Superhyperfine couplings

F superhyperfine couplings in the axially symmetric frozen solution spectrum [ReNF4] showing two pairs of sextets in parallel and perpendicular part. 

The isotropic g value, hyperfine coupling constant, and 13C superhyperfine coupling constant are found to be 1.981, 88 G, and 6.6 G, respectively. 

One important feature of Mov intermediate species during enzymatic reaction is the presence of superhyperfine splitting from a proton. Mov model compounds with coordinated N—H groups display superhyperfine coupling of the same order of magnitude as that found in molybdenum enzymes.1002... 

In contrast, the lack of superhyperfine splitting in the ferrocata-lase-NO complex would not have been foreseen if Ls s His. As pointed out by Yonetani et al. 88), a fast relaxation of electron spin may explain this behavior. Alternatively, if catalase-NO were to be a square pyramidal complex, the spin density may not extend to the nitrogen atom of the proximal histidine, and hence no superhyperfine coupling need occur. 

Superhyperfine interactions are rarely observed in the ESR spectrum of the VO + ion because the unpaired electron interacts only weakly with the hgand nuclei, so that often the size of the coupling is less than the ESR bandwidth. This problem has been surmounted through the use of ENDOR spectroscopy. In ENDOR spectroscopy, molecules with their V=0 axes either parallel or perpendicular to the direction of the static magnetic field are selectively irradiated. In this way, the anisotropic superhyperfine coupling constants of H and " N and the " N quadrupolar coupling constants can be obtained. 

Dinuclear Cu11—Viv complexes exhibit ferromagnetic coupling for the single unpaired electron in the d9 and d1 ions.706 Other heterodinuclear complexes in which superhyperfine coupling has been observed include the dinuclear Vlv—Tl1 enzyme complexes in pyruvate kinase707 and S-adenosylmethionine synthase.708... 

A wide range of five- and six-coordinate porphyrin complexes of the types [MoO(por)]+ and trara-[MoO(por)X]0/+ ((191), Scheme 8 X including OH, Cl, Br, NCSA OR, 02CMe, 03SPh, N03, H20, dmso) have been reported. These exhibit axial EPR spectra, with g ca. 1.965-1.977 and AMo ca. 55 x 10 4 and 46 x 10 4 cm 1 for five- and six-coordinate species, respectively theoretical calculation of the g tensors has advanced considerably.595 In six-coordinate species, superhyperfine coupling to the porphyrin nitrogens (dN 2x 10 4cm ) leads to characteristic nine-line resonances. The chemistry, electrochemistry, photochemistry, and catalytic properties of oxo-Mo porphyrins have been reviewed (see Scheme 8). The structures of... 

Left two-pulse [(a) primary ESEEM] and three-pulse [(b) stimulated echo ESEEM] sequences t is the (fixed) delay time between pulses one and two and T is a variable delay time. Right frequency domain and time domain (inset) of the two-pulse EESEM spectrum of VO - vanabin, recorded at the m = — 1 /2 line, at 77 K and a pulse width of 20 ns.P l The superhyperfine coupling constant = 4.5 MHz (obtained from the N double-quantum lines at 3.9 and 7.1 MHz) is in accord with amine nitrogen provided by lysines of the vanadium-binding protein. The spin echo due to proton coupling, at 13.7 MHz, was also observed. Reproduced from K. Eukui et al., J. Am. Chem. Soc. 125, 6352-6353. Copyright (2003), with permission from the American Chemical Society. 

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