Radical cations of tetrahydropyran and 1,4-dioxane revisited: quantum chemical calculations and low-temperature EPR results

Abstract

Radical cations of tetrahydropyran (THP) and 1,4-dioxane have been radiolytically generated at low temperature in freon matrices and investigated by EPR spectroscopy. Quantum chemical calculations at DFT and ab initio MP2 levels were performed to obtain information on electronic and molecular structure, energy levels and hfs splitting constants of the species. Both THP and 1,4-dioxane radical cations are stable in CF₃CCl₃ up to 140 K. For the THP radical cation two conformers were identified in the EPR spectra, namely chair and a twist. The chair is the most stable conformer and the twist conformer was observed as an average structure, resulting from the fast interconversion through ring puckering. The neutral radical is formed from the THP radical cation by deprotonation on position 2 at high THP concentrations. In the case of 1,4-dioxane only one stable conformer (chair) of the radical cation was observed. None of the studied compounds forms a stable radical cation with boat conformation, the latter can be described only as a transition structure.