Ab initio study of the structure of aniline in the S1 and S2 ππ* states

Ab initio calculations at the CIS level of theory combined with several basis sets were performed to elucidate the geometry of the two lowest ππ* singlet states of aniline. A global picture emerges from this study. The 6-31++G* basis set was the only one to give convergence for the S1 and S2 states while, for example, the CIS/6-31G* results refer to the S2 state and the S1 optimization does not converge. Aniline in the S1 state is quinoidal; in the S2 state the ring expands relative to the ground state. Planar geometry is predicted for both states.