Electronic Properties of 4-Substituted Pyridines and Their Pyridinium Cations and Dihydropyridyl Radicals in the Ground State

Abstract

The dependence of the stabilities of 4-substituted derivatives of pyridinium cation and 1, 4- dilaydrc, -4-pyridyl radical on their substituent groups, their charge densities and hyperfine splitting constants of 4-substituted dihydro pyridyl radicals were investigated by using the INDO method. and the following results were obtained, (1) As to 4-substituted pyridinium cations, an electron donating group makes the cation more stable than an electron attracting group. On the other hand, the stability of a 4-substituted dihydro pyridyl radical depends on the electron attracting character of the substituent group. An electron attracting group makes the dihydro pyridyl radical more stable than an electron donating group. (2) The stabilization energies zIET of 4-substituted derivatives of pyridinium cation can be correlated with the Hammett constants for the substituted groups. (3) The unpaired electron orbital energy of a dihydro pyridyl radical with an electron donating group is higher than that of the radical with an electron attracting group, the former being more unstable than the latter. (4) The substituent groups have a small effect on the calculated hyperfine coupling constants of the parent part of 4-substituted dihydropyridyl radicals.