Structures and vibrations of p-methylaniline in the S0 and S1 states studied by ab initio calculations and resonant two-photon ionization spectroscopy

The structures and normal vibrations of p-methylaniline (pMA) in both the S-0 and S-1 states have been studied using ab initio quantum chemical calculations and resonant two-photon ionization (R2PI) spectroscopy. Calculated results at MP2/6-311+G** and CIS-MP2/6-311+G** levels show that the minimum energy conformations of pMA in both states have a hydrogen atom of the methyl group perpendicular to the plane of the ring. In the S-1 state, the molecule contracts along the in-plane long axis and the C-N bond exhibits a partial double bond character due to larger pi-electron conjugation as in the case of aniline, A comparison of the calculated excited and ground state vibrational frequencies has made it possible for us to successfully assign our measured R2PI spectral bands as well as the previously reported ground state features. (C) 1998 Elsevier Science B.V.