Spin–spin dipolar and exchange interactions in crystalline bisgalvinoxyl biradical

Abstract

A static magnetic susceptibility study has been carried out on a stable crystalline bisgalvinoxyl biradical in the temperature range 55–330 °K. The study has established that a negative intramolecular spin–spin exchange interaction (J) exists between two unpaired electrons in the biradical, giving rise to a ground singlet state and a thermally accessible triplet state. The singlet–triplet separation (2‖J‖/k) derived from the observed temperature dependence of the magnetic susceptibility is 490±10 °K (341±7 cm−1). The existence of the triplet state of the bisgalvinoxyl has been further confirmed by the observation of the ESR zero-field splitting of its powder sample, by reducing the triplet entities at the low temperatures. The ESR spectrum at 77 °K can be clearly analyzed as the characteristic spectrum of a nonaxially symmetrical triplet, with the zero-field splitting parameters ‖D‖=76.0±0.3G, ‖E‖=14.5±0.3G, and gxx, gyy, and gzz, equal to 2.0042±0.0002, 2.0059±0.0002, and 2.0032±0.0002, respectively. When the temperature is increased, the zero-field components are broadened, then collapsed into a single, sharper line, because of temperature-dependent intermolecular exchange interactions. The D- and E-parameters were calculated for the assumed molecular structures, using McLachlan’s spin densities of galvinoxyl, which is a monoradical half of the bisgalvinoxyl.