An electron spin resonance study of the radical anions of 5H-dibenzo[a,d]cycloheptene and related compounds

Abstract

The e.s.r. spectra of the radical anions of 5H-dibenzo[a,d]cycloheptene (I) and two related ketones 5H-dibenzo[a,d]cyclohepten-5-one (II) and 10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-one (III) have been obtained and interpreted. Coupling constants have been assigned to sets of equivalent protons for the radical anions of (II) and (III) with the aid of simple Hückel MO and McLachlan SCF calculations. In the latter radical anion the four β protons exist as two equivalent pairs at the temperature of the measurements. The e.s.r. spectrum of the radical anion of (I) varies markedly with temperature and counterion. At –70 °C the radical anion exists as a single conformer with quite different coupling constants for the two β protons. As the temperature is raised an e.s.r. spectrum is obtained (with Li⁺ as the counterion; at +48 °C) which can be satisfactorily interpreted in terms of two equivalent β protons. However the e.s.r. spectrum of this radical anion is much more complex at +45 °C when Na⁺ is the counterion.