Isotopic effects on the rotational (de-)excitation rate coefficients of ortho-CH3Cl colliding with He.

Chloromethane, CH3Cl, is the first organohalogen molecule to be detected in the interstellar medium. Using the recently generated accurate three-dimensional potential energy surface (3D-PES) for the weakly bound CH335Cl–He complex, we deduced that of CH337Cl–He. Both 3D-PESs were incorporated later into dynamical computations code for nuclear motions treatments of each isotopologue colliding with He. We determined the state-to-state (de-)excitation cross sections for transitions among the 105 lowest rotational levels of both ortho-CH335Cl and ortho-CH337Cl (up to jkɛ = 206±). For total energies up to 100 cm−1, we used the quantum close-coupling methodology, and we adopted the coupled state approach for higher energies (up to 2500 cm−1). Rate coefficients for temperatures ranging from 5 to 150 K are then deduced after averaging these cross sections over a Maxwell–Boltzmann velocity distribution. Computations show that isotopic effects are weak, validating the use of collision rates of ortho-CH335Cl for ortho-CH337Cl and vice-versa. However, we expect that our results will be helpful to accurately determine the abundances of these isotopologues in the interstellar medium and, more generally, for understanding the complex and still not fully elucidated chemistry of chlorine containing molecular species there. [ABSTRACT FROM AUTHOR]