A statistical study of ab initio π charges

Abstract

A comparative analysis of the π electron density distributions derived from ab initio MO calculations was carried out for substituted ethylenes, substituted benzenes, 4-substituted styrenes and substituted pyridines. Changes in the effective atomic charges due to substitution in the molecules were considered. Linear correlations were found between the π charge changes on all atoms including ipso atoms of unsaturated molecules. The angle coefficients of the regressions for the π charge changes of the odd against even atoms proved to be negative due to the reversal of the substituent effect in the odd positions of unsaturated compounds. The regularities were found to be invariant with respect to both the level of ab initio calculations (3G, 4-31G, 6-31G and 6-31G +) and the definition of the atomic charge (the Mulliken population analysis or the Bader quantum topology approach). Moreover, the Δq π Δq′ π ratios for any two atoms of the molecule are equal for different basis sets in the framework of one charge definition, or for the different charge definitions at one basis set. In the framework of the Mulliken population analysis the regression coefficient for the acceptor substituents in linear relationship between the π charge changes obtained at different bases (6-31G/STO-3G) is larger than the regression coefficient for the donor substituents. The π charge changes obtained at one basis (6-31G+) using the different definitions of charge (the Mulliken and Bader ones) were shown to be proportional to each other for all unsaturated atoms including ipso atoms, with the correlation coefficient more than 0.99. Data from ab initio MO calculations reported in the literature were used.