Rotational Isomers in Anhydrides from IR Spectroscopy and MO Calculations

Abstract

In this study ab initio MO calculations were used to calculate the relative energies of various types of C-O rotational isomers in acetic anhydride. MO calculations were also used to predict the IR intensity ratio for the in-phase and out-of-phase carbonyl stretch bands in acetic anhydride. A linear relationship was developed between the intensity ratio and the square of the tangent of the C-O dihedral angles. This relationship was used to show that crystalline stearic anhydride has a planar skeleton and stearic anhydride in the liquid state has a C-O dihedral angle of about 32°.