Ab Initio Treatment of the Chemical Reaction Precursor Complex Br(²P)−HCN. 2. Bound-State Calculations and Infrared Spectra^†

Rovibronic energy levels and properties of the Br(²P)−HCN complex were obtained from three-dimensional calculations, with HCN kept linear and the CN bond frozen. All diabatic states that correlate to the ²P_3/2 and ²P_1/2 states of the Br atom were included and spin−orbit coupling was taken into account. The 3 × 3 matrix of diabatic potential surfaces was taken from the preceding paper (paper 1). In agreement with experiment, we found two linear isomers, Br−NCH and Br−HCN. The calculated binding energies are very similar:  D₀ = 352.4 cm^-1 and D₀ = 349.1 cm^-1, respectively. We established, also in agreement with experiment, that the ground electronic state of Br−NCH has |Ω| = (¹/₂) and that Br−HCN has a ground state with |Ω| = (³/₂), where the quantum number, Ω, is the projection of the total angular momentum, J, of the complex on the intermolecular axis R. This picture can be understood as being caused by the electrostatic interaction between the quadrupole of the Br(²P) atom and the dipole of HCN, combined with the very strong spin−orbit coupling in Br. We predicted the frequencies of the van der Waals modes of both isomers and found a direct Renner−Teller splitting of the bend mode in Br−HCN and a smaller, indirect, splitting in Br−NCH. The red shift of the CH stretch frequency in the complex, relative to free HCN, was calculated to be 1.98 cm^-1 for Br−NCH and 23.11 cm^-1 for Br−HCN, in good agreement with the values measured in helium nanodroplets. Finally, with the use of the same potential surfaces, we modeled the Cl(²P)−HCN complex and found that the experimentally observed linear Cl−NCH isomer is considerably more stable than the (not observed) Cl−HCN isomer. This was explained mainly as an effect of the substantially smaller spin−orbit coupling in Cl, relative to Br.