Rotational spectrum and properties of a complex of hydrogen cyanide and chlorine monofluoride

Rotational spectra of the five isotopomers (HCN)-N-14 ...(ClF)-Cl-35, (HCN)-N-14 ...(ClF)-Cl-37, (DCN)-N-14 ...(ClF)-Cl-35, (HCN)-N-15 ...(ClF)-Cl-35,, and (HCN)-N-15 ...(ClF)-Cl-37 of a complex of hydrogen cyanide and chlorine monofluoride were observed with a pulsed nozzle, Fourier transform microwave spectrometer. The component gases were brought together in a fast-mixing nozzle to preclude mutual chemical reaction. Groundstate spectroscopic constants B-0, D-J, chi(aa)(N-14), chi(aa)(Cl) and M(bb)(Cl) are reported. Interpretation of the spectroscopic constants with the aid of various models has led to the conclusions that HCN ... ClF is a linear molecule with nuclei in the order shown, that when HCN achieves its equilibrium position in the complex the electric perturbation of ClF is equivalent to a migration of a fraction delta approximate to 0.04 of an electronic charge from Cl to F, and that the complex is weakly bound according to the magnitude (k(sigma) = 12.3 N m(-1)) of the intermolecular stretching force constant. From a comparison of the family of complexes HCN ... XY, where XY = ClF, BrCl and Cl-2, it is concluded that all three can be described as Mulliken complexes of the weak, outer type.