Spin–spin dipolar and exchange interactions in crystalline bisgalvinoxyl biradical

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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.

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