A Quasi-Diabatic Representation of the 1,21A States of Methylamine.

Abstract

We report a full 15-dimensional two-state quasi-diabatic potential energy matrix (PEM) for the 1,21A states of methylamine (CH3NH2) suitable for the description of its two distinct photodissociation channels, CH3NH2(11A) + hν → CH3NH2(21A) → CH3 + NH2 or CH3NH + H. The PEM is fit to ab initio electronic structure data (energies, energy gradients, and derivative couplings) obtained from multireference configuration interaction single and double excitation wave functions at 7732 geometries, using a diabatic representation based on symmetry-adapted polynomials. The root-mean-square error is 78.34 cm-1 for the energies and 2.98% for the gradients. The computed T0(21A) = 42 461 cm-1 (41 669 cm-1), D0(CH3NH+H) = 33 639 cm-1 (34 250 cm-1), and D0(CH3+NH2) = 28 543 cm-1 (29 300 cm-1) are in good agreement with the experimental values given in the parentheses. The absorption spectrum of CH3NH2 is computed using this diabatic PEM and a normal mode based kinetic energy operator. Agreement with experimental data is quite satisfactory. Our accurate representation of the PEM over a wide range of nuclear configuration space offers the possibility for quality nonadiabatic dynamics simulations involving the CH3 + NH2 and CH3NH + H dissociation channels. Surface hopping trajectories are used to illustrate the types of nuclear motion supported by the PEM.