Isotope Effects 1968-1976

During this period, the long tradition of research on isotope effects in the Chemistry Division underwent a complete transformation. At the beginning, this field was represented by a robust independent experimental and theoretical effort. By the end, it had been folded into a new program concerned with basic research related to isotope separation using lasers and to the aspects of molecular energy transfer affecting such processes.

Vapor Pressures of Isotopic Molecules

Molecules that differ only in isotopic constitution show differences in chemical behavior that permit inferences to be drawn concerning intramolecular and intermolecular forces. For example, the enrichment of deuterium in one molecule in equilibrium with another molecule is an indication of a difference in the chemical bonding of the two molecules.

If the reduced partition function ratio of a pair of isotopic molecules is caste in the form of an infinite power series, it becomes possible to related isotope effects directly to intramolecular and intermolecular forces and to motions of atoms. Particular forms of power series were developed that are more generally valid for wider ranges of molecular energy. In addition, when the reduced partition ratio function takes this form, many of the observed behaviors of isotopic substitution are directly predicted.1

1. "Application of Finite Orthogonal Polynomials to the Thermal Functions of Harmonic Oscillators I. Reduced Partition function of Isotopic Molecules" J. Bigeleisen and T. Ishida, J. Chem. Phys. 48 1311 (1968).

Measurements of differences in the vapor pressures of isotopes, when interpreted in terms of the statistical mechanical theory of isotope effects in condensed phases, yield new information about intermolecular forces and the perturbation of intramolecular force fields by these forces. A cryostat was constructed to measure vapor pressures that was capable of determining pressure differences of 0.01% routinely. This apparatus was used to extend and confirm earlier studies of the vapor pressures of cis- and trans-dideuteroethylene, where a large discontinuity in the vapor-pressure difference between deuterated and protiated ethylene was found at the melting point. This effect was ascribed to greater hindrance of molecular rotation in the solid than in the liquid.2

2.  "Molal Volumes of the Isotopic Homologs of Ethylene" F. Menes, T. Dorfmller and J. Bigeleisen, J. Chem. Phys. 53 2869 (1970).

Kinetic Isotope Effects in Hydrogen Abstraction Reactions

Measurements were made of isotope effects in the reactions of trifluoromethyl radicals with isotopic hydrogen molecules:

CF3 + H2  → CF3H + H
CF3 + D2  → CF3D + D
CF3 + HD → CF3H + D
CF3 + DH → CF3D + H

Experimental rate constant ratios were compared for simple models of the transition state, with theoretical values derived by two methods: the completely empirical bond energy-bond-order (BEBO) approach and the semiempirical London-Eyring-Polanyi-Sato method. In addition, the effect of tunneling (i.e., the quantum mechanical phenomenon in which the hydrogen atom can leak through the reaction barrier rather than cross over it) was investigated. Experimental results lie between the values calculated with and without tunneling corrections, implying that tunneling does contribute to the rate constant, but that both methods overestimate the contribution of tunneling. It was encouraging, however, that the general success of the calculations confirms the validity of a simplified three-atom model for the transition state in hydrogen abstraction reactions.3

3. "Photolysis of Hexafluoroacetone in the Presence of H2, D2, and HD. Kinetic Isotope Effects in the Reaction of CF3 with Molecular Hydrogen" C.L. Kibby and R.E. Weston, Jr., J. Chem. Phys. 49 4825 (1968).

For the subsequent story of where research about isotope effects continues, see Laser-Induced Chemistry.

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Last Modified: February 9, 2016