Vibrational spectra, bond strengths anharmonicity, conformation and substitution effects in polyatomic molecules
This thesis is mainly concerned with the measurement of infrared isolated MH stretching, frequencies, v IS/MH (M belonging to Groups IV and V elements of the periodic table) and the extraction of useful chemical information from these v IS/MH values. In the first group of studies, v IS/CH frequencies in acetyl compounds, CHD₂ COX (X = halogen, methoxy and cyanide); ethyl cyanide; halomethyl methyl ethers; methyl arsines and ethyl germanes are measured. In nearly all cases (except the ethyl cyanide), the CH bonds within the same methyl or methylene group are of differing strengths. The effects of substitution and conformation on v IS/CH are investigated. In the second group, v IS/GeH, values are obtained in a series of alkyl, halomethyl and methoxy germanes. Abundances of conformers in ethyl and isopropyl germanes are examined through their v GeH spectra. For the alkyl germanes, a correlation between v IS/GeH and the sum of Taft δ* values is obtained. This is different from those for halogenated or phenyl germanes. Earlier conclusions about dπ - pπ bonding between Ge atom and substituent are implausible. In the third group, the fundamental (v IS/MH) and the first overtone (2v IS/MH) frequencies in silanes, germanes, phosphines and arsines are measured. Anharmonicities and dissociation energies were calculated, based on the diatomic=approximation. Finally, v IS/CH and 2v IS/CH in ethane, propane, isobutane and neopentane are obtained. 3v IS/CH are also obtained in ome cases. The decrease in the anharmonicities calculated from v IS/CH / 2v IS/CH and 2v IS/CH / 3vIS/CH values suggest the possibility of Fermi resonance. The anharmanicities calculated from v IS/CH / 2v IS/CH values differ significantly from the local mode anharmonicities of the ordinary species.