We carried out quantum chemical calculations for the aryl-ring rotations in phenylporphyrin and ortho-methoxyphenylporphyrin to determine the reaction path and the potential-energy barrier so that one can provide an insight into how such rotations can occur. It was found that along the reaction path the structure changes from a planar porphyrin-ring at the stable state to a considerably distorted one at the transition state, and that the small potential-energy barrier, which is consistent with experiments, largely stems from the deformation of the porphyrin ring. An increase in the potential-energy barrier resulting from the substitution at the ortho position of the aryl group was also found to originate mainly from an additional deformation of the porphyrin ring at the transition state. These findings contrast with the calculated result for the aryl-ring rotation in the biphenyl, which the direct steric hindrance between the aryl rings rather than the deformation makes large contributions to the overall potential energy barrier. We concluded that the deformation of the porphyrin ring is essential for the occurrence of the aryl- ring rotation in the arylporphyrins.

DOI:10.1016/S0166-1280(02)00336-6