Vibrational Spectroscopy of Hydrogen Cyanide Clusters

Article abstract of DOI:10.1021/j100321a041

Molecular beams of monomer HCN and its clusters, generated by supersonic helium expansions, were crossed with the output of an F-center laser operating in the C-H stretching region.The laser was scanned in its low-resolution mode Δν_1/2 = 10 GHz (0.3 cm^-1), from 3100 to 3400 cm^-1 while the molecular beam energy was monitored with a liquid helium cooled bolometer.Excitation spectra of monomer HCN and vibrational photofragmentation spectra of HCN clusters were recorded.Bands associated with unresolved vibration-rotation envelopes were assigned to dimer and trimer transitions.Features corresponding to tetramers and higher polymers were observed but remain unassigned.Under some expansion conditions a photofragmenting solidlike hydrogen cyanide feature was observed.A band corresponding to one of the bound C-H stretching vibrations in the linear trimer was studied under high resolution (15 MHz).For the few comparisons possible, our C-H stretching frequencies are in agreement with other gas-pase measurements.Our results are also consistent with matrix isolation studies when allowances are made for the large solvent shifts possible.Three theoretical approaches were used to predict trends in the C-H stretching frequencies on hydrogen bonding.These are normal-coordinate analysis, a classical electrostatic model, and ab initio quantum mechanics calculations.A critique of these theoretical models is presented.Finally, the observed vibrational predissociation lifetime of the linear trimer is discussed in terms of simple propensity rules.