Ab initio studies on structure, vibrational spectra and infrared intensities of HCN, (HCN)₂ and (HCN)₃

Equilibrium structure, harmonic force field and dipole moment derivatives of HCN and (HCN)₂ were investigated with the aid of ab initio methods applying extended basis sets and including electron correlation at the CPF level. Vibrational spectra and infrared intensities of HCN, (HCN)₂ and of several isotopically substituted species are reported. Using our previous SCF calculations on (HCN)₃ as a starting point a more approximate treatment of correlation effects on structure, vibrational spectra and infrared intensities of (HCN)₃was carried out. For the linear trimer (HCN)₃ we predict a structure with two nearly equal intermolecular distances in qualitative agreement with SCF results. Infrared intensities of individual modes change dramatically upon complex formation. Besides the expected increase in the intensity of C-H vibrations involved in hydrogen bonding, particularly strong intensity enhancements were observed for C=N vibrations in the dimer and trimer in agreement with experimental data by Maroncelli et al. and with the known C=N intensity in solid HCN.