SUBSTITUENT EFFECTS IN MONOSUBSTITUTED BENZENES - SEMIEMPIRICAL SCF STUDIES OF THE HAMMETT CONSTANTS

Semi-empirical SCF calculations using the AM1 Hamiltonian were shown t o account for the Hammett sigma constants and other properties of a re presentative series of 22 monosubstituted benzenes in terms of the ato mic charges at the ipso, ortho, meta and para positions. The trends of the correlations with experimental sigma constants were found to foll ow the normal expectations of physical organic chemistry. Additional s upport for the AM1 charge distributions was obtained in correlations w ith the C-13 chemical shifts of 12 of the test molecules. On the other hand, no correlations were found between sigma constants and computed force constants for bending peripheral H atoms out of the plane of th e aromatic ring, supporting the view that Hammett sigma constants refl ect charge distributions and not secondary effects expressed in the st iffness of bonds.