Molecular Orbitals and CH 3 , CH 2 , and CH Deformation Group Frequencies

A linear relationship has been found between the wavenumber of the CH 3 symmetrical deformation vibration and the electron density on the CH 3 carbon as calculated from CNDO/2 molecular orbital theory. Other CH deformation vibrations are also related to the electron density on the carbon and, as a result, can be correlated with the CH 3 symmetrical deformation wavenumber. These include ν̄(CH 2 def), ν̄(CH 2 wag) and both components of ν̄(CH wag). The splitting of ν̄(CH 3 sym def) in isopropyl and t-butyl groups has long been known. It is shown here that the effect is due to an interaction force constant relating to the CH 3 symmetrical deformation vibrations of two or three different neighboring CH 3 groups. The origin of the interaction is thought to be an H,H′ repulsion between hydrogens on the different CH 3 groups.