Deuterium isotope effects and hyperconjugation

Abstract

There now exists ample published evidence which indicates that reactions with rate-determining steps which involve carbonium ion or partial carbonium ion formation are slowed by the substitution of a deuterium atom for a hydrogen atom in a hyperconjugating position. In each of the several cases where data are available it seems that the isotope rate effect is paralleled by an isotope activation energy effect although entropy factors also contribute. It seems fairly clear from theory that the isotope activation energy effects can only arise from (a) changes in vibration frequency of the hydrogen and deuterium atoms on activation, (b) tunnelling of the hydrogen atom or (c) anharmonicity effects. The second effect must be unimportant in these reactions although it might be a major factor in reactions where hydrogen bonds are directly broken. Several lines of evidence indicate that anharmonicity effects although small are present and may confuse the quantitative interpretation of the isotope rate effects. Thus the interpretation of secondary isotope rate effects is complicated but the application of the time-honored classical organic chemical method of comparing the effects in a large number of different compounds indicates that hyperconjugation is the predominant cause of these secondary isotope rate effects. If we further assume that the variations in these effects in closely related compounds are due to variations in hyperconjugation, the following conclusions are tentatively indicated: (1) There are steric effects on hyperconjugation. (2) There are substituent effects on hyperconjugation; conjugating and hyperconjugating substituents increase the extent of hyperconjugation of a single CH bond. (3) There are solvent effects on hyperconjugation.