Collisional Excitation and Weak Maser Action of Interstellar Methanimine.

The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH2NH) by cold H2 have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root-mean-square error of ∼1-2 cm-1 in the region of the potential well whose depth is -374.0 cm-1. The rotationally inelastic rate coefficients have been combined with spectroscopic data to generate the nonequilibrium spectrum of CH2NH toward the interstellar molecular cloud Sgr B2(N). The transition 110 → 111 at 5.29 GHz is found to be inverted, and the predicted weak maser emission spectrum is in excellent agreement with new observations performed with the 100 m Green Bank Telescope.