Vibrational spectroscopic studies on some intermolecular complexes of the halogens.

Abstract

The work described in this thesis may be divided into three parts. The first part is an investigation into the changes which occur in the pyridine spectrum when the molecule complexes with the halogens or ICN. It has been shown that intensity changes are due to charge transfer and other (electrostatic) interactions. Normal co-ordinate calculations show only minor changes in vibrational mixing on complexation (see below).The second part of this work is a detailed investigation of the changes in the IX acceptor spectra on complexation with pyridine, dioxan, and other donor solvents. Studies on the variations of temperature and concentration were carried out to determine the nature of the absorptions in the very far-infrared (<150 cm. (^-1)) for these complexes. It has been shown that for the 'weaker' interactions (e.g. dioxan-I (_2)) the bands arise from a 'collisional' mechanism. However, for the longer-lived pyridine-X^ complexes the band is composite. This is partly due to a well-defined v (D-I) mode broadened by vibrational relaxation in a polar medium of excess donor. There is also a contribution from the Poley-Hill 'libration' of the complex dipole in solution. Band shape studies show that the rotation of the pyridine donor in solution is considerably restricted by intermolecular forces. The acceptor band profile reflects only vibrational and/or translational effects. The third part describes normal co-ordinate calculations for the 'tri-atomic' (D-X-Y) and 'whole molecule' models of the pyridine-IX (X = Cl, Br) complexes. The pyridine-d (_5) – IX complexes were used to calculate the interaction constant for the D-X-Y system. This information was used with the pyridine force field to calculate the frequencies and normal co-ordinates for the 'whole complex' molecule. It is shown that the frequency shifts observed on complexation are caused largely by the G matrix changes when the X-Y molecule is complexed. It was found that some mixing between the essentially D-I-X vibrations and the 'ring' modes does occur. The pyridine normal co-ordinates, however, change little on going to the complex, and the massive intensity changes thus appear to be due to electronic redistribution during vibration.