Observation and rovibrational analysis of the intermolecular HCl libration band ν16 of HCN-HCl, DCN-HCl and H13CN-HCl

The high-resolution far-infrared absorption spectrum of the intermolecular HCl libration band nu(1)(6) (nu(B)) of the gaseous molecular complex (HCN)-C-12-HCl and the two isotopically substituted species (HCN)-C-13-HCl and (DCN)-C-12-HCl is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 205 K using an electron storage ring source. The rotational structure of the nu(1)(6) band has the typical appearance of a perpendicular type band of a linear polyatomic molecule. The structure is analyzed using a standard semi-rigid linear molecule model including l-type doubling to yield the band origin nu(0), together with values for the upper state rotational constant B', the upper state quartic centrifugal distortion constant D'(J) and the value for the l-type doubling constant q(6). The values for the ground-state spectroscopic constants B '' and D ''(J) for (DCN)-C-12-(HCl)-Cl-35 and (HCN)-C-13-(HCl)-Cl-35 are determined for the first time by ground state combination difference analyses. A number of nu(1)(6) + nu(1)(7) - nu(1)(7) and nu(1)(6) + 2 nu(7)(2) - 2 nu(7)(2) hot bands are observed in the spectra and the sum of the anharmonicity constants X-6,X-7 + g(6,7) is estimated. The observed decrease of the rotational constant B together with the simultaneous increase of the quartic centrifugal distortion constant D-J upon excitation of the HCl libration mode indicate that the hydrogen bond in the molecular complex is significantly destabilized upon intermolecular vibrational excitation. The calculated harmonic force constants for the intermolecular hydrogen bond stretching vibration nu(sigma) for the ground state and the excited HCl libration state indicate that the excitation of the HCl libration mode destabilizes the intermolecular interaction between HCN and HCl by almost 20%. The hydrogen bond is elongated by 0.030 angstrom upon excitation of the nu(1)(6) mode.