BOUND-STATES OF HEHCN - AB-INITIO CALCULATION AND HIGH-RESOLUTION SPECTROSCOPY

Several rotational levels in the lowest excited bending state of HeHCN have been observed at hyperfine resolution by electric resonance spectroscopy near 100 GHz. The observed transitions correlate to the j = 1 <-- 0 transition in the limit of free internal rotation. The ground state has been characterized by using millimeter wave/microwave double resonance. One-photon transitions in the ground state are not observable using electric resonance, due to poor focusing of the nominally j = 0 levels. Ground-state J = 1 <-- 0 and J = 2 <-- 1 transitions were measured at 15893.6108(41) and 31325.2443(82) MHz, respectively. Quadrupole coupling constants eq(J)Q were determined to be 0.1118(15) MHz for J = 1 and 0.199(12) MHz for J = 2. We have calculated rovibrational energies and wave functions arising from an ab initio intermolecular potential, calculated at the MP4 level using a large basis set containing bond functions. The potential is characterized by a well depth of 25 cm(-1) at the centers of mass separation R = 4.27 Angstrom. The global minimum occurs at the collinear He-H-C-N configuration, and the minimum energy rises monotonically, with large angular-radial coupling, as the HCN orientation angle theta increases from 0 to pi. Calculated and observed transition frequencies, including hyperfine structure, agree to within 10%. We have used the calculated Coriolis interaction energy to deperturb the measured ground-state spectroscopic constants. This procedure permits estimates of vibrationally averaged structural parameters. We find, for the ground state, [R(-2)](-1/2) = 4.23 Angstrom. Very large amplitude radial motion results from zero-point energy that is 75% of the 25-cm(-1) well depth. The hyperfine data reflect very weak anisotropy in the potential, with [P-2(cos theta)] = 0.092 (J = 1) and [P-2(cos theta)] = 0.115 (J = 2). These values are very close to [P-2(cos theta)] = 0, characteristic of a free internal rotor. The centrifugal distortion of eq(J)Q indicates that, as in the other rare gas-HCN complexes, significant angular-radial coupling causes the HCN to align with the intermolecular axis in the rotating complex.