Zero-Field Splittings in Some Triplet-State Aromatic Molecules

Abstract

Zero-field splitting constants for the lowest triplet states of naphthalene, benzene, triphenylene, and coronene were calculated using configuration-interaction Hückel molecular orbitals. The mixing coefficients for different configurations were used as parameters to fit experimental data. The two-, three-, and four-center dipolar interaction integrals were computed using both the Mullikan—Hameka approximation and the Gaussian wavefunction method. The latter method gave better results. When more and more multiple-center integrals were included, the results essentially converged. It was concluded that inclusion of all two-center Gaussian integrals sufficed in such calculations. First-approximation triplet wavefunctions were formed according to the conventional choice of configurations obtained from two highest filled and two lowest empty orbitals. Where these two orbitals are degenerate as in molecules with threefold symmetry the third highest filled and the third lowest empty orbital were found to be significant. The effect on zero-field splitting due to extended mixing of doubly excited and independently excited configurations was investigated. The introduction of negative spin correlation in triplet state due to configuration interaction was shown. Spin correlation and its positive and negative nature between various pairs of atoms at different interatomic distances were computed. Atom pairs were then grouped under two types (starred and unstarred) which behaved as an entity in a specific manner during a specific configuration interaction.