Ab initio through-space/bond interaction analysis of the long C-C bonds in Bi(anthracene-9,10-dimethylene) photoisomers

The bi(anthracene-9,10-dimethylene) photoisomer has remarkably long C-C single bonds. To examine the lengthening of the C-C bond, we propose a novel procedure for quantitatively analyzing orbital interactions in a molecule at the level of the ab initio molecular orbital method. In this procedure, we can cut off the specific through-space/bond interactions in a molecule by artificially increasing the absolute magnitude of the exponents in a Gaussian function. Then, the spatial orbital interactions were perfectly cut off, and, each term that makes up the total energy, that is, the nuclear-electron attractions, the electron-electron repulsions, and the nuclear-nuclear repulsions cancel each other. Several model molecules of the photoisomer were analyzed by this procedure. It was found that the orbital interaction between the p orbital on the benzene ring and the σ* orbital on the C-C bond in question, σ → σ* electron transfer through π orbital, weakens the C-C bond efficiently when these orbitals were located in the "periplanar" conformation.