Electron-Nuclear Hyperfine Coupling

The unpaired electron in these two molecules is now in exactly the right orbital for the present purposes, since d —Px bonding is usually invoked in order to stabilise such antibonding or non-bonding orbitals. As we have seen both the nuclear quadrupole coupling constant and the anisotropic electron-nuclear hyperfine coupling constant are dominated... 

For those familiar with NMR spectroscopy it may be helpful to realize that the ESR g-shift is comparable with the NMR chemical shift. Similarly, electron-nuclear hyperfine coupling can be compared with nuclear-nuclear spin-spin coupling in NMR. (In systems containing more than one unpaired electron per molecule, electron spin-electron spin coupling is, of course, important. For doublet-state radicals, this coupling does not arise it is of great importance in triplet state molecules and in many high-spin transition metal complexes.)... 

Group 3 (Sc, Y, La) metallofullerenes exhibit ESR hfs, which provides us with important information on the electronic structures of the metallofullerenes. Typical ESR-active monometallofullerenes are La Cs2, Y Cs2, and Sc C82- The ESR hfs of a metallofullerene was first observed in La Cs2 by the IBM Almaden group (Johnson et al., 1992) (Eigure 15) and was discussed within the framework of an intrafullerene electron transfer. The observation of eight equally spaced lines provides evidence of isotropic electron-nuclear hyperfine coupling (hfc) to La with a nuclear spin quantum number I = 7/2. The observed electron g-value of 2.0010, close to that measured for the Ceo radical anion (Allemand et al., 1991 Krusic et al., 1991), indicates that a single unpaired electron resides in the LUMO of the carbon cage. They also observed hyperfine... 

Marks and co-workers (12) have studied the alkyl substituted compounds 7-16. Assuming that INDO/2 molecular orbital calculations on alkyl radicals can reasonably predict experimental electron-nuclear hyperfine coupling constants, a, they have calculated the a values for each of the alkyl substituents. Taking the ratio of the contact shifts of the ortho positions in 7 and vinylic position in 16 as equal to the ratio of calculated a values and the ratio of the geometry factors as equal to the ratio of pseudocontact shifts, Marks and co-workers could solve for the contact and pseudocontact shifts in 7 and 16. Factoring the... 

The possible contribution of electron-nuclear hyperfine coupling (HFC) as a mechanism for intersystem crossing between TT encounter pairs of different multiplicity was strongly suggested by recent resonance-Raman-spectroscopic determinations of k2 = 5.3 X 10 Af s and 2.1 X 10 s (Eq. 67)... 

Combines sensitivity of EPR and high resolution of NMR to probe in detail ligand superhyperfine interactions with metal center and to identify specific type of ligand Complementary technique to ENDOR for measuring very small electron-nuclear hyperfine couplings... 

In addition to the spatial structure of the reaction center (RC) of Rhodopseudomonas (Rps.) viridis obtained from X-ray crystallography (1) a knowledge of the electronic structure is required for a basic understanding of the functional details of the RC. This can be obtained for the radical ions formed in the charge separation process by EPR and ENDOR techniques (2) which yield electron-nuclear hyperfine couplings (hfc s). From the hfc s a map of the valence electron spin distribution over the molecule is obtained. In this work we studied the intermediate electron acceptor radical anion I", a monomeric bacteriopheophytin (BPh) b (3)> Fig. 1,that was trapped at low temperature (77 K) in the RC ( ). EPR and ENDOR results on I" had been reported earlier (5). Improved instrumental design enabled us to measure additional hfc s with higher accuracy. 

The rates of the electron transfer processes in reaction centers (RC s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC s has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis and for Rhodobacter (Rb.) sphaeroides,- The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques. The information is contained in the electron-nuclear hyperfine couplings (hfc s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules. 

The electronic structure of the cation radical can be obtained by measuring the electron-nuclear hyperfine couplings (hfc s). After assignment to specific molecular positions, a map of the valence electron spin density distribution over the molecule can be... 

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