MOLECULAR ZEEMAN AND NATURAL VIBRATIONAL OPTICAL ACTIVITY STUDIES OF SELECTED MOLECULES

Tam, Cheok Neng

Abstract

This thesis consists of two independent parts in the field of vibrational optical activity. Part I describes the magnetic vibrational circular dichroism (MVCD) studies of some small molecules in the gas phase and the fullerenes in solution while Part II presents experimental work on the natural vibrational circular dichroism (VCD) and Raman optical activity (ROA) of some medium-sized organic molecules.

MVCD spectra of some small gas phase molecules are studied. Molecular g-values for these molecules are determined and compared to the accepted values in the literature. Together with the theoretical simulation of MVCD spectra, MVCD is proven to be an alternative technique for the determination of the molecular Zeeman effect. MVCD is then applied to the determination of g-values for C₂H₂ in several excited vibrational states, including the v₄ + v₅ bending combination level of C₂H₂, C₂HD and C₂D₂, which are found to be quite different from the g-value in the ground vibrational state. The rotational g-values for the $nu_3$ fundamental C-D stretching bands for C₂HD and C₂D₂ have also been determined by MVCD which are virtually the same as their vibrational ground state g-values. Thus, the vibrational effect on the rotational g-values for acetylene and its isotopomers is discovered.

MVCD of C₆₀ in solution in the 1183 cm^-1 and 1430 cm^-1 t_1u modes are measured. The MVCD intensity seen is partly attributed to the vibronic coupling between the ground vibronic state and the low-lying T_1u electronic excited states through the t_1u vibrational modes. In addition, the MVCD of C₇₀ in solution has also been measured.

Backscattering Raman spectroscopy and ROA experiments are described in detail. The performance of the new backscattering ROA spectrometer is also discussed.

The experimental Raman, ROA, IR-absorption and VCD spectra of 5-methylpyrrolidinone in the mid-ir region are measured and compared to the theoretically predicted spectra from ab initio calculations and normal mode analysis. The above compound is found to consist of 2 conformers (axial and equatorial) at room temperature with a bigger proportion of the equatorial conformer.

Similar studies of natural vibrational optical activity are also carried out for 2,3-trans-dideuteriosuccinic anhydride and 3S-cis-3,6-dimethyl-1,4-dioxane-2,5-dione. By comparing their experimental spectra and the spectra calculated by ab initio quantum mechanical methods, their absolute stereochemical configurations are determined.