Correlation between Hammett substituent constants and directly calculated pi-conjugation strength

The results of an energy decomposition analysis of ortho-, meta-, and para-substituted benzylic cations and para-substituted benzylic anions H2C-C6H4Rq (R = H, F, CN, Me, OH, NH2, NO2, CHO, CO2H; q = +, -) are presented and discussed. The calculated values for the pi bonding between CH2(q) and C6H4R show for substituents which have pi orbitals a linear correlation with the Hammett sigma(p), sigma(+)(p), and sigma(m) constants.