X-ray data, Watson and Crick might not be as famous as they are.

(laughter)

Having the prestige of a Nobel Prize helped him in one major category. Dr. Pauling’s liberalism and courage, courage in fighting causes had caused him to have his passport banned by the American State Department. But he carried on undiminished in these endeavors.

In nineteen fifty-eight, he presented a petition of eleven thousand scientists to the Secretary of the UN, asking for a ban on nuclear tests in the atmosphere, and received the Nobel Peace Prize in nineteen sixty-two on October 10th, the day that the Americans and Russians signed the first partial nuclear test ban treaty. Knowing Dr. Pauling, I think that signature probably meant more to him than receiving his second Nobel Prize. He left Caltech in the middle sixties to join the Center for Democratic Institutions at Sa-Santa Barbara, spent a brief time at UC San Diego, and is now at Stanford University, where he is head of the Pauling Institute.

In recent years, he has devoted himself to theories of vitamin C in relation to cure of colds and po-potentially a cure of cancer, or at least a prevention of cancer. And these studies have been very controversial, and I would, many other people, I’d hesitate to mention that. With Linus Pauling, I think controversial is a compliment.

He is always a leader in his field. He is well aware that his, uh, by being way ahead of his time, he is frequently controversial. I don’t yet know the final resolution of that, but I talked to a friend recently who said, “Linus Pauling’s hunches are better than most scientists’ data.”

(laughter)

I can only end by adding my own experience in this subject because I was flying back from Washington to Berkeley in the late 1960s and happened to talk to the person next to me and asked him what he did, and he told me he was an FBI agent. He then asked me what I did, and I said I was at Berkeley. And I thought-

(laughter)

I thought- I thought I saw him wince.

(laughter)

And then he asked me what my profession was, and I said I was a chemist. And he said, “Oh.” He said, “Do you know Linus Pauling?”

(laughter)

And I didn’t know quite what to do, and he said–

(laughter)

I said, “Yes.” And he said, “Well, what do you think of him?” So I said, “Well, I think he’s probably the greatest science, greatest chemist of our time.” And I sort of put up my hands for the hand clap.

(laughter)

And he said, “Professor, you’re absolutely right.” He said, \”You know, my wife and I were troubled by colds all of our life. We’ve taken vitamin C.

He’s cured us. He’s certainly the greatest scientist of our time.\” Uh, so any man who can conquer both the left-wing causes and the FBI, I think has no fields left to conquer. Dr. Pauling is no stranger to Berkeley, and therefore I can’t say welcome to the campus for the first time,

[00:08:01] DR. KOSHLAND:
but I can say because of his many contributions to science and to humanity, it is our honor to welcome you here for the first of your two Hitchcock lectures.

(applause and cheering)

(applause and cheering continues)

[00:08:30] DR. LINUS PAULING:
Well, thank you, Dr. Koshland, and I thank, uh, the university, too, for giving me the opportunity to come back to this great university. Chemistry is wonderful. Uh, I feel sorry for people who don’t know anything about chemistry.

They are missing an important part of life, an important source of happiness- So, satisfying one’s intellectual curiosity. The whole world is wonderful, and chemistry is an important part of it.

You know, substances change into other substances. That’s what attracted me to chemistry when I was thirteen years old and saw the first chemical reactions that I realized were chemical reactions. You have iron that rusts to iron oxide or hydroxide.

And Lavoisier, back, uh, nearly just about two hundred years ago, showed that you can burn diamonds to carbon dioxide. Uh, Cavendish showed that, uh, hydrogen and oxygen combined together to produce water about the same time, two hundred years ago. Uh, then there’s phosphorus.

Uh, it’s hard to imagine the excitement caused by the discovery of phosphorus, perhaps comparable to that caused by Sputnik.

(laughter)

The alchemist, Dr. Hennig Brand, who apparently was a remarkable man, was the first man to discover a new element. He discovered phosphorus. This remarkable white waxy substance would glow in the dark.

Uh, it was exhibited to dukes and other important persons and was called cold fire, and it aroused very great interest. And how did Brand make phosphorus? I’m sure if you haven’t read, heard about how he did it, you wouldn’t be able to guess.

He made it out of urine.

(chuckles)

And no one knows why he carried out, uh, his procedure. Uh, it involved, uh, using a still. Distilling is, is at high, moderately high temperature.

And, uh, the alchemists were very much interested in distillation. They had d-designed a hundred different kinds of stills, and they were always distilling things. Uh-

There was a Benedictine monk, I’ve forgotten his name, who, uh, was the first to make ethanol, ethyl alcohol. He distilled wine and got ethanol. And, uh, he dissolved, uh, he, uh, leached out, uh, herbs with this ethyl alcohol and made a, a product that was a popular medicine for centuries.

It was called Benedictine. It still is reasonably, uh, popular. That was the first, uh, liquor, uh, to be made.

Well, Dr. Brand, uh, took a large volume, uh, hundred gallons of, of a lot of liters, I’ve forgotten just how many there are to a gallon, of urine, evaporated it down to dryness, put it in a still, and heated it for a while, and pretty soon he had a white waxy, uh, liquid that could be poured out into molds as, uh, sticks of white phosphorus. I’m still astonished that you can get that, uh, strange element by this, uh, procedure. And, uh, I said nobody knows why Dr. Brand was doing this, but, uh, we can, uh, make a reasonable surmise about it.

He was interested, as other alchemists were, in trying to change lead into gold. Well, he may have had a rational argument. Doctors check the urine, they did even then, to learn something about a patient.

A sick person has a bluish-gray color complexion, something like lead. A healthy person is rosy-cheeked. As a person gets well, his color changes.

So they thought, uh, lead might just be sick gold, and silver too. And if we could cure what is wrong with the lead and silver, uh, we might get gold. Couldn’t the color be restored, uh, to the metal?

Then you would have gold. Well, Dr. Brown didn’t know that he was breaking the phosphorus-oxygen bonds in phosphate, permitting phosphorus-phosphorus bonds to form. We have a pretty good understanding of these bonds now, but it took nearly 300 years to get it.

Perhaps there are some physicists here this afternoon. I like physicists.

(laughter and applause)

Uh, they’re pretty smart men Men and women.

(laughter)

But they don’t enjoy life, uh, so much as they should. Uh, because they don’t know very much about chemistry. Dirac said long ago, the physicist Dirac said that “The Schrödinger wave equation covers a large part of physics and the whole of chemistry.”

(gasp)

That’s the trouble with physicists.

(laughter)

They don’t know much about the world, they just know the equations. We chemists are fortunate that chemistry, structural chemistry was well-developed, pretty well-developed before the Schrödinger wave equation was discovered. If that had not occurred, chemistry would be in the same state as nuclear physics today.

(laughter)

The equations could be solved, giving numbers that agree with experimental observations, but there wouldn’t be much understanding. Well, modern chemistry, or almost modern chemistry, the first stage of modern chemistry, began with Lavoisier about 1789. Uh, could I have the first slide, please?

See, I’ll, I’ll read it. It says Antoine Laurent Lavoisier. He was in better focus with the environment than…

Uh, Antoine Laurent Lavoisier. Let’s see if I can focus that. There.

Uh, 1743 to 1794. 1794 was when his head was cut off-

(laughter)

In the French Revolution.

(laughter)

Guillotined, he was.

(laughter)

In 1789, he published his Elementary Treatise on Chemistry. There was, he made a distinction between elements and compounds. The ancients had talked about elements, fire, water, air, something else, I forgot.

[00:16:55] AUDIENCE MEMBER:
Earth,

[00:16:56] DR. LINUS PAULING:
yes.

(laughter)

Lavoisier said there are 30 elements, and seven of them had not yet been isolated. Uh, the, uh, alkali metals, for example, he knew that they existed. The oxides and other compounds were known.

He analyzed many compounds quantitatively. Then there came Dalton, John Dalton, 1766-1844. He had the idea, the essentially modern idea, that substances consist of atoms, that there are atoms of iron, cobalt, nickel, sulfur, phosphorus, and s-so on, oxygen, nitrogen, and, uh, carbon.

And, uh, he checked this idea by making quantitative studies and got some evidence in the law of multiple proportions, that in carbon dioxide there is a gas, there’s twice as much oxygen combined with a certain amount of carbon as in carbon monoxide, and this you could explain by saying that there were two atoms of oxygen, to, uh, in carbon dioxide, and only one in the carbon monoxide. And, uh, he wrote the first table of atomic masses, atomic weights, they’re called. Then came Berzelius, and it takes 15 years or 20 years, and for each of these steps.

Jöns Jacob Berzelius, seventeen seventy-nine to eighteen forty-eight. In eighteen fourteen, he published his theory of chemical proportions and the chemical action of electricity. The chemical effects of electricity had been known for a couple of decades.

Alkali compounds had been electrolyzed. The, uh, several of the alkali metals, sodium, potassium, and calcium too, another metal made by electrolysis. Berzelius had the idea that, uh, some– since, uh, these alkali metals were formed at the, you know, positive pole, that some, uh, elements have a positive electric charge, maybe the negative pole.

I’ve forgotten how these are named, these poles. Uh, some atoms have a positive charge of electricity and some have a negative charge of electricity, and since positive and negative charges attract one another, these elements, these groups or atoms would be pulled together and you would have compounds. He made quantitative chemical analyses of two thousand substances and introduced the modern symbols for the elements using the letters, initial letters, and the– this dominated chemistry, chemical thinking for a good number of years.

Actually, 36 years. Edward Frankland came along then in England in 1850 when he was 25 years old. He was born 1825, died in 1890.

He, uh, carried out some experiments and interpreted them with a, a new idea, the idea of valence, that, uh, uh, tin might form two valence bonds or four valence bonds. He– the theory– the Encyclopedia Britannica for 1911 said, “The theory of valence, thus founded, has dominated the subsequent development of chemical doctrine.” That was up to 1911.

Well, Kekulé contributed. He was born in 1829 and died in 1896. In 1858, he had the idea that the carbon atom has four bonds, is quadrivalent.

Another, a Scot named Couper, had the same idea at about the same time, but did not go uh continue to make important contributions to chemistry. Kekulé had the idea also of chains or rings of carbon atoms, and ultimately the Kekulé structure for benzene with alternating single and double bonds. Then we have the next important contribution again, let’s see, eighteen fifty-eight to seventy-four, sixteen years.

Jacobus Hendricus van ‘t Hoff, when he was twenty-two years old, published a paper in which he said that it was called Chemistry and Space, I think, in Dutch language. a paper in which he said that the carbon atom sticks its four single bonds out to essentially toward the corners of a regular tetrahedron. Le Bel, a Frenchman a few years older, had the same idea about the tetrahedral character of, uh, single-bonded carbon.

Well, it’s shown in this slide of the methane molecule. We know that, uh, methane has this structure. I had a friend who took his PhD at the University of Munich in 1926 or 7, 27, I think.

Uh, and his thesis was a spectroscopic study that led him to con- conclude that methane is a flat square with the carbon in the middle of the square.

(laughter)

Well, it was a misinterpretation. It was a valiant effort to interpret, to do something in molecular spectroscopy, but wasn’t right. It’s tetrahedral.

The chemists knew it was tetrahedral. The- ethylene molecule, uh, with its double bond can be described as two tetrahedra with a common edge, and this immediately explains the existence of these stereoisomers, cis and trans dichloroethylene, for example, where, uh, the one-two cis-trans, the one-two cis or one-two trans, where the two chlorine atoms on different carbon atoms are either on the same side of this, uh, rectangle or on opposite corners. Then what, uh, van ‘t Hoff and Le Bel were explaining, the optical activity of substances.

Here we have, uh, an amino acid, a D-amino acid and an L-amino acid. The carbon atom forms bonds to four different groups, and you can do this in two different ways when you put the groups at the corners of a tetrahedron. Proteins in human beings and other living organisms are made up of L amino acids.

Uh, we don’t have a form of life on Earth and with, uh, uh, proteins made up of D amino acids. And I think it was just the toss of the die that, uh, led to this decision. Perhaps life got started with both L and D, but pretty soon one happened to win out over the other.

Well, there. I put this slide in of hexamethylenetetramine to show, uh, tetrahedral character, the tetrahedral character of atoms. The six carbon atoms are tetrahedrally bonded to the neighbors, and the four nitrogen atoms have bonds at three tetrahedron corners to the carbon atoms.

This is the first structure of an organic compound to have been determined precisely, the bond lengths and bond angles. It was done by Roscoe Gilkey Dickinson, X-ray crystallographer, with whom I began my graduate work in Pasadena in 1922, who taught me the science of X-ray, uh, diffraction, uh, of bicrystals for the determination of their structures as it was practiced then. It was a very powerful, valuable technique, this X-ray diffraction.

And of course, after a while, other organic compounds as well as inorganic compounds began to be determined. Well, here I have Alfred Werner, about 1900, who, uh, he was born 1866, died 1919. Who was thinking about, uh, the various, uh, complexes, inorganic complexes that metals form.

He determined that some have a tetrahedral arrangement, such as the zinc tetracyanide ion. Some have a square planar arrangement. For example, the bivalent palladium tetrachloride or, or platinum tetrachloride ion, and some are octahedral, such as, uh, palladium, quadrivalent palladium, quadrup positive palladium, hexachloride ion.

This was a very important contribution to inorganic chemistry. Now we get into the electronic period, the, the start of the, uh, sort of sesqui stage, one and a half stage of development of modern chemistry. J.J. Thomson at Cambridge, 1856 to 1940, in 1897, discovered the electron and was able to announce that it was about, uh, a thousandth to two thousandth as heavy, as massive as the hydrogen atom.

He had some ideas about chemical bonds and shared electrons, too. Well now, I’ve come to the point that I’ve been looking forward to, uh, Berkeley, the contributions from Berkeley in this field. Uh, they were great ones, too.

Uh, much of modern chemistry, structural chemistry, originated here in Berkeley. In 1913, this got started, a paper by William C. Bray and Gerald Branch, who were, had been brought to the chemistry department here by G. N. Lewis when he had come, ah, just two or three years before.

(coughing)

They proposed that, that valence be split into t-two, separate, uh, quantities. Two separate ideas. Polar valence, uh, what I call oxidation number, I’m not sure.

Polar number, oxidation number, and the, the total valence. This, we would say now, was distinguishing between electrostatic valence or largely ionic valence and covalence. They have an illustration in their paper of ammonium chloride, in which they had the hydrogen atoms positive, the chlorine atom negative, and nitrogen as minus three.

Total polar number n equal minus three, and they said the total valence of nitrogen is five. Then G.N. Lewis, the same year, in fact, uh, the same issue of the journal, the paper that just followed the Bray and Branch paper, said, uh, that, uh, there were three kinds of compounds or bonds: uh, polar, non-polar, and metallic compounds. He said, uh,

(clears throat)

he, he didn’t think you should try to introduce the partial— he talked about partial polarity of bonds and suggested that sometimes there was no need to put in arrows like this, as Bray and Branch were doing all the time. It was better sometimes just to draw a line. These lines went back actually to Frankland’s time in the 1850s as, uh, representing the valence bonds.

Well, next, of course, there’s G.N. Lewis’s famous 1916, uh, paper. In it, he said electrons, uh, in most atoms can be put at the corners of a cube, that sometimes these cubes share edges with one another, not corners, but edges, a pair of electrons. Shortly after that, he got around to saying that the chemical bond consists of two electrons held jointly by two atoms.

He emphasized partial ionic character, uh, of bonds, a partial polar character. Here’s a picture of G.N. Lewis sitting on the running board of a car.

(laughter)

Uh, Arthur A. Noyes, Arthur Amos Noyes, head of the– with whom G.N. Lewis had, uh, worked in MIT. Uh, Noyes had his laboratory of physical chemistry, and G.N. Lewis was the assistant director for a while, came from there to Berkeley. So Noyes is, is there the two people in the back of the car are, uh, Dr. Jimmy Bell, who taught freshman chemistry at CIT, and Mrs. Bell.

I don’t know who the other person is. I’ve ridden in that car a number of times. A.A. Noyes took me and a couple of other graduate students to the desert, Palm Springs, a little hamlet out in the desert there.

And, uh, also down to Corona del Mar, uh, to the, the beach where later he bought, uh, or the Institute bought a building that became a marine laboratory of California Institute of Technology. Well, uh, to continue with Lewis, there was an argument to the effect that Lewis’s ideas about atoms were incompatible with the Bohr theory, where the electrons went around in orbits. Lewis said, “Well, the Bohr theory just isn’t right, because we know that the electrons are static or they’re in the at molecules there, forming chemical bonds between… that hold the nuclei together.”

Well, in a sense, he was right, uh… I’ll come back to that because here’s Irving Langmuir come along in nineteen nineteen. And, uh, this is important to me.

Uh, what this slide says, Langmuir in eighteen eighty-one to 1957. In 1919, he had two or three papers in the Journal of the American Chemical Society and a couple of notes in Science in 1920, and he gave many lectures. These were long papers with a much more detailed, uh, discussion of individual substances than in G.N.’s 1916 paper.

Lewis- Uh, Langmuir, uh, introduced something into chemistry, the word covalent. This was the first time that, uh, anyone had spoken of covalence and covalent bonds.

He introduced the idea of formal charge, saying that you can split the electron pair, that is the chemical bond, between the two atoms and then count up how many electrons there are on each atom and what its resultant charge would be. An important idea. He was really very clever.

He said, nitrous oxide is not a ring the way some people had written it. It isn’t N-O-N with oxygen in the middle. It’s N-N-O with an oxygen on one end, a nitrogen atom in the middle, and a nitrogen atom on the other end.

Formal charge minus one for the N nitrogen, plus one for the middle nitrogen, zero for oxygen. He introduced the principle of electroneutrality, that the charges on atoms need to be zero or plus one or minus one, that is almost zero. That, uh, it-

You can take an electron off of an atom. The first ionization energy, uh, for some atoms isn’t awfully, some s- metals isn’t very great. You can take an electron off.

It takes much more energy to pull a second electron off, so it’s unlikely that that would, uh, be the situation in a stable molecule or a crystal. This was a very valuable principle, overlooked. He had the idea that nickel tetracarbonyl is octavalent, that the nickel atom forms, uh, four double bonds with oxygen, and this was required rather than G.N. Lewis’s structure with single bonds around the nickel atom, uh, by his electroneutrality idea.

Now, it was this 1919 paper that got me started on structural chemistry, the nature of the chemical bond. I had been, uh, up to 1918. Let’s see.

Sixteen, seventeen, eighteen, seventeen, eighteen, nineteen, nineteen. I had had two years of chemistry and chemical engineering at Oregon State University, Oregon Agricultural College. I didn’t have money enough to come back the next year, and after a month when I was working as, uh, putting down blacktop pavement, paving plant inspector in Southern Oregon, I got a telegram offering me a job to teach sophomore quantitative analysis at Oregon State.

So I taught that 1919 to ’20. And, uh, I had a desk in the chemistry library, a small room, 15 feet square, with s- books and journals in it. No one ever came into it.

I read the journals, the Journal of the American Chemical Society, and when I read Irving Langmuir’s papers, I was pretty excited. And I went back and read his 1916 paper. So that, uh, and Langmuir was, I think, made important contributions.

Here we have Max Born, 1822 to 1970. In 1918 to 1920, he developed the theory of ionic crystals. He, he evolved the idea of the Born cycle, where an ionic crystal such as sodium chloride is vaporized to sodium ions and chlorine ions, then the electron is taken off the chlorine and put on the sodium to make atoms, and the sodium, uh, combines with other sodiums to make metallic sodium, the chlorine with chlorine to make Cl2 molecules.

And, uh, you there knew experimentally a good number of energy quantities here so that you could get a cycle. Uh, the one thing that wasn’t known experimentally, except from this cycle, is the energy of the crystal, the forces, the energy with which the positive and negative ions attract one another. Born stimulated a young, uh, physicist named Madelung, uh, to, uh, learn, find out how to calculate values of the Madelung constant by summing over the pairs of electric charges in the crystals.

And he evaluated the electron affinity of cl- fluorine, chlorine, bromine, iodine, uh, by making use of the Born cycle. Pretty important, uh, contributions. Ionization energies had been determined experimentally.

The Franck and Hertz experiment on electron bombardments, uh, bombardment, for example. Well, here I get back to Berkeley. There was an undergraduate student here, uh, in 1920 who became a graduate student, got his PhD degree, came down to Pasadena as a postdoctoral fellow, was an assistant professor at Stanford for a while and wandered around until he became retired, came back to Palo Alto, and, uh, died 13 months ago.

Maurice Loyal Huggins, very clever fellow. In 1920, he had an unpublished idea about bicovalent hydrogen. He says that this was the hydrogen bond, but, uh, uh, it isn’t…

Nobody knows just what this idea was. His undergraduate thesis, so far as can be discovered, no longer exists. The library here doesn’t have it, and nobody has it.

And, uh, Maury Huggins didn’t have it. Well,

(laughter)

but, uh, G.N. Lewis mentioned it and, uh, Latimer and Rodebush mentioned it, so he had some idea about bicovalent hydrogen. He worked on the electronic structure of crystals such as silicon and related aluminum nitride and so on, and set up an early set of covalent radii. Later on, he made, he had a number of other interesting ideas in chemistry.

So here we come to two men who became professors, were professors of chemistry under G.N. Lewis. Here, Wendell Latimer and Worth Rodebush. Well, there’s the Latimer Laboratory.

Great big building full of chemists up here. Uh, they published an important paper in the Journal of the American Chemical Society in 1920. It was essentially the discovery of the hydrogen bond.

In my book, 1939, On the Nature of the Chemical Bond, I think I ended the book, the last sentences or sentence or two, the last paragraph in this book, by saying that the hydrogen bond is no doubt a very important structural feature so far as living organisms are concerned in biology. Well, it turned out to be very important, all right. So, uh, Latimer and Rodebush discovered it.

They made the first clear statement about the hydrogen bond, that water molecules, water has its peculiar properties, uh, the boiling point, melting point, and boiling point much higher than for the heavier analog, uh, hydrogen sulfide, hydrogen selenide, hydrogen telluride. It explained a lot of things, why ice floats, for example. They also mentioned that, uh, acetic acid forms dimers when it’s dissolved in chloroform, or in the gas phase, there are dimers present and…

Well, this slide shows up at the top, uh, the HF2 minus ion. You can make compounds such as KHF2, and, uh, for a… When I was a boy, hydrogen… hydrofluoric acid was written as H2F2.

Probably the existence of this, these compounds such as KHF2 gave chemists the idea that it wasn’t just HF as hydrochloric acid is HCl, but rather the dimer. Well, Joel Hildebrand, he’s about the only one of these old fellows who my, uh, essentially of my period who is still around. Joel Hildebrand carried out some very nice experiments on vapor density of hydrogen fluoride, and was able to show that there are polymers H5F5 and H6F6, which…

H5F5, H6F6, these ring structures, they have one more hydrogen bond than a straight chain structure would have, and, uh, they are important. Hildebrand also, no doubt, while he was teaching freshman chemistry, the forty thousand students that he thinks he had in his classes, was introducing these ideas as they came along. The idea of a polar number or oxidation number is a very important one for teaching freshman chemistry, I must say.

What did I do? I passed over the picture of acetic acid dimer. Here we have ice.

Well, there it is. There. Uh, these pictures were drawn by my associate, no longer with us, uh, Roger Hayward, an architect who had a surprisingly good knowledge of physics and chemistry and the artistic sense.

Uh, Here we have ice, a rather open structure in which each water molecule is surrounded by only four neighbors to which it’s connected with hydrogen bonds. Giauque showed in 1935 that ice has a residual entropy at very low temperatures. Uh, I was able to work out…

This shows my physics background. I was able to work out a, an equation, a formula, uh, for the residual entropy, which came out just right, point eight oh calories per mole degree. Uh, What do I have now?

Well, diaspore. One of my students, uh, Fred Ewing, determined the structure of this AlOHO. Uh, every oxygen is connected by a hydrogen bond, uh, to another oxygen.

You have OHO pairs. The hydrogen bonds are indicated by those struts in this drawing. Uh, the aluminum is in the center of the octahedra, each octahedron indicated here, and aluminum atom, Um, oxygens at the corners.

And another structure that Fred Ewing worked out about 1935 is, uh, is, uh, lepidocrocite, the somewhat analogous compound of iron. The iron is at the center of the octahedra, and here, uh, half of the oxygen atoms are just oxygen atoms bonded with several bonds to iron atoms, and the other half, uh, formed two hydrogen bonds. So it is FeO and then OH, with the w- that second oxygen being the one involved in hydrogen bond formation.

Well now, uh, forgetting about this last pair of slides, the, uh, diaspore and lepidocrocite, so we come to, uh, c-another contribution by G.N. Lewis, his great book on valence. Nineteen twenty-three, Valence and the Structure of Atoms and Molecules by Gilbert Newton Lewis, Professor of Chemistry in the University of California. It was– it went considerably beyond his 1916 paper, in some ways not catching up with Langmuir in 1919, however.

He said, for the first time quite precisely, “The chemical bond is at all times and in all molecules merely a pair of electrons held jointly by two atoms.” He said.

(laughter)

(background chatter)

I may be pressing the wrong button. Let’s see. If we regard the orbit as a whole and not the position of the electron within the orbit, then we may think of each electron orbit as having a fixed position in space.

The average position of the electron in the orbit may be called the position of the electron. So he was saying, “I was right after all. Uh, the atom, the electrons are there in between the nuclei.”

Well, referring to chlorine, he said that, “We may be sure, however, that each of the outer shells should be represented by a pair of electrons at each corner of a tetrahedron,” the tetrahedral carbon atom. Well, uh, there we are. There is a man here named Parson, a student, I think, graduate student, who wrote a paper in 1915 in which he said that the electron has a magnetic moment.

It’s a little magnet, and that two of them tend to pair up with one another. Uh, he published a paper, I think two papers in Proceedings of the Farada– the, uh, what was it?

Uh- An institution in Philadelphia.

[00:48:19] AUDIENCE MEMBER:
Franklin Institute. Hmm? Franklin Institute.

[00:48:21] DR. LINUS PAULING:
Franklin, the Franklin Institute. Uh, pretty clever too. He says, “It was at my suggestion that he, Parson, 1915, attributed a fixed magnetic moment to his magneton, the electron.

It seems unlikely, but perhaps not impossible, that an electron possesses any magnetic properties except when it is part of an atom or molecule.” Well, in this case, G.N., uh, missed the boat. He rejected a good idea, uh, about the electron having a magnetic moment of its own, which it took another, uh, several years before Goudsmit and Uhlenbeck discovered the electron spin.

In nineteen twenty-three, in his book, and G.N. didn’t mention the electroneutrality rule, which was pretty important. Uh, for example, Lewis described, uh, osmium tetroxide, uh, by putting these single bonds in, which puts a formal charge of four minus on the osmium atom. Langmuir, with the electroneutrality rule, said that there are four double bonds and that the atom is neutral.

It has eight electrons outside of the noble gas shell. Well, G.N. complained that, uh, from 1919 on, chemists kept talking about the Lewis-Langmuir theory. He said, “Any contributions that Dr. Langmuir has made to what was said or implied in my 1916 paper should be attributed to him alone.”

He said, “It has been a cause of much satisfaction to me to find that Dr. Langmuir has not been obliged to change the theory which I advanced.”

(laughter)

Well, in fact, Langmuir, I think, had done quite a lot in,

(laughter)

in, uh, contributing to the development of the theory. Uh, but G.N. Lewis perhaps didn’t even read the papers very carefully, uh. Here, uh- he wrote this structure for the carbonate ion, which is essentially what I write, too.

Uh, he said, “But the crystalline structure of solid carbonates and nitrates seems to indicate that the three oxygens are symmetrically placed with respect to the central atom.” The concept of resonance didn’t present itself to him, that the double bond could resonate around among the three positions so as to give the symmetry. W. A. Noyes Sr. at the University of Illinois back at that time was trying to make two kinds of nitrogen trichloride.

Uh, one of them, one with nitrogen positive and one with nitrogen negative. Well, nitrogen and chlorine have the same electronegativity. Uh, the bonds are essentially pure covalent bonds without ionic character.

Um, it’s also a pretty explosive substance, dangerous to handle, and the W.A. Noyes didn’t ever succeed, and no one ever will succeed. As Lewis pointed out, it’s hardly to be expected. He said, “It’s unnecessary to assume the existence of two distinct forms.”

He said, “A molecule may pass from the extreme polar to the extreme non, non-polar form, not per saltum, but by imperceptible gradations.” Well, now, uh, I’m… I come to my first, uh, theoretical paper about chemical bonds.

It’s there, uh, with Sterling Hendricks, uh, mountain climber, uh, who died recently, not by climbing mountains, I guess just by getting old. Uh, he was at CIT, in a sense was my first graduate student because Dickinson had gone off to Europe on a Rockefeller Fellowship. We calculated the difference in energy of isosteric, isomeric ions and molecules, uh, from the difference in the repulsion of the kernels.

G. N. had introduced the concept of the kernel. In carbon, for example, the two K electrons plus the nucleus constitute the kernel. And, uh, we predicted values for the difference between, say, HCN and CNH, the hydrogen being on the carbon end or the nitrogen end.

We predicted 14.2 kilocalories per mole, and, uh, predicted a number of other values. There are a couple of experimental values and, uh, several theoretical calculations, quantum mechanical ones,

(laughter)

and our predictions are pretty good. For some reason, I left that out of the nature of the chemical bond in 1939 and later editions, and I wondered why. Just oversight, I think.

I’d sort of forgotten about it. Well, at that time, the, the predictions were pure predictions. No, experimental values.

Now we come to a major step forward: quantum mechanics. Well, here I am, uh, let’s see. Øyvind Burrau in Copenhagen in 1927, um, a year after the Schrödinger wave equation, made an essentially exact quantum mechanical treatment of the hydrogen molecule ion, uh, the one-electron bond.

(clears throat)

Now I say the one electron jumps back and forth between the two nuclei and holds the atoms together. I don’t know what happened to Birge. I’ve never have heard anything about him except, uh, this one paper.

The same year, Condon, Ed Condon, who… Didn’t he take his PhD here? Work with Birge? Is there the Birge-Condon?

No. I think he was an undergraduate here. If not…

Well, I think he took his PhD here at Berkeley. Forgot to look that up. He, uh, he died in nineteen seventy-four, born in nineteen two.

He gave the first treatment of the hydrogen molecule by using Burrau’s results and introducing an energy term for the repulsion of the electrons. Pretty good. There’s a drawing showing, uh, two electrons with opposed spins in accordance with Pauli principle up and down, holding the two hydrogen nuclei together.

This was the start of the molecular orbital method. In the same year, uh, Walter Heitler and Fritz London, I don’t have the dates of their birth and death, both, uh, they both died, uh, carried out the treatment of the hydrogen molecule as involving resonance between two normal hydrogen atoms. You have an electron with positive spin on one and essentially the normal state 1s orbit, and with negative spin on the other.

But there’s another structure where the two, uh, electrons have interchanged positions, and, uh, the fact that you have two structures introduces a, a term that you can call a resonance integral in the energy, and this is the prototype of the valence bond method. The actual structure of, uh, the hydrogen molecule is a sort of intermediate one, perhaps three-quarters the Heitler-London sort, and well, perhaps half and half. It has about, it has some, um, some of the, uh, Burrau, uh, structure where the two electrons are on the same atom or the Condon structure.

In 1927, the same year, I had a long paper in the Proceedings of the Royal Society, a quantum mechanical treatment of the electronic structure and properties of many electron ions and atoms. There were later treatments by Hartree and Slater a few years later, which have pretty well taken over from the screening constant treatment that I used. In JACS 1927, I communicated a set of ionic radii, essentially theoretical, a little empirical information put in them.

And, uh, 1928 in PNAS, uh, the idea that, uh, you can hybridize the S and P orbitals for the carbon atom and get tetrahedral orbitals explain the, uh, tetrahedral structure of carbon. Well, uh, my calculations then were too complicated, uh, that caused me to reach that conclusion. I didn’t ever publish them because, uh, they just were so complicated that I didn’t have confidence in them.

1928 and following years, there are many people: Slater, Hund, Hückel, Mulliken, Rumer, and of course I was involved, who applied quantum mechanical ideas to the problem of the nature of the chemical bond. There’s been so much done. My time is up, I guess.

Uh, I’ll go rather rapidly through this. 1931, a simplified theory of hybrid bond orbitals. It took me three years, partially, a good bit of the time working on determining crystal structures, of course, but every once in a while trying to solve that problem about the tetrahedral carbon atom.

When I thought of the, uh, of a simplified theory. The radial part of the wave function of s and p is much the same as the, uh, for 2s as for 2p in the outer part of the orbital. Uh, so that I said, let’s assume that they are the same and just look at the angular parts.

These are just surface harmonics, tesseral harmonics. So they’re easily handled, just functions of theta and phi. You can make very simple calculations that you can understand.

They lead immediately to the conclusion that the hybrid that gives the strongest bond, strongest bonds is of such a nature that four of them form at tetrahedral angles. These are S and P orbitals, diagrammatically illustrated in space, the way they’re distributed around the nucleus. The S spherically symmetrical, the P’s pointing in, uh, three different directions.

And the best hybrid orbital looks like this. It has a big lobe in one direction, much better for forming bonds. The strength of the bonds, you can calculate very easily.

The strongest bonds are at one hundred and nine point four seven degrees, the tetrahedral angle, you can form four of them. Similarly, it was possible to conclude, to make a very simple calculations showing the stability of dsp squared octahedral orbitals, square orbitals, and so, ugh, and to discuss also the relation- Between, between the relation between, uh, uh, structure and magnetic properties. Well, here, I show the SP orbital, best SP orbital above, and the best SPD orbital below.

The SPD orbital has two, uh, bond angles at which you can have the, the best spd orbitals, seventy-three point one five degrees and a hundred and thirty-three point six two degrees. And these two angles have the same significance for inorganic chemistry that the tetrahedral angle has for organic chemistry. For example, here is the Hoard polyhedron.

Uh, this, uh, uh, tetragonal dodecahedron discovered by Lynn Hoard, who in molybdenum octacyanide ion. The angles there are all close to seventy-three degrees or a hundred and thirty-three degrees. And here, just as an example of an enneacovalent compound, cobalt has nine orbitals and nine electrons outside of the argon shell.

And here it forms single bonds. Each cobalt forms, uh, single bonds with each of three carbonyl groups, uh, forms double bonds with them, and then three single bonds, which are one with another cobalt atom and two to carbon atoms, which have, uh, benzene groups attached to them. In 1931, I, um, I published a paper about the three-electron bond, Uh, and, uh, included, uh, I guess it wasn’t in that paper, but when I was up at Berkeley giving my lectures as I did every spring, I said that there should be a, an ion, the, a radical, the superoxide radical.

I didn’t say exactly that. I said there should be this radical, and I said, “What should it be called?” And, uh, by that time, we had shown that it existed by measure– we had measured the magnetic susceptibility of a substance called potassium tetroxide, K2O4, and shown that it really was KO2.

At any rate, uh, Bray, the same Bray I’ve mentioned, and, uh, Erman Eastman, whom I haven’t mentioned, each said that it ought to be called superoxide. So we called it superoxide radical. Well, it has a history, of course.

So, fifteen years ago, an enzyme called superoxide dismutase was discovered. You have this enzyme in all the cells of your body to destroy the superoxide radical that otherwise might attack proteins and nucleic acids and cause damage. Uh, there are even people who sell at a high price superoxide dismutase to readers of Prevention and other health journals to take by mouth, without saying that, of course, it’s a protein and will be digested when it gets into the stomach.

Well, uh, this superoxide dismutase, uh, is, uh, the enzyme about which the most papers have been written in recent years, more about it, uh, than about any other enzyme. Well, I think I’ll… It’s time that I brought this to an end.

They’re like, “I don’t have time to discuss the electronegativity scale.” I must say, when I was a boy, eighteen, nineteen years old, I kept wondering why some elements combine with one another very vigorously, with each other very vigorously, and others hardly at all. Why you have noble metals and very reactive metals and non-metals.

The electronegativity scale is the answer to that, I’m pleased to say. Well, I come to the end by showing a moderately large molecule, uh, myoglobin, about two thousand five hundred atoms. Its structure was determined by John Kendrew a quarter century ago, and, uh, now a hundred new protein structures are being turned out each year.

Several hundred are known, much information. And I’m pleased to say that all the atoms here, all the bond angles conform to that simple calculation with sp cubed. For the orbitals, hybrid orbitals, the bond angles are close to a hundred and nine point four seven degrees for single bonds.

Everything works out, uh, well. The… Well, uh, uh, this recent work has given more understanding about the nature of life.

Uh, that’s what I’m… Uh, the title of my next lecture on Thursday is, uh, Chemical Bonds in Biology. But as I thought about it, I thought I should have said that I would talk about the nature of life, and that’s what I shall talk about on Thursday.

(crowd applauding and cheering)

[01:05:57] DR. KOSHLAND:
Do you want to answer a couple of questions, or would you rather just-

[01:06:00] DR. LINUS PAULING:
Oh, fine with me.

(crowd applauding and cheering)

[01:06:17] DR. KOSHLAND:
I think Dr. Pauling said he will answer a couple of questions, and so I would have a couple if we could, um… And just talking to Dr. Pauling, he’s a great teacher. He’ll answer every– It doesn’t have to be cosmic to have a question. I, I thought you were unduly nice to a physicist.

(laughter)

[01:06:42] DR. LINUS PAULING:
No questions?

[01:06:44] DR. KOSHLAND:
Well, Dr. Pauling will be here during the week, and we’ll meet informally, so some of you can come up briefly and talk to him now, and then he’ll be in the Department of Chemistry and Biochemistry department during the week, and we’ll be back here on Thursday for his second lecture. Thank you again for a great stimulating one.

(applause)

[01:07:07] DR. LINUS PAULING:
Well, thank you.