Detection of O2(1Δg) phosphorescence emission, λmax = 1270 nm, following laser excitation and steady-state methods was employed to determine the total rate constant, kT, and the chemical reaction rate constant, kR, for reaction between 5,6-disubstituted-2,3-dihydropyrazines and singlet oxygen in several solvents. Values of kT ranged from 0.26 × 105 M-1 s-1 in hexafluoro-2-propanol to 58.9 × 105 M-1 s-1 in N,N-dimethylacetamide for 5,6-dimethyl-2,3-dihydropyrazine (DMD) and from 5.74 × 105 M-1 s-1 in trifluoroethanol to 159.0 × 105 M-1 s-1 in tributyl phosphate for 5-methyl-6-phenyl-2,3-dihydropyrazine (MPD). Chemical reaction rate constants, kR, for DMD are similar to kT in polar solvents such as propylencarbonate, whereas for MPD in this solvent, the contribution of the chemical channel to the total reaction is about of 4%. Dependence of the total rate constant on solvent microscopic parameters, α and π*, for DMD can be explained in terms of a reaction mechanism that involves formation of a perepoxide exciplex. Replacement of the methyl by a phenyl substituent enhances dihydropyrazine ring reactivity toward singlet oxygen and modifies the dependence of kT on solvent parameters, specially on the Hildebrand parameter. These results are explained in terms of an additional reaction path, involving a perepoxide-like exciplex stabilized by the interaction of the negative charge on the terminal oxygen of the perepoxide with the aromatic π system.
Áreas temáticas de ASJC Scopus
- Química orgánica