Electron Distribution in 1,2,3-Benzotriazole and 1,2,3-Triazole Anion Radical Isomers: An EPR and DFT Study

The anion radicals of N1- and N2-alkylbenzotriazoles and alkyltriazoles (alkyl = methyl or isopropyl) have been generated by low-temperature potassium metal reduction in tetrahydrofuran. Electron paramagnetic resonance (EPR) analysis and density functional theory calculations reveal that the electron spin distribution within the triazole ring of these systems is markedly different. The magnitude of the electron-nitrogen couplings along with the calculated spin densities reveals that the N2-alkylbenzotriazole and N2-alkyltriazole anion radicals have significantly greater electron spin residing within the N₃ portion of the triazole ring compared with that of the respective N1 isomers. These differences impact the overall geometry of the triazole ring where both N2-isomers lose planarity upon reduction. Experimental and computational results reveal that the N2-methyltriazole anion radical has the largest concentration of electron spin residing in the N₃ moiety compared to that of the other three anion radicals studied. Significant anisotropic line broadening is observed in the EPR spectrum of the N2-methyltriazole anion radical, which is a consequence of the large nitrogen hyperfine couplings and sufficiently slow rotational motion of this species in solution.