An ENDOR study of the geometry of the lowest triplet state of benzil

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Abstract
We present the results of our optically detected ENDOR investigation of the lowest triplet state of benzil. The interaction between the total electron spin and nuclear spins placed at strategic positions throughout the molecule was used to probe the molecular geometry. 13C substitution at the carbonyl carbons led to the dihedral angle, while proton A tensors were used to deduce the ring twist. The geometric information resulted from a point–dipole calculation which reproduced the 13C and proton A tensors by optimizing geometry and spin density distribution. A dihedral angle of 157° was found, substantially larger than the ground state value of 111°, though significantly different from a t r a n s‐planar configuration. The ring twist angle was found to be 24°.