Ab initio analysis of the vibrational spectra of conformers of some branched alkanes

Abstract

A scaled ab initio HF/6-31G force field has been optimized that provides an accurate description of normal mode frequencies and eigenvectors of branched alkanes. This force field reproduces 159 observed non-CH stretch bands of 10 conformers of 2-methylpropane, 2,2-dimethylpropane, 2-methylbutane, 2,2-dimethylbutane, and 3-methylpentane with an rms deviation of 6.1cm−1. A number of modes have been reassigned based on this vibrational analysis. The force field serves as the basis for the development of a spectroscopically accurate molecular mechanics energy function for saturated hydrocarbon chains.