Natural bond orbital (NBO) analysis of the angular group induced bond alternation (AGIBA) substituent effect

The geometries, natural charges, and resonance structures of 11 monosubstituted benzene derivatives were analyzed at the B3LYP/6-311++G(d,p) and HF/6-311++G(d, p) levels of theory. The following angular substituents were chosen: OCH3, CH2CH3, OH, SH, NHCH3, NHNH2, N=O, CH=CH2, N=CH2, N= NH, and CHO. The analysis of resonance structures was performed by using two different methodologies: harmonic oscillator stabilization energies (HOSE) and natural resonance theory (NRT). Also, the natural bond orbital (NBO) donor-acceptor stabilization energies for different resonance structures were calculated. We found that for all the substituents, the purely geometric resonance stabilization parameter (HOSE) is linearly correlated with quantum chemically derived resonance structure weight (NRT) of a given structure. Also, the calculations provide qualitative support for the earlier assumption of a through space angular group induced bond alternation (AGIBA) effect. Copyright (C) 2010 John Wiley & Sons, Ltd.