Vibrational spectra, force fields and structures of polyatomic molecules
This Thesis reports the results of detailed isotopic studies carried out on three main groups of compounds, MMe2(M= Zn,Cd,Hg), MH 4(M= Si,Ge,Sn) and B(OMe)3 (Me&61 12CH 3,13CH3,12CD3). The infrared and Raman spectra of four isotopic species, 12CH 3, 13CH3, 12CD3, 13CD3, of dimethyl zinc, cadmium and mercury have been recorded, the fundamental frequencies corrected for Fermi resonance and anharmonicity and the empirical force field of each compound calculated. By incorporating interaction force constants determined by ab initio methods, semi-empirical force fields have also been calculated for these three compounds. A comparison of the scaled ab initio force fields with the empirical force field shows that properly constructed basis sets which employ effective core potentials are capable of producing results of similar quality to near Hartree-Fock calculations on molecules containing lighter atoms. The absolute infrared intensities in the 12CH3 isotopes of dimethylzinc, cadmium and mercury and in (12CD3)2Zn have been measured and the associated electrooptical parameters and atomic polar tensors calculated. The electrooptical models which appeared capable of explaining hydrocarbon infrared intensities have been shown to be invalid for these organometallic compounds. Atomic polar tensors have proved to be the more satisfactory means of representing infrared intensities generally. The absolute infrared intensities of SiH4, GeH4 and SnH4 have been remeasured in order to clarify the confusion over previous conflicting results. The unusual intensity patterns and the technological importance of these compounds in chemical vapour deposition made more precise intensity data desirable. The link between the gas phase intensities of these compounds and their gas-crystal shifts has been discussed and the electrooptical parameters and atomic polar tensors have been calculated for all three compounds. Three isotopes of trimethoxy boron; B(O12CH3)3, B(O13CH 3)3 and B(O12CD3)3 have been prepared and their infrared and Raman spectra recorded. The new 13C shifts allowed the spectra to be analysed in greater detail than was previously possible. A rudimentary force field of trimethoxy boron has also been calculated.