The vibrational spectra of molecular ions isolated in solid neon. IX. HCN+, HNC+, and CN−

Abstract

When a Ne:HCN sample is codeposited at ∼5 K with a beam of neon atoms that have been excited in a microwave discharge, infrared absorptions of HNC, HCN+, and CN− appear. The absorptions of HNC isolated in solid neon are considerably closer to the gas-phase band centers than are the previously reported argon-matrix absorptions. The CH-stretching fundamental of HCN+ is identified, and the two stretching force constants of HCN+ are determined, using the isotopic data obtained in this study and the CN-stretching frequencies of HCN+ and DCN+ obtained in earlier photoelectron spectroscopic studies. In the inert, nonpolar environment provided by the neon matrix, the infrared absorption of CN− appears at 2053.1 cm−1, very close to the position obtained in two recent ab initio calculations. HCN+ photoisomerizes to HNC+ with a threshold in the near-infrared spectral region, consistent with an earlier ab initio calculation of the position of the transition state for this process. The reverse photoisomerization has also been observed at somewhat higher energies. All three vibrational fundamentals of HNC+ have been identified, and the stretching force constants have been determined. In all but the most dilute samples, HCNH+ is also stabilized in detectable amounts. Electron photodetachment from CN− results in charge neutralization of HCN+ and HCNH+ and in growth of the infrared absorption of CN.