An Infrared Study of Intermolecular Interactions in Gaseous Hydrogen Bonded Molecular Complexes

Abstract: Intermolecular interactions are responsible for the deviations of gases from ideality, determine the three- dimensional structure and folding dynamics of biological molecules, and the binding of molecules to surfaces as in heterogeneous catalysis. The hydrogen bond is the most important intermolecular interaction. The most simple intermolecular interaction free from solvent and lattice effects arises when two chemical species are allowed to combine into a weakly bound gaseous molecular complex. In the present doctoral thesis the intermolecular interactions in a number of simple binary prototype molecular complexes held together by hydrogen bonds are investigated. The molecular complexes are studied by means of static gas-phase high-resolution absorption spectroscopy in the far- and near-infrared spectral regions. The high-brightness source of synchrotron radiation from the electron storage ring MAX-I at MAX-lab in Lund is used as a far-infrared radiation source. The far-infrared absorption spectra of the hydrogen-bonded molecular complexes OC-HCl, HCN-HCl and ammonia-HCN are recorded and provide information about the intermolecular hydrogen bond vibrations of the molecular complexes (Papers III-VI) introduced upon the complex formation. These floppy intermolecular hydrogen bond vibrations are directly correlated to the relative translation (stretching of the hydrogen bond) and the internal rotations of the subunits (bendings of the hydrogen bond). The spectral analyses allow the characterization of the intermolecular potential energy surface for these molecular complexes over a wide range of geometries away from the equilibrium structure. The near-infrared absorption spectra of the hydrogen-bonded molecular complexes HCN-HCN and HCN-HCl are recorded and provide information about the modification of the intramolecular force fields within the donor subunits by the incorporation into the bimolecular complex (Papers I and II).