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Crystallography in Canada 


I. David Brown


I. D. Brown

Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada  
Reprinted from CNCC Newsletter No 1 (Canadian National Committee for Crystallography) September 2009




I. David Brown

I studied for my bachelor's degrees in General Science and Honours Physics at King's College, London, UK, at the time when Rosalind Franklin and Maurice Wilkins were determining the structure of DNA. For graduate studies I moved to the Royal Institution (RI) where the group working with the director, Sir Lawrence Bragg, was unraveling the first crystal structure of a protein. For my PhD under the supervision of Jack Dunitz, I solved the crystal structures of two organic complexes of Cu(I), doing most of the calculations by hand, but experimenting with writing machine-language code for a computer with around a hundred and twenty memory locations which had to store both program and input. When Jack accepted an appointment at the ETH, I followed and spent my final PhD year in Zurich. In 1959, as was customary at the time, I crossed the Atlantic to take up a post doctoral position at McMaster University with Howard Petch. My job was to finish building a single-crystal neutron diffractometer at the newly opened McMaster swimming pool reactor. During this time I developed an interest in the structure of inorganic compounds, a term which I use to include any non-metallic crystal that contains no C-H bonds. This was an area much neglected except by mineralogists who were, however, focused on natural minerals. Solid state physicists were not prepared to work with anything more complex than a cubic crystal, preferably containing only one kind of atom, while chemists thought of a crystal as a kind of freezer in which a dormant compound could be indefinitely stored until it was needed to carry out some real chemistry in solution. Those who called themselves 'inorganic chemists' were more interested in preparing organic molecules that could bind to a transition metal atom. The neglect of inorganic solid-state chemistry arose in part from the lack of an effective model for the systematic description of infinite structures. Most descriptions were based on an eclectic mixture of words such as 'packing of spheres', 'ionic interactions', 'covalent bonds', 'space group symmetry',' polyhedral packing' and 'structure type'. Unlike the chemical bond descriptions used in organic chemistry, descriptions of inorganic structure tended to be vague and unconvincing. 

 

In 1960 I was joined at McMaster by Chris Calvo, who in his short life, published a series of high-quality structures of phosphates that still compare favourably with structures published today. We set up a small x-ray laboratory with a generator and a precession camera where we were later joined by Colin Lock in Chemistry and by Doug Grundy in Geology. Subsequently this laboratory was expanded and provided structure determinations for other faculty members, while students had the opportunities to learn how to determine crystal structures. Today this laboratory is managed by Jim Britten, and is equipped with the latest x-ray diffractometers and software. 

 

After my postdoctoral years I joined the McMaster Physics Department where I spent my first ten years wandering in my own scientific wilderness. Although I gained experience solving structures for my chemistry colleagues I could not see where my research was going. As many can corroborate, it is difficult in the wilderness to know what one is looking for, or even if there is an answer to the question that has yet to be discovered. When Gay Donnay came to McMaster to show how Pauling's electrostatic valence model could be improved by taking account of the correlation between bond valence and bond length I failed to pick up on the clues. My epiphany came a couple of years later in 1971 when Bob Shannon arrived at McMaster on a year's leave from duPont. Within an hour of our first meeting he had convinced me of the importance of Donnay and Allmann's work. The rest of that year we spent extending and simplifying their approach. In our paper we showed how bond valences could be calculated from a wide range of experimentally observed bond lengths, how they are normalized by arranging that they sum to the valence (oxidation state) of each atom, how they can be used to check the validity of newly determined structures, and how they provide a quantitative measure of the strength of a bond that, unlike the bond length, is independent of the nature of the atoms involved. 

 

At the end of his stay, Bob Shannon left me six boxes of computer cards on which he had laboriously punched the unit cells, the symmetry operators and atomic coordinates of over four hundred inorganic crystals, but this database was useless because there was no way to retrieve the information it contained. The experience made me realize the importance of creating a well-structured, machine-readable database if I wanted to carry out systematic studies. A large part of my subsequent career has been devoted to simplifying crystallographic data retrieval, first by creating an annual bibliography of inorganic crystal structures (BIDICS 1969-1981), and later proposing a common file structure to allow the interchange of crystal structure databases between programs and laboratories (this has now evolved into CIF). At the same time Guenter Bergerhoff and I started the Inorganic Crystal Structure Database (ICSD). By 1985 the ICSD had reached the point where Daniel Altermatt and I were able to use it to prepare a systematic set of bond valence parameters. 

 

Subsequently I expanded the bond valence model to show how it can be used in structure modeling of both solids and liquids, how it predicts chemical stability and reactivity including aqueous solubility, and how the bonding geometry can be analyzed into separate contributions from chemical bonding, steric strain and electronic anisotropies. An invitation from Henk Schenk to present a series of lectures in Amsterdam in 1994 led to the publication in 2002 of my book 'The Chemical Bond in Inorganic Chemistry' (OUP) which brings all these threads together. 

 

In 1986 I acted as local chair for the ACA meeting held at McMaster, the last ACA meeting held on a university campus. Subsequently I persuaded the ACA to establish a Canadian Division and I served as its first representative on the ACA Executive, an experience which convinced me of the importance of the ACA in bringing together North American crystallographers. 

 

My students and academic colleagues will probably remember most affectionately the daily morning coffee breaks when my group would meet together informally. We discussed many topics on these occasions, including the problems of structural chemistry, sometimes focusing on the particular problem of an individual, sometimes raising more general questions. On these occasions we would let our imaginations run wild, but always challenged each other to test and justify each new idea. These gatherings were the forge in which the bond valence model was hammered out.