Semi-empirical molecular orbital calculations on the interaction between singlet oxygen and amines: modeling charge transfer quenching

Abstract

Semi-empirical molecular orbital calculations were performed using MINDO/3 to examine the mechanism of quenching of singlet (excited) molecular oxygen by amines. Amines with reported ionization potentials and singlet oxygen quenching rate constants were modeled with singlet oxygen at various distances from the amine nitrogen, and the enthalpies of formation of the resultant supramolecules were graphed vs. the nitrogen-oxygen separation. Primary amines gave a distinct energy minimum at a nitrogen-oxygen separation of about 1.54 Å and a nitrogen-oxygen-oxygen “bond” angle of about 119°. The electron density at nitrogen and the distal oxygen in the minimum energy complex, relative to that in the individual molecules, indicated a substantial (approximately 0.3 esu) transfer of charge from nitrogen to oxygen, consistent with a charge transfer complex. Secondary amines showed a less distinct energy minimum at the same nitrogen-oxygen separation, whereas tertiary amines gave only an inflection point. Because a charge transfer quenching mechanism requires intersystem crossing (singlet to triplet) during complexation, the upper limit of the energy of the charge transfer complex was also calculated by specifying a triplet state. This was substantially higher in energy than the singlet complex in the case of primary and secondary amines, but slightly lower in energy for tertiary amines. This calculated upper limit for the enthalpy of activation of intersystem crossing (quenching) via a charge transfer complex correlated well ( r = −0.97) with the logarithm of the quenching rate constant for a series of amines. The close proximity (1.54 Å) required between nitrogen and oxygen in the charge transfer complex explains the sensitivity to steric hindrance in the vicinity of nitrogen observed for singlet oxygen quenching rates by aliphatic amines. These data are consistent with a charge transfer mechanism of quenching of singlet oxygen by amines.