Ronald J. Gillespie; the VSEPR model; and molecular symmetry
As a student, I had no knowledge of this model, and much of the information about the shapes of molecules we had to learn one by one. Once in possession of the model, many of the structural variations could be inscribed into the realm of a set of rudimentary rules. I was introduced to the model in the discussion following my first ever seminar presentation in the United States. It was in January 1969; I was on my way to the University of Texas at Austin, and was invited to Indiana University in Bloomington by Russel A. Bonham. I talked about the structures of some simple molecules containing sulfur as we determined them by gas-phase electron diffraction in Budapest. I was trying to observe trends in the structural variations and even interpret them, when Russ asked whether I had tried to apply VSEPR considerations to them. I was taken by this model so much that I penned a long letter to my wife-and-graduate student back in Budapest, and illustrated it with loops of electron pairs in color (the new knowledge of this attractive model kept me up all night following my seminar, or it may have been just jet leg).
When, after 1 year, I returned home, I wrote a long article in a popular-science magazine about the model, but doing so, I ran into difficulties. As I tried to use the simplest molecules to illustrate the applicability of the model, I observed conspicuous discrepancies, and this prolonged the completion of the article. For example, the model in its original formulation predicted a gradual decrease of the bond angles in the series of molecules CF4 → NF3 → OF2 just as was observed in CH4 → NH3 → OH2. While there was a decrease in CF4 → NF3, this was not the case in NF3 → OF2. The deviation from prediction was similar in SiF4 → PF3 → SF2 and even more conspicuous in CCl4 → NCl3 → OCl2 and in SiCl4 → PCl3 → SCl2.
I wrote a letter to Gillespie about my observations, which he could not resolve either, but suggested not rushing into publishing anything about them, because he was just bringing out his book about his model, and advertising problems about it at this time would not be fortunate. Years of frustrations followed, culminating in some quantum chemical calculations, which pointed to the necessity of more complete considerations of the interactions among all electron pairs. This meant following the variations not only of the bond angles, but also the angles involving the lone pairs of electrons. This consideration did not necessitate the introduction of any additional rule; it only meant a fuller assessment of the very interactions the VSEPR model was based on in the first place. Needless to say, Ron was not enthusiastic about augmenting the considerations that went beyond the original formulation.
I stayed in mail-contact with Ron throughout these years, and we met in person for the first time only in the early 1990s. In the mid-1980s, I raised the question about a new book on the VSEPR model, and it turned out that my suggestion coincided with Ron’s thinking about a new book as well. We decided to produce it jointly and the roles were clearly if tacitly defined from the start. I was to be the junior author doing much of the writing and Ron’s role would be polishing what I wrote. This became a rewarding experience in that I learned that every single page must be didactically perfect—we were aiming at producing a textbook. It was also a frustrating experience as Ron was on sabbatical leaves most of the time; sailing the inter-coastal waterways on the East Coast of the United States and I had to be vigilant to make sure that my ever improving drafts would be delivered in the right marina at the right time.
Our joint writing was also made difficult by Ron’s jealously guarding his model from any attempts of showing its limitations. One of those limitations was the model losing its validity with increasing ligand size relative to the size of the central atom. It was a little triumph when some regularity in non-bonded distances could be mentioned in the book. The irony of this is that years later, Ron became enamored with the importance of non-bonded interactions in determining molecular shapes.
Another, yet more general problem was that Ron was and has always been somewhat embarrassed by the tremendous success of his seemingly unsophisticated model and would have preferred to see its sophistication enhanced. So he wanted to stress the quantum mechanical foundations of the model, which would have made it into a theory and, also, less applicable without involved computations. In contrast, if one would have to carry out quantum chemical computations each time instead of applying simple VSEPR considerations, the utility of the model would fast disappear. To me the highest compliment was when Ron told me that by the time we completed the book—it took almost 5 years for us—I may have known more about it than he. What I took away from the experience was what I learned about didactics from the great pedagogue that Ron has been and my pledge never get involved in co-authoring a book with anybody again (family members excepted).
The VSEPR model has had double significance. One is in chemistry education and the other is as a research tool. It is very difficult if at all possible to assess the importance of the VSEPR model in education. It has certainly made it easier for students at any level of chemistry instruction to grasp the notion of and variations in molecular structure, meaning the geometry of molecules in this case. Although the Nobel Prize should not be made into any kind of absolute measure, in a way it has become one, and had Gillespie’s VSEPR model been awarded one, I doubt it would have elicited any protest. In fact, it could have been among the recently rather rare pure chemistry awards countering the growing tendency of using chemistry Nobel awards for achievements in discoveries of biological importance. The criteria for the Prize according to Alfred Nobel’s Will have double requirements. One is a general stipulation referring to having conferred the greatest benefit on mankind. The other is specific to each of the five categories although the distinctions are slight. For chemistry it sounds, “to the person who shall have made the most important chemical discovery or improvement.” The VSEPR model has benefited mankind in facilitating the dissemination of chemical literacy, and it was a discovery or at least an improvement of significance. Chemistry is not an easy subject; neither is it among the favorite ones for most pupils. Yet Gillespie’s model certainly made its important aspects more user-friendly. If this is true to any extent, and I believe it is to a great extent, it has been beneficial to mankind.
Concerning molecular symmetry, it has no limits in principle, whereas, in classical crystallography, crystal symmetries excluded five-fold symmetry. Following the discovery of quasicrystals and in terms of generalized crystallography, there has been an expansion of recognized symmetry possibilities in condensed state structures as well—of course, they have always been present in nature. It is noteworthy, however, that different symmetries may occur with different frequencies and play different roles in structures. Thus, for example, symmetry planes may hinder maximum space utilization in the condensed phase, that is, when the symmetry plane would relate two molecules, resulting in considerable empty space between them. The presence of two-fold symmetry, on the other hand, facilitates space utilization. In molecular symmetries there is no such preference and the symmetry plane carries no disadvantage either, and has a high occurrence, especially in the structures of simple molecules. Molecular symmetries may have great importance for biological function, although this has yet been understood in few cases only. The most conspicuous example is the two-fold symmetry of the double helix structure of DNA directly related to the copying mechanism for this molecule. In contrast, the two-fold symmetry of the molecule of the photosynthetic reaction center has been admired, but its function, if it has one at all, has not yet been understood. Symmetry is a most useful concept and it may have functional utility, but when observed in molecules, especially in small molecules, it may just be an expression of equalities of interactions between identical parts of the system. It may be counterproductive to read too much into it in most cases. Symmetry considerations and the structural consequences of VSEPR often point in the same direction and both are helpful in systematizing structures and understanding their variations. With the VSEPR model, Ronald Gillespie placed a most useful tool into the hands of chemistry educators and researchers and for this and his other contributions we celebrate him on the occasion of his remarkable anniversary. It is instructive to review his path to his achievements.
Ron was born in London in 1924. When he was 11 years old, he was awarded a scholarship to the local grammar school; this eased the burden of paying for his education, but his parents, who were not very well off, still had to pay for school uniforms, sports clothing, and train fares. He was an average student during the term, but his performance was outstanding on the final exams, especially in science. In preparation for university, he studied chemistry, physics, pure mathematics, and applied mathematics. He was much better in chemistry and physics than in mathematics, but he worked hard to bring up his math. His physics teacher gave boring lectures by simply reading the textbook aloud, but the chemistry teacher gave them a lot of exciting lab work. Ron graduated from school in 1942 and started a 2-year chemistry course at University College London. They were evacuated to the University of Wales in Aberystwyth at the beginning of Ron’s studies. Toward the end of the second year, as they were moving back to London, the head of the chemistry department, Christopher Ingold asked him whether he would like to stay and do research. Thus, Gillespie became a member of Ingold’s team working on aromatic nitration. Ron made remarkable progress in his research when he found cryoscopic evidence for the nitronium ion. The interaction with Ingold was infrequent; Ingold came by the lab once a month, in the company of Ted Hughes; he was taciturn; but even his remoteness appeared inspiring. Having worked with the nitronium ion, Gillespie worked a lot with sulfuric acid, leading his interest toward other superacids. Ingold let him working quite independently, but helped him writing up his papers. Several of them were actually written by Ingold, but appeared under Gillespie’s name as sole author. Not long before Gillespie got his Ph.D. degree in 1949, Ingold casually asked him to give a course on molecular properties, which meant also an appointment as Assistant Lecturer at the University.
In 1953, Gillespie got a Commonwealth Fund Fellowship to work in the United States; he spent a year at Brown University; and toured the United States and Canada at the end of the year. It was then that the idea of moving to North America occurred to him. What Gillespie perceived before his American sojourn as encouragement, upon having returned to University College, he felt that Ingold’s tight control of everything was hindering advancement. He was trying to strike out in new directions and was moving increasingly into inorganic chemistry in 1957. At the same time, he developed a fruitful interaction with Ronald Nyholm, a newly appointed faculty member from Australia. Their discussions led to a joint review of inorganic stereochemistry in which Gillespie dealt with the main groups and Nyholm with the transition metals. Working on this review inspired Gillespie to formulate the rules that later became the VSEPR model. On another trip to North America, he visited McMaster University, which was on its way of rapidly developing into a major research school from a small Baptist college. They made him an offer, which he could not refuse; his package contained double of his London salary, a Raman spectrometer, and an NMR spectrometer. In London, under Ingold, instrumentation was compartmentalized under different individuals, and there was not even a good NMR unit that Gillespie might have had access to.
Gillespie moved to Canada in 1958 and has stayed at McMaster University ever since. He was elected Fellow of the Royal Society of Canada in 1965 and Fellow of the Royal Society (London) in 1977. His research over the decades concentrated on superacids, sulfuric acid, fluorosulfuric acid, and hydrogen fluoride, and generally, fluorine chemistry. As for the VSEPR model, he was more of a promoter than doing extensive research in it. He also became engaged in textbook writing for general chemistry. In recent decades, the VSEPR model has taken off, and is often mentioned in the literature without even referring to Gillespie. This is the fate of important discoveries as the Periodic Table often figures without Mendeleev’s name or the double helix without spelling out Watson and Crick. The expression VSEPR and Gillespie’s name have become synonyms.
- Gillespie RJ, Hargittai I (1991) The VSEPR model of molecular geometry. Allyn & Bacon, BostonGoogle Scholar
- Hargittai I (2002, 2003) The road to Stockholm: Nobel Prizes, science, and scientists. Oxford University Press, Oxford, UKGoogle Scholar
- Hargittai I (2003) Candid science III: more conversations with famous chemists. Imperial College Press, LondonGoogle Scholar
- Hargittai M, Hargittai I (2009) Symmetry through the eyes of a chemist, 3rd edn. Springer, LondonGoogle Scholar