A Theoretical Approach to Substituent Effects. Structural Consequences of Methyl Hyperconjugation. Methyl Tilt Angles and Carbon-Hydrogen Bond Lengths

The effect of methyl hyperconjugation on the tilt angle and C-H bond lengths of methyl groups has been examined using ab initio molecular orbital theory. A perturbation molecular orbital approach is developed to explain the results in two model systems, methylborane and methylamine. The qualitative model is then used to rationalize the results observed in the set of molecules CH₃X=Y for X = CH and N and for Y = CH₂, NH, and O. The results are interpreted in terms of two-electron interactions between the occupied ir(Me) orbitals on methyl and any adjacent low-lying vacant orbitals as well as between the vacant ι*(Me) orbitals on methyl and any adjacent high-lying occupied orbitals. Although the possibility of four-electron interactions cannot be discounted, it is significant that all the results presented in this paper may be understood in terms of two-electron interactions alone. Methyl tilt angles and C-H bond lengths provide a sensitive and experimentally accessible probe for hyperconjugative interaction for a wide variety of molecules containing methyl groups.