An ab initio structural and spectroscopic study of acetone—An analysis of the far infrared torsional spectra of acetone‐h6 and ‐d6

The far infrared torsional spectra of acetone (CH3)2CO and (CD3)2CO have been determined from ab initio calculations, and the main features of the experimental data assigned. For this purpose, the potential energy surface for the double methyl rotation was determined with fully relaxed geometry into the RHF and RHF+MP2 approximations using a 6–31G(p,d) basis set. The energy values, as well as the kinetic parameters obtained from the optimized geometry, were fitted to double Fourier expansions as functions of the rotational angles in seven terms. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G36 nonrigid group, which factorize the Hamiltonian matrix into 16 boxes. The energy levels and torsional wave functions for each symmetry specie were then obtained diagonalizing each blocks separately. Intensities were obtained from the calculated electric dipole moment variations and the nuclear statistical weights, and were combined with the torsional frequencies to predict the spectra. The calculated band patterns show a multi- plet structure and reproduce the main features of the experimental data. The torsional bands of the infrared active ν17 mode were found to be clustered into quartets, (A1→A2, G→G, E1→E1, E3→E4), for the v=0→v=1 fundamental, and (A2→A1, G→G, E1→E1, E4→E3) for the v=1→v=2 first sequence transitions. The G→G transitions were found to be the more intense. The correlation between the calculated and observed spectra allows for an assignment of the major bands.