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Sidney W. Benson

Distinguished Professor of Chemistry, Emeritus
Physical Chemistry

A.M., Ph.D., 1941, Harvard University
A.B., 1938, Columbia College
Office: LHI 121
Phone: (213) 740-5964
Fax: (213) 740-6679

Research Focus

Kinetics and Thermochemistry of Free Radicals

Dr. Benson has had a continuous research effort in exploring the kinetics and thermochemistry of free radicals for more than 40 years. His most recent development has been the construction of a Very Low Pressure Reactor (VLPR) in which single, elementary reactions of free radicals can be observed directly using mass spectrometric detection.
By studying elementary reactions such as:
Cl + RH -> R + HCl
and its inverse at low pressures where no competing reactions occur, it has been possible to obtain rate constants and equilibrium constants for such systems (R = CH3, C2H5, vinyl, etc.). These latter make it possible to obtain bond dissociation energies and heats of formation of radicals to +/- 0.1 kcal/mole, an order of magnitude improvement over prior efforts. These results are of great importance in understanding atmospheric chemistry and combustion.
One of the last frontiers in theoretical chemistry has been the qualitative prediction of heats of formation from molecular structure. Dr. Benson and his research associates have recently completed studies from which they were able to develop simple empirical laws permitting them to estimate heats of formation of solid salts and their hydrates to an uncertainty of about 2 kcal/mole.
These efforts and similar ones on bond energies are still underway. The empirical models being developed may be able to account for the discrepancies in estimated heats of formation which arise from non-bonded interactions between atoms separated by 2, 3, or 4 other atoms.
Joint projects with colleagues in Chemical Engineering are underway to investigate the processes occurring on catalytic and reactive surfaces. Despite simple pictures which have sometimes been presented, a catalyst is a species which undergoes complex chemical reactions with reactants. In a catalytic process, many steps occur in the last of which the catalyst is regenerated. This is true in organic, in biological, and in inorganic systems.
Two processes being looked at intensively are the kinetics whereby crystals of semiconductors such as silicon or gallium arsenide are grown from vapor phase molecules and the conversion of HCl to chlorine on metal oxide catalysts.
Theoretical projects are being undertaken involving the structure of water solutions of nonpolar, polar and ionic solutes. Also underway are new methods of calculating binding in complex atoms and ions. The methods involve a change from the Hartree-Fock type approach in that significant electron correlation is introduced into the basic wavefunction.

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