Calculation of the geometry of the complex of spatially hindered quinones and phenols from E.S.R. spectra of radical pairs

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Abstract
Geometry of the binary complex of 3,6-di-t-butyl-o-quinone and 2,4,6-tri-t-butyl phenol is investigated. Photolysis of this system produces two types of radical pairs with unequal zero-field splitting tensors. The values of the components, D and E, for each type of radical pair were calculated by diagonalizing the dipole-dipole interaction matrix; the elements of this matrix were calculated by using spin density distributions, either known from the experiment or obtained theoretically. The four parameters (three coordinates of the centre of the benzene ring in the phenoxy radical, and one Euler angle) were found by variational search techniques, through comparing the experimental and calculated values of the zero-field splittings. It is shown that benzene rings of the two radicals are displaced with respect to one another by approximately 1·6 Å and rotated relative to one another by 86·5°. This configuration is interpreted in terms of interaction between the t-butyl groups, hydroxyl and carbonyl groups.