Calculation of the zero-field splitting parametersDof benzenoid hydrocarbons and their dependence upon the molecular structure

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Abstract
The zero-field splitting parameter D of the lowest excited triplet states of benzenoid hydrocarbons is calculated within semi-empirical π-theory. The calculated D values with an average absolute deviation of only 0·003 cm-1 from the experimental values are governed mainly by the contributions of all bonds D 01. By analysing D 01 it is shown that D depends crucially on the size and symmetry of the molecule as well as on the degree of localization of the molecular orbitals which are singly occupied in the leading configurations of the triplet wavefunction. Furthermore it is demonstrated that there is no general correlation between the degree of localization of benzenoid units and their contribution to D. The relationship between the probability density or the mean separation of the two unpaired electrons and D is investigated.