Abstract
To understand the role of π‐electron delocalization in determining the conformation of the NBA (Ph–NCH–Ph) molecule, the following three LMO (localized molecular orbital) basis sets are constructed: a LFMO (highly localized fragment molecular orbital), an NBO (natural bond orbital), and a special NBO (NBO‐II) basis sets, and their localization degrees are evaluated with our suggesting index D~L~. Afterward, the vertical resonance energy Δ__E__^V^ is obtained from the Morokuma's energy partition over each of three LMO basis sets. Δ__E__^V^ = Δ__E__~H~ (one electron energy) + Δ__E__~two~ (two electron energy), and Δ__E__~two~ = Δ__E__~Cou~ (Coulomb) + Δ__E__~ex~ (exchange) + Δ__E__~ec~ (or ΣΔ__E__~n~) (electron correction). Δ__E__~H~ is always stabilizing, and Δ__E__~Cou~ is destabilizing for all time. In the case of the LFMO basis set, Δ__E__~Cou~ is so great that Δ__E__~two~ > |Δ__E__~H~|. Therefore, Δ__E__^V^ is always destabilizing, and is least destabilizing at about the θ = 90° geometry. Of the three calculation methods such as HF, DFT, and MPn (n = 2, 3, and 4), the MPn method provides Δ__E__^V^ with the greatest value. In the case of the NBO basis set, on the contrary, Δ__E__^V^ is stabilizing due to Δ__E__~Cou~ being less destabilizing, and it is most stabilizing at a planar geometry. The LFMO basis set has the highest localization degree, and it is most appropriate for the energy partition. In the NBA molecule, π‐electron delocalization is destabilization, and it has a tendency to distort the NBA molecular away from its planar geometry as far as possible. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 809–824, 2006