Interpretation of Substituent Angular Parameters of Monosubstituted Benzenes by Means of ab initio STO-3G Fully Optimized Molecular Structures and Charges Densities. Part VI of the Series: “Crystallographic Studies and Physicochemical Properties of π-Electron Systems”

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Abstract
The angular variations of molecular structures calculated for 17 monosubstituted derivatives of benzene by use of ab initio analytical gradient optimization at the STO -3G level have been correlated by multiple linear regression analysis with Mulliken σ- and π-electron charge densities at corresponding ring carbon atoms. Variations of angles γ at meta positions are solely due to π-electron effects (R = 0.961). Changes in angles β at ortho positions are independent of σ- or π-electron effects but may be related to steric effects (R = 0.719) alone or additionally to charge densities on ortho protons (for dual regression R = 0.846). Angles a at ipso atom s depend significantly on both, σ- and π-electron charge densities in the ratio of 70% to 21% with R = 0.974 for dual regression. Variations of angles δ para-positions depend on all three effects considered (with correlation coefficient for a triple regression R = 0.951) in the ratio 37.6% :2 8 .3% : 34.1% for steric effects, σ- and π-electron charge densities, respectively.

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