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The equilibrium isotope effect (EIE) for the interconversion of the two chair isotopomers of 1-trideutero-1,3,3-trimethylcyclohexane was predicted using geometry and vibrational force constants derived from electronic structure theory at HF, B3LYP, and MP2 levels as input for the program THERMISTP. Agreement between theory and previously reported NMR results is very good (experimental K(eq) = 1.042 +/- 0.001 vs K(eq) = 1.0409 at MP2/6-311G* level, K(eq) = 1.0503 at HF/6-311G*, and K(eq) = 1.0417 at B3LYP/6-311G* level, all at 17 degrees C). In order to investigate the origin of this isotope effect, the calculated EIEs for the monodeuterated isotopomers were analyzed. It has been shown that the hydrogen atom on an axial methyl group which is unusually close to its counterpart on the other axial methyl is responsible for the large (steric) isotope effect in the compound studied.

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)

The ability of simple levels of ab initio molecular orbital theory to describe with reasonable accuracy the energetics of isotopic exchange processes is demonstrated. Three levels of ab initio molecular orbital theory have been surveyed. The first two levels are single-determinant Hartree-Foch methods utilizing the 3-21G split-valence and 6-31G* polarization basis sets. The third level, which is computationally the most complex, uses the 6-31G* basis set but allows for partial account of electron correlation by way of Moller-Plesset perturbation theory terminated at second order. Theoretical and spectroscopic equilibrium constants for reactions XH + XD reversible XD + H/sub 2/ where XD is a hydride of the first row of the periodic table are tabulated. (BLM)

The C13 kinetic isotope fractionation in the dehydration of formic acid by concentrated sulfuric acid was studied over the range --30.0 to 0.0 . The observed temperature dependence was examined by the gamma-barl' method of Bigeleisen and Wolfsberg (both with and without consideration of the effects of loss of bending frequencies) and by the semi-empirical" method of Yankwich, Weber, and Ikeda. Calculated values of the temperature-independent factor in the isotopic rate constant ratio lie within 1% of unity; the calculated sums of the differences upon activation of the diagonal cantesian force constants correspond especially well with the value expected for C- O bond rupture when bending frequency losses are taken into account, being near 7 md.A.-I.

Results of some experiments to measure the isotopic exchange equilibrium constants for D/H, {sup 13}C/{sup 12}C, and {sup 15}N/{sup 14}N involving a neutral organic molecule A and its anion radical A{sup {minus}} are reported. It is pointed out that the reported magnitudes of the carbon and nitrogen isotopic effects are at variance with conclusions based on the statistical mechanical theory of the equilibrium isotope effects. Theoretically calculated values of the isotopic exchange equilibrium constants with nitrobenzene and fluorenone as the neutral molecule are tabulated. 7 refs., 2 tabs.