Collisional excitation of CN(X2Σ+) by para- and ortho-H2: Fine-structure resolved transitions.

We present a new four dimensional potential energy surface (PES) for the CN(X(2)Σ(+))-H2 system. Both molecules were treated as rigid rotors. Potential energy was obtained from the electronic structure calculations using a partially spin-restricted coupled cluster with single, double, and perturbative triple excitations method. The four atoms were described using the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis sets augmented with mid-bond functions for improved description of van der Waals interactions. The global minimum is characterized by the well depth of 121.36 cm(-1) for the linear CN⋅⋅⋅H2 structure. The zero-order corrected dissociation energies D0 are 27.73 cm(-1) and 38.75 cm(-1) for the complex with para- and ortho-H2, respectively. These theoretical results obtained using our new PES are in excellent agreement with experimental values [Y. Chen and M. C. Heaven, J. Chem. Phys. 109, 5171 (1998)]. We perform fully quantum close coupling calculations of the rotationally inelastic cross sections of CN in collisions with para-H2 and ortho-H2 at low and intermediate energies. Corresponding rate coefficients were compared with experimental results of Brunet et al. [J. Chem. Phys. 116, 3617 (2002)]. A good agreement between theoretical and experimental results was found. Fine-structure resolved cross sections were then obtained through a recoupling technique. Significant differences exist between para- and ortho-H2 results. The propensity rules between fine-structure levels are also studied, and it is shown that the cross sections for Δj = ΔN transitions are much larger than those for Δj ≠ ΔN transitions, as expected from theoretical considerations.