Results from semi-empirical molecular orbital calculations on a series of pyridine and benzene esters and thioic S-esters (closed-shell) and their associated anion radicals (open-shell) using the MOPAC computer program, the AMI Hamiltonian and the RHF and UHF methods, have been compared with electrochemical and spectroscopic experimental data obtained in acetonitrile solution containing 0.1 M supporting electrolyte (Et4NPF6). The experimental data considered an reversible half-wave potentials obtained by differential pulse voltammetry and/or cyclic voltammetry, hyperfine splitting constants obtained by in situ electrochemical EPR experiments and absorption spectra obtained by in situ electrochemistry-UV-Vis spectroscopy.