A phenomenological use of benzene tip to probe aromaticity

Abstract

Interaction of a C–H tip of benzene with a monocyclic aromatic compound studied using M062X functional in DFT, yields a tilted T-shaped conformation with a tilt angle θ. For a set of ten monohetero five-membered rings C4H4X (X=BH, AlH, SiH2, CH2, O, S, SiH−, PH, NH, CH−), θ−1 is found to be linearly related to NICS(0), Mo-Schleyer resonance energy and Shannon’s aromaticity in excellent manners. Consequently θ can be used as an index of aromaticity of the system with which benzene is interacting. The larger the θ, the larger the aromaticity. The angle θ, which can vary from 90° to 180°, seems to have a threshold value of ∼115°; a θ value below 115° means non-aromaticity. The index shows that borazine, the inorganic benzene is aromatic. It reveals that cyclopropane is aromatic while cyclobutane is not. The present approach faces problems in polycyclic aromatic hydrocarbons as the benzene probe tends to span two adjacent rings. Finally, to test the efficacy of Truhlar’s functional M062X in DFT in describing non-bonded interactions, the case of benzene dimer has been studied. It is found that M062X/LanL2DZ or M062X/6-311++G(2d,p) can reproduce the experimental binding energy of the benzene dimer quite well, while B3LYP and BP86 cannot.