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Resolving a puzzling anomaly in the spin‐coupled generalized valence bond description of benzene
Author(s) -
Xu Lu T.,
Cooper David L.,
Dunning Thom H.
Publication year - 2020
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.26185
Subject(s) - chemistry , wave function , molecular geometry , valence bond theory , bond length , geometry , valence (chemistry) , benzene , basis set , ionic bonding , distortion (music) , computational chemistry , atomic physics , molecular physics , molecule , crystallography , physics , ion , mathematics , density functional theory , crystal structure , amplifier , optoelectronics , organic chemistry , molecular orbital , cmos
In an earlier study of benzene, Small and Head‐Gordon found that the spin‐coupled generalized valence bond (SCGVB) wave function for the π system predicted a distorted (non‐D 6h ) geometry, one with alternating CC bond lengths. However, the variations in the energy were very small and the predictions were made using a very small basis set (STO‐3G). We re‐examined this prediction using a much larger basis set (aug‐cc‐pVTZ) to determine the dependence of the energy of benzene on the distortion angle, Δθ CXC (Δθ CXC = 0° corresponds to the D 6h structure). We also found a distorted geometry with the optimum Δθ CXC being 0.31° with an energy 0.040 kcal mol −1 lower than that for the D 6h structure. In the optimum geometry, adjacent CC bond lengths are 1.3861 Å and 1.4004 Å. Analysis of the SCGVB wave function led us to conclude that the cause of the unusual non‐D 6h geometry predicted by the SCGVB calculations seems to be a result of the interaction between the Kekulé and Dewar components of the full SCGVB wave function. The addition of doubly ionic configurations to the SCGVB wave function leads to the prediction of a D 6h geometry for benzene and a dependence on Δθ CXC essentially the same as that predicted by the complete active space self‐consistent field wave function.