Optically detected magnetic resonance spectra of the lowest triplet states of benzophenone, 13C-benzophenone, and three 4,4′-dihalobenzophenones

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Abstract
We describe measurements of the complete angular dependence of the optically detected magnetic resonance (ODMR) spectra of the lowest triplet states of benzophenone, carbonyl‐13C‐benzophenone, 4,4′‐difluorobenzophenone, 4,4′‐dichlorobenzophenone, and 4,4′‐dibromobenzophenone in 4,4′‐dibromodiphenylether host crystals at liquid helium temperatures. The principal axes and values of D (including the absolute signs) and g are reported and used to evaluate the spin–spin and spin–orbit contributions to the fine‐structure tensor of the parent molecule. It is found that the dipolar contributions to D in 3(n,π*) benzophenone are at least a factor of 2 less than recent ab initio estimates of the spin–spin contributions to D in 3A2 formaldehyde. Carbon‐13 hyperdfine splittings have been observed in the ODMR spectra of 13C‐benzophenone; an analysis of these yields estimates of the orbital spin densities ρ2sC(O)?0.008 and ρ2pxC(O)?0.14 and suggests that the C1–C(O) –C1′ fragment is planar. The fine‐structure parameters of triplet benzophenone are strongly influenced by halogen atoms in the para ring positions. The ODMR spectra of the 4,4′‐difluoro and 4,4′‐dibromo derivatives also exhibit ring halogen atom hyperfine structure from which the estimate ρ2pπC4?0.10 is obtained. Taken together, the results of this investigation suggest that as much as 50% of the total spin density in the lowest triplet state of benzophenone itself resides on the aromatic rings, thereby accounting for the reduction in the dipolar contributions to D in the parent molecule and for the changes in the principal values (and directions) of D which occur in its heavy‐atom substituted derivatives.