Methyl torsional fine structure in the high-resolution S₀→ T(nπ*) excitation spectrum of acetophenone and its three methyl-deuterated isotopomers

The cold-beam excitation spectrum of the S₀ → T(nπ*) 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-d ₁, 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 -d ₂. 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(nπ*) share the same torsional equilibrium configuration, relative to which the S₁ (nπ*) equilibrium configuration is rotated by 60°. However, both nπ* 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(nπ*) 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.