Analysis of isotope effects by perturbation theory technique 18O/16O isotope effects in alkyl ethers

Abstract

A statistical mechanical perturbation theory approach has been used to analyse ¹⁸O/¹⁶O isotope effects in a series of alkyl ethers having a transferable valence force field. It was found that the contributions from only the stretches and bends involving isotopic substitution are sufficient to evaluate the reduced isotopic partition function ratio, (S₂/S₁)f, which is within 0.3% of the exact value obtained from the usual approach of normal-mode vibrational frequency calculations. Four such contributions (corresponding to one CO stretch and three ∠ HCO, ∠ CCO and ∠ COC bends) have been identified and listed. Besides obtaining a fairly accurate value of (S₂/S₁)f in so simple a manner, the perturbation theory approach gives a clearer understanding of the origin of isotope effects.