Ab initio MO Study of Hydrogen Bonding and Spectral Characteristics of HCN-H2O-HCN Trimer: Comparison between Dimer and Trimer

Hydrogen bonding of HCN-H2O-HCN trimer has been studied by means of ab initio molecular orbital (MO) calculations. The changes in intermolecular interaction energy and vibrational frequency induced by addition of HCN molecule to HCN-H2O or H2O-HCN dimer are especially focused. The distances of hydrogen bonds in the trimer are calculated to be shorter than those in the corresponding dimers. The hydrogen bond distances are hence shortened by addition of another HCN. The interaction energies of the hydrogen bonds are also increased by addition of HCN. The dipole moment of the trimer is smaller than the sum of the dipole moments of the separate moieties (HCN + H2O + HCN). This is opposite of the previous result for H2O-HCN-H2O. The spectral shift of the stretching modes induced by hydrogen bond formation has been predicted by vibrational frequency analysis. The vibrational frequency of the asymmetric stretching mode of HCN included in the H2O-HCN part of the trimer is remarkably red-shifted from that of the H2O-HCN dimer. This information is expected to be useful for experimental detection of the trimer.