Theoretical studies on radicals.

This thesis is concerned with the calculation of spin density distributions in molecular systems with an unpaired electron. The methods employed are briefly discussed, and the Unrestricted Hartree Fock method with annihilation is adopted as the principle tool for the investigation carried out. The effect of the hyperfine coupling constants of the solvent employed is studied for p-benzosemiquinone and its methyl substituted analogues. Three variants of the Pariser-Parr-Pople method for ? electron systems are employed. Solvation is represented by a change in the oxygen parameters and the various other parameters involved are optimised. Excellent agreement with experiment is obtained. The structures of the pyrazine-lithium ion pair and the monoprotonated form of the PYRAZINE anion are studied using the CNDO/2 method. The results obtained for the lithium ion pair are in disagreement with experiment and this is attributed to the neglect of the solvent. The use of annihilation operators in the calculation of spin density distributions is investigated. The error involved in applying the annihilator to the wave function that minimises the Unrestricted Hartree Fock energy, rather than applying the variation principle to the annihilated wave function, is found not to be significant. The possibility of employing a basis set of Gaussian functions in ab initio calculations of hyperfine coupling constants is considered. Two such calculations on the hydroxyl radical are reported results compared with those obtained by other authors