Structures and vibrations of o-methylaniline in the S-0 and S-1 states studied by ab initio calculations and resonant two-photon ionization spectroscopy

Ab initio calculations show that the optimized structure of o-methylaniline (OMA) in the ground state is non-planar with the amino part having sp(3) h ybridization-like character due to the existence of lone-paired electrons o n the nitrogen atom. In the S-1 state, the C - N bond of OMA exhibits a par tial double bond character, indicating an enhanced interaction between the ring and the NH2 group. In addition, the NH2 and the CH3 substituents are r epelled from each other such that the hindrance of the methyl rotation by t he amino group is greatly reduced. Our experimental results from jet-cooled resonant two-photon ionization (R2PI) spectroscopy show that most active t ransitions are related to the in-plane ring vibrations and the methyl rotat ions. Spectral features corresponding to the in-plane substituent-sensitive stretching and bending vibrations of OMA in the S-1 state are observed for the first time. All the observed spectral bands have been successfully ass igned on the basis of our computed results and comparison with those of oth er relevant substituted benzenes. (C) 1999 Elsevier Science B.V. All rights reserved.