Role of hyperconjugation in secondary .beta.-deuterium isotope effects

The effect of hyperconjugation of the magnitudes of secondary BETA-deuterium isotope effects has been investigated by ion cyclotron resonance spectroscopy and a b initio molecular orbital theory. Isotope effects measured for ion-molecule equilibria involving formation either of a nonbonded pair of electrons or of a vacant orbital adjacent to a methyl probe have been compared to theoretical calculations which have been restricted to consideration of changes in CH stretching force constants alone. Although this simple theoretical model successfully mirrors the observed BETA effects in the former systems in which the methyl group acts to accept excess electron density, it fails to properly account for the observed effects in those systems where the methyl probe is adjacent to an electron-deficient center. (2 tables)