McMaster Chemical Extracts '99

Ronald Gillespie: A Lifetime in Chemistry

Professor Emeritus, Ronald Gillespie, F.R.S., F.R.S.C., F.R.S.C. (U.K.), F.C.I.C. celebrated his 75th birthday on August 21, 1999. Professor Gillespie's distinguished career is highlighted by the discovery and characterization of super acid media, the earliest identification of many polyatomic cations of nonmetals, and early studies of noble gas fluorocations. However, he is perhaps best known for the ubiquitous rules of the Valence Shell Electron Pair Repulsion (VSEPR) model of molecular geometry. His legacy is thus familiar to anyone who has taken freshman chemistry.

Professor Gillespie's investigations into superacid media began with his Ph.D. research under the supervision of C.K. Ingold at London University. There, he used cryoscopic measurements (which reveal the number of ions and/or molecules in solution) to demonstrate the formation of the nitronium ion, NO2+, upon dissolution of nitric acid in sulfuric acid. His later technically difficult electrical conductivity measurements of 100% sulfuric acid gave the definitive description of the solvent which would provide the first superacid medium. For example, it was demonstrated that the electrical conductivity of pure sulfuric acid is predominately associated with the Grotthus chain mechanism of proton transfer, rather than simple transport of H3SO4+ and HSO4- ions. This results because the high viscosity of the pure acid (27 times higher than that of water) greatly reduces the mobility of solvated ions. The pure solvent studies provided the foundation for subsequent investigations of new strong acids such as HSO3F, HB(HSO4)4 and HSO3F-SbF5, best known as "magic acid". The new superacids permitted preparation of stable solutions of important intermediate species such as nitronium and acylium ions, and carbocations. They are now essential tools of inorganic and organic chemistry, providing acidity far beyond the reach of the most acidic aqueous solutions.

Superacid media provided the tool for preparing a wide range of polyatomic cations. Gillespie and co-workers were quick to use the modern physicochemical and spectroscopic tools, as they became available, to characterize the newly identified species. For instance, Gillespie provided the first conclusive evidence of the polyatomic nature of the I2+ and I42+ species. Other exotic species prepared by Gillespie include Br2+, Br3+, S42+, S82+, S192+, Se42+, Se102+, Te42+, Te62+ and Te4Se42+, both in solution and in crystalline salts. The structure of many of these species, determined using X-ray crystallography, brought many surprises and extended our understanding of chemical bonding.

In a remarkable application of SbF5 or AsF5 in SO2, Gillespie prepared unique mercury compounds such as Hg2.91SbF6 and Hg2.85AsF6 which are composed of infinite cationic chains of mercury atoms. These conducting ionic materials exhibit anisotropic conductivity with current traveling along the mercury chains, but not in other directions. The HF- SbF5 superacid system was used to prepare noble gas fluoride cations, including XeO2F+, XeOF3+, XeF3+, KrF3+ and Kr2F3+. Characterization of these and other species involved extensive pioneering use of NMR and Raman spectroscopies. In fact, Gillespie joined the McMaster Chemistry Department in 1958 from University College, London, where he was a Lecturer, on the condition that a commercial NMR spectrometer capable of running 19F and 1H spectra be purchased for his use. The instrument, a Varian HR60, operating at 56.4 MHz for 19F, and equipped with a "hot-wire" plotter, was one of the first commercial NMR spectrometers in Canada and was installed during the summer of 1959.

The VSEPR model of molecular geometry was advanced in 1957, when together with Ronald (later Sir Ronald) Nyholm, Gillespie published "Inorganic Stereochemistry" in Quarterly Reviews of the Chemical Society (volume 11, page 339). G.N. Lewis first described valence in terms of the electron pair bond and the octet rule. The VSEPR model extends the Lewis picture to account for molecular geometry in terms of Pauli principle repulsion of electron domains (either bonding or nonbonding) about each atom. The extraordinary simplicity and success of this model made it an essential component of the freshman chemistry curriculum around the world. The model was elaborated and applied extensively in two books authored by Gillespie: "Molecular Geometry", published in 1972, and "The VSEPR Model of Molecular Geometry", published in 1989 (co-authored by I. Hargittai).

Gillespie has continued his research full time since his retirement in 1989. He collaborates with Richard Bader and many of Bader's former students and postdoctoral fellows in conducting atoms in molecules (AIM) analyses of bonding in a wide range of molecules. His goals include understanding and accounting for exceptions to the VSEPR model. It has been seen that such exceptions occur when nonbonding electrons fail to localize into distinct electron domains. In addition, recent observations have spawned the Ligand Close Packing model of bonding in molecules with highly polar bonds. In such molecules, which include many main group fluorides and oxofluorides, the molecular geometry is determined largely by the close packing of anions (or atoms which can be viewed as such) around the central, less electronegative atom. This model accounts for the remarkable constancy of F to F distances in AFn species.

The widely known VSEPR model was first developed as an aid to teaching. It grew from Prof. Gillespie's longstanding interest in chemical education. His many contributions to teaching have been recognized by the Manufacturing Chemists' College Chemistry Teaching Award, the Union Carbide Award of the Chemical Institute of Canada and the McMaster Student's Union for Excellence in Teaching. In addition, he has published two innovative first year chemistry texts: Chemistry, with Professors Humphreys, Baird and Robinson, and Atoms, Molecules and Reactions: An Introduction to Chemistry, with Professors Humphreys, Eaton and Robinson. These texts will be very familiar to many alumni who used them in their freshman introduction to chemistry.

Related Links:
R.J. Gillespie Research Group
R.F.W. Bader Research Group
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