Optically Detected Magnetic Resonance Spectra Research Articles

ChemInform Abstract: OPTICALLY DETECTED MAGNETIC RESONANCE SPECTRA OF THE LOWEST TRIPLET STATES OF BENZOPHENONE, 13C‐BENZOPHENONE, AND THREE 4,4′‐DIHALOBENZOPHENONES

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 parameter...

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

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.

Zero-Field Splitting of the Triplet State in Zinc Etioporphyrin

The zero-field splitting (ZFS) of the 3Eu state of Zn porphyrin is treated theoretically. Three types of interaction are discussed: (i) spin-orbit coupling Hso, (ii) spin-spin coupling Hss, and (iii) crystal-field coupling Hcef. Four coupling cases are defined depending on the relative sizes of these three terms. Experimental data on ZFS by optically detected magnetic resonance (ODMR) are given for Zn etioporphyrin. The data can be explained assuming either Hso ≪ Hss or Hcef ≫ Hso. In either case we find D ≈ 0.035 cm−1, a value obtained earlier by EPR studies on the triplet. For a 3Eu case, in addition to the usual E parameter there is a parameter E′. The ODMR experiments show that | E | + | E′ | ≲ 0.005 cm−1. The ZF ODMR spectrum is unusually broad. This can be explained by the effect of Hcef in the case Hso ≪ Hss or by the effect of Hso in the case Hcef ≫ Hso.