THEORETICAL INVESTIGATION OF ASYMMETRIC METHYL ROTOR DYNAMICS IN PARTIALLY DEUTERATED ACETOPHENONES

Ab initio molecular orbital calculations at the HF/4-21G level, are reported of the structure, the molecular force field, and the methyl torsional potential energy curve of acetophenone in an attempt to interpret the observed tunneling and rotamer splittings in the S0 --> T(n-pi*) excitation spectra of methyl-deuterated acetophenone-d1 and -d2. It is found that the angular dependence of the torsional kinetic energy operator does not contribute significantly to the rotamer splittings, which instead are mainly due to the angular dependence of the overall zero-point energy. The calculated zero-point energy differences between the two rotamers in the ground state are 14 cm-1 for acetophenone-d1 and 12 cm-1 for -d2. To account for the observed rotamer splittings of 7.5 and 4 cm-1, respectively, the corresponding zero-point energy difference in the triplet state must be about 9 cm-1 for both isotopomers. Comparison with experimental results for isotopomers of acetaldehyde indicates that these values are reasonable and support the notion that the level of theory used yields relevant results in the case at hand. The similarity of the zero-point difference between the rotamers in the ground and excited state contrasts strongly with the large difference in torsional barrier: 700-800 cm-1 for the ground state and 95 cm-1 for the triplet state of acetophenone-d0; the latter value is derived from the tunneling splitting observed in the excitation spectrum. These contrasting results are due to the fact that the barrier is an electronic effect whereas the rotamer splitting is a purely vibrational effect.