Abstract
A linear relationship has been found between the wavenumber of the CH<sub>3</sub> symmetrical deformation vibration and the electron density on the CH<sub>3</sub> 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<sub>3</sub> symmetrical deformation wavenumber. These include ν‾(CH<sub>2</sub> def), ν‾(CH<sub>2</sub> wag) and both components of ν‾(CH wag). The splitting of ν‾(CH<sub>3</sub> sym def) in isopropyl and <i>t</i>-butyl groups has long been known. It is shown here that the effect is due to an interaction force constant relating to the CH<sub>3</sub> symmetrical deformation vibrations of two or three different neighboring CH<sub>3</sub> groups. The origin of the interaction is thought to be an H,H' repulsion between hydrogens on the different CH<sub>3</sub> groups.
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