title: Theoretical study on the photodissociation of methylamine involving S1, T1, and S0 states.
abstract: Various photodissociation pathways of methylamine involving the three lowest electronic states, namely, singlet ground S0 state, singlet first excited S1 state, and triplet ground T1 state, were studied by the (MS-)CAS(8e,8o)PT2/6-31++G** method. All critical points, i.e., minima, transition states, minimum energy conical intersections, and minima on the seam of crossing, were explored systematically by the global reaction route mapping (GRRM) strategy utilizing the anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods. On the basis of obtained structures, we discuss the photodissociation mechanism of methylamine in the experimental excitation wavelength range 222-240 nm in detail. Especially, the T1 potential energy surface was explored systematically for the first time. The N-H bond rupture is a primary channel on the S1 state. Along the N-H dissociation path on S1, there is a low-energy conical intersection (CI), and through this CI the system can go back to the S0 state; from the CI the system can directly dissociate to CH3NH + H or reproduce the original CH3NH2 on S0. There is a seam of crossing between S0 and T1 in a partially dissociated CH3---NH2 geometry, and through this seam the system may go up to the T1. On the T1 state, a roaming-like pathway giving CH4 + NH (X(3)Σ(-)) products was found, which would explain the recently proposed intersystem crossing mediated roaming dynamics.
authors.affiliation: Fukui Institute for Fundamental Chemistry, Kyoto University , 34-4 Takano Nishihiraki-cho, Sakyo, Kyoto 606-8103, Japan.
authors.forename: Hongyan
authors.initials: H
authors.lastname: Xiao
authors.forename: Satoshi
authors.initials: S
authors.lastname: Maeda
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma