METHYL TORSIONAL FINE-STRUCTURE IN THE HIGH-RESOLUTION S0-]T(N-PI-STAR) EXCITATION SPECTRUM OF ACETOPHENONE AND ITS 3 METHYL-DEUTERATED ISOTOPOMERS

The cold-beam excitation spectrum of the S0 --> T(n-pi*) origin band of acetophenone shows two maxima separated by about 1.2 cm-1, interpreted as due to methyl tunneling. Consecutive deuteration of the methyl group leads to three maxima in acetophenone-d1, two maxima in acetophenone-d2 and a single broad maximum in acetophenone-d3, with maximum separations of about 7 cm-1 in -d1 and about 3.5 cm-1 in -d2. These separations are due to the occurrence of two rotamers for isotopomers without threefold permutation symmetry. The spectra are analyzed in terms of hindered rotor potentials. It is shown that S0 and T(n-pi*) share the same torsional equilibrium configuration, relative to which the S1 (n-pi*) equilibrium configuration is rotated by 60-degrees. However, both n-pi* states have very shallow torsional potentials with barrier heights of 70 and 90 cm-1 for T and S, respectively, compared with a barrier of about 800 cm-1 in the ground state. Hence, relative to S0 the two potentials are quite similar despite the shift. The potential terms entering upon partial deuteration are quite similar for S0 and T(n-pi*) and hence depend only weakly on the barrier height. They are ascribed to kinetic coupling of the asymmetric rotor of the CH2D or CHD2 group with other modes in the molecule.