A comparative analysis of the internal rotation in acetone and trans-dimethylglyoxal. Application to the fundamental frequencies and torsional modes

Abstract

The large-amplitude internal rotations in trans-dimethylglyoxal (DMG or biacetyl) were considered as a pair of interacting methyl rotors which were completely decoupled from the torsion of the acetyl groups around the central C-C bond. The potential energy surface for methyl rotation was determined by performing ab initio RHF calculations for seven conveniently chosen conformations of the methyl groups. In these calculations a 6-31G(d,p) basis set was used, and the geometry of each conformation fully optimized. The set of energy values were fitted to a symmetry-adapted Fourier series as a function of the rotation angles, which was used to construct a potential energy function for the double rotations and combined with the kinetic energy. The energy levels and the transition energies calculated for DMG were compared with the hot band intervals observed in the warm-jet laser fluorescence excitation spectrum. The acetyl torsion around the central bond was studied in a similar way and the frequencies were compared to the far-IR spectrum. The effects of interacting methyl groups were explored by comparing the dynamics of the remotely situated methyl groups in the DMG system with the sterically coupled methyl groups of acetone. © 1993.