The geometry and intermolecular binding of HCN…BrC1 probed by rotational spectroscopy

Ground-state rotational spectra of several isotopomers of a linear complex formed between HCN and BrCl were observed by pulsed-nozzle, F-T microwave spectroscopy and analysed to give the spectroscopic constants B₀, D_J, _χ(Br), _χ(Cl), and M_bb(Br). The order HCN…BrCl of the nuclei was established from the changes in the B₀ values on isotopic substitution. The small fractional electronic charge (δ ≈ 0.05) transferred from Br to Cl on complex formation and the intermolecular stretching force constant (k_σ ≈ 11 N m⁻¹) estimated from the spectroscopic constants indicates a weak interaction of HCN and BrCl, with little perturbation of the charge distribution of BrCl.