Article pubs.acs.org/JPCA
Theoretical Study on the Photodissociation of Methylamine Involving S1, T1, and S0 States Hongyan Xiao,†,‡ Satoshi Maeda,*,§ and Keiji Morokuma*,†,∥ †
Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihiraki-cho, Sakyo, Kyoto 606-8103, Japan Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan ∥ Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States ‡
S Supporting Information *
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)PT2/631++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 lowenergy 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 (X3Σ−) products was found, which would explain the recently proposed intersystem crossing mediated roaming dynamics. barriers along both the N−H and C−N fission reaction coordinates are small due to an avoided potential crossing of Rydberg-valence configurations.2 Butler and co-workers also reported pathways 1−4 by the photofragment translational spectroscopy study under collisionless conditions at the 222 nm3 and suggested that the N−H fission takes place through a conical intersection (CI) leading to the ground-state products, whereas the C−N rupture occurs on the electronic excited-state surface. The C−H rupture and H2 dissociation processes leading to channels 2 and 4, respectively, were not discussed well compared to pathways 1 and 3, and H2 elimination was assumed to proceed via a four-center transition state. Dunn and Morokuma further examined and confirmed the three mechanisms for pathways 1−3 using CASSCF and MRSDCI methods.4 In addition, the complete spectroscopic characterizations and vibrational structures of the à state (the first excited singlet state) of CH3NH2 and its isotopomers were reported.6,7 Kim and co-workers8 also investigated the
1. INTRODUCTION Methylamine (CH3NH2) is the simplest primary amine, which plays an important role in various fields of chemical science. Because of its simplicity and importance, the structural and spectroscopic properties, photodissociation dynamics and pathways have been widely investigated.1−12 Furthermore, the photodissociation of CH3NH2 might provide helpful insights and implications in understanding photoinduced processes of various compounds containing amine moieties. In 1963, Michael and Noyes detected four major pathways 1−4 following the broad-band excitation in the 194−244 nm and determined the corresponding quantum yields of pathways 1−4 to be 75%, ∼7.5%,
