Multiple trapping sites and symmetry splitting in cryogenic matrices: Infrared spectroscopy of HCN in Ar, Kr, and Xe

Infrared spectra of HC14N and HC15N trapped in cryogenic matrices of Ar, Kr, and Xe and of DCN in Xe have been measured. In addition to the fundamental frequencies, overtone and combination bands along with their anharmonic constants are reported. In the Kr and Xe matrices several of the bands exhibit multiple peaks. Changes in the bands with temperature, concentration, annealing, and host indicate that there are two trapping sites. In Xe the degeneracy of the C–H bend ν2 is split in one site. The spectral data in Xe may be understood in terms of HCN in a substitutional site of the face-centered-cubic rare gas lattice with some molecules oriented along the 〈111〉 axis and others along the 〈110〉 axis. The low symmetry about the 〈110〉 axis removes the degeneracy between the two bending coordinates of an HCN oriented along it. Two sites occur in Kr. Although a splitting of the ν2 degeneracy is not resolved, the same sites are probably involved in both Kr and Xe. A single unsplit site is observed for Ar.