Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. The present paper focuses on the calculation of rate coefficients for rotational excitation of the hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) molecules in their ground vibrational state in collision with He. The calculations are based on new two‐dimensional potential energy surfaces obtained from highly correlated ab initio calculations. Calculations of pure rotational (de)excitation cross‐sections of HCN and HNC by He were performed using the essentially exact close‐coupling method. Cross‐sections for transitions among the eight first rotational levels of HCN and HNC were calculated for kinetic energies up to 1000 cm−1. These cross‐sections were used to determine collisional rate constants for temperatures ranging from 5 to 100 K. A propensity for even Δj transitions is observed in the case of HCN–He collisions whereas a propensity for odd Δj transitions is observed in the case of HNC–He collisions. The consequences for astrophysical models are evaluated and it is shown that the use of HCN rate coefficients to interpret HNC observations can lead to significant inaccuracies in the determination of the HNC abundance, in particular, in cold dark clouds for which the new HNC rates show that the j= 1 − 0 line of this species will be more easily excited by collisions than HCN. An important result of the new HNC–He rates is that the HNC/HCN abundance ratio derived from observations in cold clouds has to be revised from >1 to ≃1, in good agreement with detailed chemical models available in the literature.

Modelling of molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for (de‐)excitation by collisions with the most abundant species. We calculate rate coefficients for the rotational and hyperfine (de‐)excitation of the hydrogen cyanide (HCN) by collisions with H2 (j= 0), the most abundant collisional partner in cold molecular clouds. The scattering calculations are based on a new ab initio potential energy surface for the HCN–H2 collisional system, averaged over the H2 orientations. Close‐coupling calculations of pure rotational cross‐sections are performed for levels up to j= 10 and for total energies up to 1000 cm−1. The hyperfine cross‐sections are then obtained using a recoupling technique. The rotational and hyperfine cross‐sections are used to determine collisional rate coefficients for temperatures ranging from 5 to 100 K. A clear propensity rule in favour of even Δj rotational transitions is observed. The usual Δj=ΔF propensity rules are observed for the hyperfine transitions. The new rate coefficients are compared with the previous results obtained for the HCN molecule. Significant differences are found, mainly due to the use of H2 as a collisional partner instead of He. The new rate coefficients will significantly help in interpreting HCN emission lines observed with current and future telescopes.

The low-temperature rotational (de-) excitation of C 3 (X 1 AE þ g ) by collisions with He ( 1 S ) is studied using an ab initio potential energy surface (PES). This PES has been calculated using the single-and double-excitation coupledcluster approach with noniterative perturbational treatment of triple excitations [CCSD( T)] and the augmented correlation-consistent triple-basis set (aug-cc-pVTZ ) with bond functions. This PES is then incorporated in full close-coupling quantum scattering calculations for collision energies between 0.1 and 50 cm À1 in order to deduce the rate constants for rotational levels of C 3 up to j = 10, covering the temperature range 5Y15 K.

Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. From a new, highly correlated, two‐dimensional potential energy surface, rotational excitation of the C2H(X2Σ+) molecule by collision with He is investigated. State‐to‐state collisional excitation cross‐sections between the 25 first fine structure levels of C2H are calculated for energies up to 800 cm−1 which yields after thermal averaging rate coefficients up to T= 100 K. The exact spin splitting of the energy levels is taken into account. The recoupling technique introduced by Alexander & Dagdigian allows us to deduce the corresponding temperature‐dependent hyperfine state‐to‐state rate coefficients. Propensity rules are discussed.