Magnetic resonance studies of cation radicals from chromans. Part 1.—Electron spin resonance and ENDOR spectroscopy of some tricyclic chromans

Abstract

The e.s.r. spectrum of the cation radical from 2,3,4,7,8,9-hexahydro-2,2,5,7,7,10-hexamethylbenzo[1,2-b:4,5-b′]dipyran is temperature dependent and shows the alternating line-width effect from interconversion.

ENDOR and TRIPLE resonance measurements provided precise values of hyperfine coupling constants and their relative signs at the slow limit. These data enabled us to simulate spectra from the slow to the fast limits, which are observed at ca.–70 and ca. 100 °C, respectively. The activation parameters for the interconversion process were found to be +28(± 4) kJ mol^–1, +50(± 3) kJ mol^–1 and +71(± 12) J mol^–1 K^–1 for the free energy, enthalpy and entropy, respectively, at 298 K. The e.s.r. spectrum of the cation radical from 2,3,4,5,6,7-hexahydro-2,2,7,7,9,10-hexamethylbenzo[1,2-b:4,3-b′]dipyran is also temperature dependent, but the slow limit is observed at room temperature and the alternating line-width effect is masked: activation parameters for the interconversion were found to be +42(± 6) kJ mol^–1, +29(± 3) kJ mol^–1 and –46(± 9) J mol^–1 K^–1 for the free energy, enthalpy and entropy, respectively, at 298 K. A third tricyclic chroman, namely 2,3,6,7-tetrahydro-2,2,4,6,6,8-hexamethylbenzo[1,2-b:4,5-b′]difuran, gives a cation radical which has a temperature-independent e.s.r. spectrum, owing to interconversion, which is rapid on the e.s.r. time-scale.