SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS ON THE INTERACTION BETWEEN SINGLET OXYGEN AND AMINES - MODELING CHARGE-TRANSFER QUENCHING

Semi-empirical molecular orbital calculations were performed using MIN DO/3 to examine the mechanism of quenching of singlet (excited) molecu lar oxygen by amines. Amines with reported ionization potentials and s inglet oxygen quenching rate constants were modeled with singlet oxyge n at various distances from the amine nitrogen, and the enthalpies of formation of the resultant supramolecules were graphed vs. the nitroge n-oxygen separation. Primary amines gave a distinct energy minimum at a nitrogen-oxygen separation of about 1.54 Angstrom and a nitrogen-oxy gen-oxygen ''bond'' angle of about 119 degrees. The electron density a t nitrogen and the distal oxygen in the minimum energy complex, relati ve to that in the individual molecules, indicated a substantial (appro ximately 0.3 esu) transfer of charge from nitrogen to oxygen, consiste nt with a charge transfer complex. Secondary amines showed a less dist inct energy minimum at the same nitrogen-oxygen separation, whereas te rtiary 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 tra nsfer complex was also calculated by specifying a triplet state. This was substantially higher in energy than the singlet complex in the cas e of primary and secondary amines, but slightly lower in energy for te rtiary amines. This calculated upper limit for the enthalpy of activat ion of intersystem crossing (quenching) via a charge transfer complex correlated well (r = -0.97) with the logarithm of the quenching rate c onstant for a series of amines. The close proximity (1.54 Angstrom) re quired between nitrogen and oxygen in the charge transfer complex expl ains the sensitivity to steric hindrance in the vicinity of nitrogen o bserved for singlet oxygen quenching rates by aliphatic amines. These data are consistent with a charge transfer mechanism of quenching of s inglet oxygen by amines.