LUMO energy, correlation with half-wave

The essential parameters which determine the electrochemical process are the electron affinity of the neutral compound, which correlates with the energy of the LUMO, the energies of interaction with the solvent and counterions, the electron-electron repulsion energies and stereochemical factors. A precondition for an electrochemical study is that the chemical reaction which may occur, e.g. with the solvent, is much slower than the electron transfer process, and that the electrochemical reaction is reversible 66). Correlation of half-wave potentials with the energies of Huckel LUMO s has been one of the early successes of the Huckel model 8>2°.67-88>. The power of the electrochemical method in the study of polycyclic anions has been demonstrated recently 69a). Studies on reactions occurring during electrochemical reductions report reductive alkylations of polycyclic systems and their mechanism 70,69b). 

The experimental observations on the mechanism(s) of electroreduction of 2-thio-pyrimidines have been interpreted on the basis of their electronic structures as calculated with the aid of the CNDO/2 and Huckel procedures l42). The energies of LUMO (lowest unoccupied molecular orbitals), calculated for pyrimidine and its 2-oxo-and 2-thioderivatives, were compared with the reduction half-wave potentials (Table V). These show that the presence of a carbonyl or thione substituent at C2 enhances the electron acceptor properties of the molecule, which are correlated with formation of a dimer susceptible to photooxidation. 

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