Van der Waals interaction of HNCO and H2: Potential Energy Surface and Rotational Energy Transfer.

Abstract

Isocyanic acid (HNCO) is the most stable of all its isomers; it has been observed repeatedly in many different conditions of the Interstellar Media, and its chemistry is poorly known. To quantitatively estimate the abundance of HNCO with respect to other organic molecules, we compute its rotational quenching rates colliding with H2, the most common gas in the gaseous Interstellar Media. We compute ab initio the van der Waals interaction HNCO-H2, in the rigid molecules approximation, with a CCSD(T)-F12a method. On the fitted ab initio surface, inelastic scattering cross sections and rates are calculated for a temperature range of 7-200 K, with the coupled-states quantum time-independent formalism. The critical densities are high enough to yield rotational temperatures of HNCO differing significantly from the kinetic temperature of H2, especially so for the shorter wavelengths observed at the ALMA interferometer. It is found that the quenching rates for collisions with ortho- or para-H2 differ greatly, opening the possibility of far from equilibrium populations of some rotational levels of HNCO.