A REMPI and ZEKE spectroscopic study of a secondary amide group in acetanilide

Vibrationally resolved REMPI and ZEKE spectra of acetanilide are presented with assignments based on CASSCF/cc-pVDZ ab initio calculations and symmetry considerations of different methyl rotor states. ZEKE spectra selectively recorded via intermediate rotor states of a particular symmetry allow us to directly measure the splittings of the 0a1 and 1e rotor levels in the S0 state. Methyl group rotational features in the REMPI and ZEKE spectra are assigned from a simulation of methyl internal rotor levels in a one-dimensional model using a periodic V3-V6 potential, and barrier heights for methyl group rotation are extracted for the S0, S1, and D0 states. Additional vibrational features corresponding to displacements of the amide group and side-chain are identified and analyzed by comparison to formanilide which was studied previously (Ullrich, S.; et al. Phys. Chem. Chem. Phys. 2001, 3, 5450; Angew. Chem. Int. Ed. 2002, 41, 166).