Transformations of 5-membered heterocyclic radical cations as studied by low-temperature EPR and quantum chemical methods

Abstract

Intramolecular transformations of 3-pyrroline (2,5-dihydropyrrole, 2,5-DHP) radical cation radiolytically generated in Freon matrices were investigated using low temperature EPR spectroscopy. DFT quantum chemical calculations made it possible to assign the observed transients and to explain the observed transformations. The primary nitrogen-centred radical cation 2,5-DHP⁺˙ is thermally stable, but on illumination with visible light of wavelengths λ > 600 nm it is transformed by intramolecular H-shift into 2,4-DHP⁺˙. The latter is stable at 77 K but its further intramolecular rearrangements through 2 → 3 and 4 → 3 H-shifts can be induced by illumination with visible light of shorter wavelengths (400 nm < λ < 600 nm). Both transformations proceed simultaneously, with similar yields of the two isomers of the DHP radical cation, namely 2,3-DHP⁺˙ and 3,4-DHP⁺˙. The overall transformation pattern is analogous to that established earlier for 2,5-dihydrofuran radical cation. In irradiated F-113 solutions of 2,5-DHP the spectrum of a neutral radical was observed and assigned to one of the two most stable possible structures of pyrrolinyl radicals.