Structures and vibrations of o-methylaniline in the S0 and S1 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 sp3 hybridization-like character due to the existence of lone-paired electrons on the nitrogen atom. In the S1 state, the C-N bond of OMA exhibits a partial double bond character, indicating an enhanced interaction between the ring and the NH2 group. In addition, the NH2 and the CH3 substituents are repelled from each other such that the hindrance of the methyl rotation by the amino group is greatly reduced. Our experimental results from jet-cooled resonant two-photon ionization (R2PI) spectroscopy show that most active transitions are related to the in-plane ring vibrations and the methyl rotations. Spectral features corresponding to the in-plane substituent-sensitive stretching and bending vibrations of OMA in the S1 state are observed for the first time. All the observed spectral bands have been successfully assigned on the basis of our computed results and comparison with those of other relevant substituted benzenes.