G tensor components

Determination of g-tensor components from resolved 327-670 GHz EPR spectra allows differentiation between carotenoid radical cations and other C-H jt-radicals which possess different symmetry. The principal components of the g-tensor for Car"1 differ from those of other photosynthetic RC primary donor radical cations, which are practically identical within experimental error (Table 9.2) (Robinson et al. 1985, Kispert et al. 1987, Burghaus et al. 1991, Klette et al. 1993, Bratt et al. 1997) and exhibit large differences between gxx and gyy values. 

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

The valence CAS calculations overestimate the correlation corrections dramatically and predict rotational g tensor components which are much too small for all molecules. In CH4 it predicts even the wrong sign for the correlation correction. Similarly, MP2 is not able to reproduce the sign or size of the correlation correction. For HF and the in-plane component in H2O, the MP2 contribution is too small and for all the other molecules or tensor components the MP2 contribution is much too large and has even the wrong sign. 

On the other hand, the results of MP2 and valence CAS calculations are not useful. MP2 predicts correlation correction which are not only much too large but have also the wrong sign for most g tensor components studied here. In the remaining cases the correlation correction is either much too large (CH4) or much too small (HF and H2O gip). The valence CAS calculations similarly overestimate the correlation effects and predict in general much too small g tensor components. SOPPA and SOPPA(CCSD) performs thus clearly better than MP2 or valence CAS calculations for the rotational g tensor in the studied molecules. 

P700 can be created by light or chemical oxidation (ferricyanide) in PS I. To learn more about the electronic structure of this species different groups studied P 7o0 by EPR at different mw frequency to resolve the g tensor components.191 195 The results were compared with those obtained for monomeric Chi a +.195,196 it could be shown that the g-tensor could only be fully resolved at very high mw frequency (> 300 GHz)193 unless fully deuterated material was used.191 Another possibility is the investigation of P7J0 in PS I single crystals, which in addition yields information on the g-tensor principal axes in the molecular frame.192,197... 

Interesting observations have been made by Witt et al.m upon replacement of Thr(A739) in C. reinhardtii. This amino acid is hydrogen bonded to the 13 -keto group of PA. The ENDOR spectra showed clear changes of the major hfcs. In contrast to the histidine mutants, the hfcs are decreased. A similar mutant, TA(A739), has also been shown to have different g-tensor components than WT in C. reinhardtii. 95... 

The g-tensor components are usually not resolved for a triplet state of a porphyrin or chlorin molecule at X and Q band frequencies and only the zero field splitting (ZFS) parameters are obtained. This situation changed with the advent of high field/high frequency EPR techniques. The first data of 3P700 and 3Chl a were published by Poluektov et a/. 194 the spectra obtained at D-band are shown in Fig. 5. 

Recently, high field EPR allowed resolution of the g-tensor components of triplet states (see Chapt. 2.1, 3.1). Pashenko et al.23i studied 3P680 in a PS II RC preparation by EPR at 130 GHz over a temperature range of 50 to 90 K. The... 

Tyrosine Y/. Yb has been studied in detail by EPR and related techniques in preparations where YD was converted to a phenylalanine and the Mm-cluster removed. The early work has been reviewed3,418 Yb shows larger g-tensor components than Yb which is best seen in the highly resolved high field EPR experiments of Un et al.30-419, see Fig. 6. 

The experimental data in Table 7.7 indicates that the values of two g-tensor components are close for the structurally different PCs. Such is the case for the gzz component, whose value is the closest to the free-electron value g — 2.0023. The gyy component also exhibits little changes with changes in the composition of the silicon coordination sphere. This component lies in the symmetry plane of A-Si (0-Si=)2, in which the A-Si bond is located. The third component gxx proves to be most sensitive to the structure of coordination sphere. It is directed perpendicular to the symmetry plane of the paramagnetic center A-Si (Q-Si=)2 and, thus, is... 

Ag is the variation of the g-tensor components from 2.0023. This interaction arises from the coupling of the paramagnetic moment to the lattice through spin-orbit coupling as was derived by Moriya by extending Anderson s superexchange model " to include the spin-orbit effect. 

Because of our reliance here on ESR results, it is perhaps worthwhile to briefly summarize their benefits and limitations The spectra of randomly-oriented molecules (the general case) in matrices can provide the total spin (S), the g tensor components, the hyperfine tensor components with magnetic nuclei, and sometimes... 

The ratio of A/t can be determined experimentally from the g-tensor component of the Ru(ni) complex... 

The relative order of the g tensor components of signal B suggests a dz2 ground state for the single electron, and consequently a distorted tetrahedral [53] or trigonal-bipyramidal symmetry [54,55] for the corresponding Ni complex. Unfortunately, because of the poorly... 

Then the g-tensor components, ignoring the orbital reduction factor, become... 

When the g-tensor components are distinguished, one can use the fact that the scalar product of two vectors can be written in terms of the spherical tensor components, hence... 

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