Chemical Bonding in Monosubstituted Aromatic Molecules from Full-Valence Modern Ab Initio Valence Bond Calculations

Modern ab initio valence bond theory is used to understand the bonding and electronic structure of nine monosubstituted aromatic molecules: fluorobenzene, aniline, toluene, phenol, benzonitrile, benzaldehyde, styrene, nitrobenzene, and benzoic acid. All sigma bonds and in-plane valence lone pairs are treated at the Generalized-Valence-Bond-Perfect-Pairing level while all the pi electrons are described at the Spin-Coupled level including up to all possible spin couplings. Through progressive restrictions on the spin function space, it is shown that in all the molecules considered there is no "pi bonding" between the substituents and benzene ring pi electrons. There are, however, quasi-classical electronic effects that indeed influence the aromatic electron sextet and the sigma bonds in different ways depending on the substituent. These effects are inferred not only by comparing the intrinsic energies associated with the aromatic electron sextet within the group function approach but also by considering the relative position of the centroids of the sigma and pi nonorthogonal singly occupied optimized orbitals. Thus, a comprehensive picture of the electronic effects of the substituents in the benzene ring is presented, identifying and separating the influences associated with the pi and sigma electrons. The obtained model is then compared against experimental physical and reactivity data, giving sound insights into the origin of the electronic effects of substituents on the aromatic ring. Therefore, within a modern ab initio valence bond context, the idea of resonance as a root cause or natural language to describe electronic effects on aromatic molecules should be revised.