Internal rotation in nitrosomethane and acetaldehyde: incremental effect of deuterium substitution on the potential for methyl torsion

Abstract

Torsional splittings and zero-point conformational energy differences have been fitted for all forms of nitrosomethane and acetaldehyde substituted with deuterium in the methyl group. A single degree of freedom potential function, ▪, has been used with the isotopic data accommodated by allowing incremental changes in the potential according to the position of substitution. Coefficients up to and including V 6 have been considered. This approach has allowed rationalisation of the unusually small amount of cis-gauche perturbation observed in the microwave spectrum of CHD 2CHO. The incremental behaviour of the potential function relates closely to the conformational dependence of the CH stretching force constants. The difference in CH force constants also accounts largely for the zero-point energy differences reported here. The same ideas applied to methylamine lead to a revision of potential coefficients for CH 2DNH 2.