In Memoriam: Paul Kebarle, 1926-2019

The university and the Department of Chemistry mourn the loss of Professor Emeritus, Paul Kebarle.

Karl Kopecky - 19 August 2019

Paul Kebarle was born in Sofia, Bulgaria on September 21, 1926 and died on July 30, 2019 in the Grey Nuns Community Hospital after a long and increasingly severe dementia.

Paul was born with scoliosis and was able to leave Bulgaria in 1948 to Czechoslovakia on the recommendation of a physician to get treatment for his condition by a specialist. Paul spent a few days there but left for Switzerland the day before the Czech government closed the country's borders. He studied hard and was able to pass the entrance examination to enrol in the Swiss Federal Institute of Technology (ETH Zürich). He graduated in 1952 with a degree in Chemical Engineering. He then came to Canada and obtained his PhD in mass spectrometry from the University of British Columbia (UBC) in 1956 after which he spent two years as a postdoctoral fellow at the National Research Council (NRC) in Ottawa. In 1958, he began his academic career at the University of Alberta.

Paul's initial research used mass spectrometry to study ion-molecule reactions of mercury- and xenon-sensitized saturated and unsaturated hydrocarbons. His research took a dramatic turn in 1963 as a result of reading a thesis on the interpretation of the electron spin resonance signals of solutions that had been irradiated with gamma rays from Co-60. He thought that the author didn't know what was going on and got the idea that it might be possible to irradiate molecules in the gas phase with alpha particles. He built a small chamber that could be filled with gas at a pressure of several hundred torr and had a microscopic slit from which gas could bleed into the high vacuum chamber of a mass spectrometer. Initial experiments were done with nitrogen gas containing small amounts of water molecules and polonium as the alpha source. Water molecules solvated the protons that were formed by the irradiation, and the clusters were stabilized by collisions with the gas. By varying the amount of water and the temperature of the chamber, very accurate values of the entropy, enthalpy, and free energy of the clustering could be calculated. This was the birth of high-pressure mass spectrometry, which was acclaimed as a major breakthrough that mass spectrometrists had been looking at for a long time.

A graduate student suggested that if water molecules solvated a proton then they should solvate positive metal ions such as lithium, sodium, and potassium. It turned out that they did, as well as anions. Paul was able to show that a relatively small number of solvent molecules, about 10, provided all the solvation energy for salts. The solvation of ammonia and protonated amines was also successfully measured. As a result, a vast number of publications ensued containing very accurate and precise data such that theoretical chemists could use the data to adjust the parameters of their equations and compute the energetics of systems for which there were no experimental data. His work on the gas phase solvation of dipolar, aprotic solvents helped to explain why solvents like acetonitrile, dimethylformamide, and dimethyl sulfoxide are superior to protic solvents for conducting reactions with anions in organic synthesis. These are classical studies, and the data are tabulated in the US National Institutes of Standards and Technology Standard Reference Database.

It has been suggested that these contributions are as basic and fundamental to thermodynamics as were the contributions of a number of Nobel Prize winners who measured equilibria in other systems. Every textbook of physical organic chemistry has compilations of the thermodynamic properties if ions and neutral molecules taken, without attribution, from the database, as do most textbooks of elementary organic chemistry. Thus, more than two generations of students studying organic chemistry have learned these concepts. For example, Kebarle determined the heats of formation of carbocations in the gas phase and showed that the 2-norbonyl cation is more stable by six kcal/mol than expected for a classical structure. This finding was an important step in ending the decades-long controversy in physical organic chemistry surrounding the "nonclassical" carbocation. The measurement of the energy of the hypervalent CH5+, and homologous cations, complemented G. A. Olah's studies of systems with hypervalent carbon in condensed media. Olah won the Nobel Prize, in part, for his work on these materials.

Kebarle also showed how hydrated protons are produced by cosmic ray bombardment of nitrogen and oxygen molecules in the upper atmosphere and are thus responsible for long range radio communications.

An outstanding breakthrough that had a profound impact in many areas of biology and physiology was his discovery that in the gas phase the benzene molecule made a stronger association with a potassium cation than did water, and he termed this interaction a cation-pi association. It was difficult for many to believe that this association could be stronger than the cation-water association, but others soon showed that other cations also formed complexes with aromatic molecules in the gas phase, such as the ammonium ion and protonated amines. Proteins contain both protonated amino acid residues and aromatic amino acid residues that can interact and contribute to protein folding. X-ray crystallographers have recognized this and cite Paul's paper on the interaction between benzene and potassium ion in the gas phase in their studies of protein structure. It was later recognized that this cation-pi interaction is involved in drug-receptor interactions as well as ion transport through membranes.

In his later work, he provided some of the most fundamental insights into protein behavior in mass spectrometry. He then made important contributions to the understanding of the processes involved in equilibration in and evaporation of electrospray clusters in electrospray ionization, a remarkable technique for ionizing high molecular weight molecules.

It should be noted that soon after Paul reported his early results from high pressure mass spectrometry many other entered the now crowded field. His work contributed greatly to improvements in the design and to the increase in sales of mass spectrometers. These instruments are now manufactured by dozens of companies with annual sales amounting to several billion dollars.

Paul Kebarle was an internationally renowned scientist, and his influence was immense. He published 265 papers, and in 2011 his h-index was an astonishing 72 (Paul ranked 223 out of the top 500 chemists at the time). His excellence was recognized with a number of awards, including section as a Member of the New York Academy of Sciences (1965), as a Fellow to the Chemical Institute of Canada (1969) and the Royal Society of Canada (1978), the Award for Excellence from the Province of Alberta (1980), CIC Medal of Chemical Institute of Canada (1986), Frank H. Field and Joe L. Franklin Award for Outstanding Achievement in Mass Spectrometry from American Chemical Society (1994), and the Fred P. Lossing Award from the Canadian Society for Mass Spectrometry (1994).

Paul will be very much missed by his family, colleagues, students, and all those who knew him.