A Novel Energy Partition for Gaining New Insight into Aromaticity and Conjugation

Abstract

To gain new insight into the nature of aromaticity and conjugation, we have developed a novel procedure for constructing a localized fragment molecular orbital basis set. It is a three-step procedure: (i) obtainment of each subcanonical FMO (fragment molecular orbital) basis set from a specific double bond fragment and its fragment molecule; (ii) the localization of the canonical FMOs; (iii) the superposition of all sublocalized FMO basis sets. On the basis of our procedure, Morokuma's energy partition provides, in the framework of ab initio SCF-MO computation at the STO-3G level, each of 46 compounds with various energy effects. The π-energy difference in each of four fictitious electronic states between the experimental and dSHgeometries shows that the delocalized π-system is practically destabilized. The π-system always prefers a distorted geometry. The role of the π-delocalization, stabilizing or destabilizing, depends on the response of the σ-framework to the π-delocalization. In the case of benzene-like and condensed-ring species, the vertical resonance energy (VRE) is always stabilizing. However, it is the σ-framework, rather than the π-system itself, that is strongly stabilized by the VRE. The energy effect Δ of the π-delocalization on the π-system of the fragment itself is generally destabilizing, and it is found to be a Boltzmann model function of the net π charge transfer (CT) energy. The VRE of [N]annulene with 4N π-electrons is more destabilizing than that of [N]annulene with 4N + 2 π electrons is stabilizing. It appears to be a prerequisite to the ring current that the π CT forms two closed circuits around the aromatic ring. In the case of benzene-like and condensed-ring compounds, the chemical shift is the Boltzmann model function of the net CT energy. As far as the VRE and chemical shift are concerned, the furan-like species appears not to be aromatic. However, the five-membered ring is the most rigid, and its hydrogen atom is a good leaving group, leading to high reactivity toward the substitution by an electrophilic reagent. The fact that 3H2 is more stable than regular hexagonal H6 and its explanation imply that the delocalized σ-system is also destabilized.