Studies of zero-field splittings in aromatic molecules

The zero-field splitting parameters corresponding to exponential-orbital wavefunctions for benzene and naphthalene have been evaluated by the Gaussian transform technique. Comparison of the results obtained with and without multicenter integrals shows significant differences; in particular, an erroneous value for E in naphthalene is found if only two-center integrals are included. For the lowest triplet in benzene, the use of accurate integrals with the wavefunction of van der Waals and de Groot for the quinoid structure improves the agreement with the experimental D* value. In naphthalene, Pariser's wavefunction and accurate integrals give a. D that is close to the measured result; however, the E determined from the same wavefunction is considerably too small in magnitude. A qualitative examination of the effect of D on substitution in benzene and naphthalene is made. For benzene it is found that most substituents should decrease D or D*; only if a weak mesomeric interaction is involved (e.g., CH3) is an increase in D* expected. In naphthalene, the effect of substituents is expected to be considerably smaller than in benzene. Comparisons with the limited available data show agreement with these conclusions.