Investigations of structure and conformation. Part 10. Electron spin resonance and INDO studies of conformational interconversion in the tetrahydrofuran-2-yl and tetrahydrothiophen-2-yl radicals

Abstract

E.s.r. spectra have been recorded, over a wide range of temperature, for the radicals tetrahydrofuran-2-yl, 2,5-dimethyltetrahydrofuran-2-yl, and tetrahydrothiophen-2-yl. The spectra for tetrahydrofuran-2-yl indicate that, even at –125 °C, pseudorotation is rapid; in contrast, the spectra from its sulphur-containing analogue show that a preferred conformation is frozen out at low temperature. For 2,5-dimethyltetrahydrofuran-2-yl, the presence of the 5-methyl group causes conformational preferences to be established. Analysis of the splittings, in conjunction with INDO calculations on a variety of possible geometries, suggests that the favoured conformation in each case is a specific envelope type. For tetrahydrothiophen-2-yl, the barrier to pseudorotation (ca. 17.0 kJ mol^–1) is considerably greater than for the oxygen-containing radicals.