Protonation of azines: An ab initio molecular orbital study

Abstract

The structure, protonation energies, isomer stabilities and vertical ionization potentials of a wide set of azines were investigated by ab initio calculations using different basis sets ranging from minimal to split-valence plus polarization. Minimal basis tends to overestimate NN bond lengths. CN and CC bond lengths are also overestimated but to a lesser degree. In general 6-31G structures are very close to the experimental ones, while 3-21G basis does not always correct the deficiencies of the minimal basis. For those azines with basic centers in contact with the one which undergoes protonation, there are strong interactions between the acidic hydrogen and the second basic center which affect the gas-phase basicity of these compounds. 3-21G and 6-31G yield almost identical absolute protonation energies, about 30 kcal mol −1 smaller than those obtained at the minimal basis set level. The inclusion of polarization functions in the basis leads to a further decrease, this effect decreasing as the number of basic centers is increased. Our results illustrate the necessity of employing fully optimized structures to obtain reliable relative proton affinities. Basis set effects on relative proton affinities are analyzed as they are quite important. The effects of successive nitrogen substitutions on the gas-phase basicities are not additive. Koopmans' theorem predicts the wrong ionization for pyridine and diazines. In general N-lone-pair interactions are well reproduced at the 6-31G*/6-31G level.