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Molecular Orbitals and CH3, CH2, and CH Deformation Group Frequencies

Volume 34, Number 1 (Feb. 1980) Page 1-6

Colthup, Norman B.


A linear relationship has been found between the wavenumber of the CH3 symmetrical deformation vibration and the electron density on the CH3 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 CH3 symmetrical deformation wavenumber. These include ν(CH2 def), ν(CH2 wag) and both components of ν(CH wag). The splitting of ν(CH3 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 CH3 symmetrical deformation vibrations of two or three different neighboring CH3 groups. The origin of the interaction is thought to be an H,H' repulsion between hydrogens on the different CH3 groups.