Rotational Excitation of Vinyl Cyanide by Collisions with Helium Atoms at a Low Temperature

Among the numerous molecular systems found in the interstellar
medium (ISM), vinyl cyanide is the first identified olephinic nitrile.
While it has been observed in various sources, its detection in Sgr
B2 is notable as the 211–212 rotational
transition exhibits maser features. This indicates that local thermodynamic
equilibrium conditions are not fulfilled, and an accurate estimation
of the molecular abundance in such conditions involves solving the
statistical equilibrium equations, taking into account the competition
between the radiative and collisional processes. This, in turn, requires
the knowledge of rotational excitation data for collisions with the
most abundant species, He or H2. In this paper, the first
three-dimensional CH2CHCN–He potential energy surface
is computed using the explicitly correlated coupled-cluster theory
[(CCSD(T)-F12] with a combination of two basis sets. Scattering calculations
of the rotational (de-)excitation of CH2CHCN induced by
He atoms are performed with the quantum mechanical close-coupling
method in the low-energy regime. Rotational state-to-state cross sections
derived from these calculations are used to compute the corresponding
rate coefficients. The interaction potential exhibits a high anisotropy,
with a global minimum of −53.5 cm–1 and multiple
local minima. Collisional cross sections are calculated for total
energies up to 100 cm–1. When the cross sections
are thermally averaged, collisional rate coefficients are determined
for temperatures up to 20 K. A propensity favoring the transitions
with Δka = 0 is observed.