Hyperfine coupling constants nitrogen

Although their chemistry is less developed than that of phosphonyl, phospho-niumyl or phosphoranyl radicals, many structural studies have been devoted to phosphinyl radicals [1]. Like their nitrogen analogs,phosphinyl radicals are 7i-type radicals (Fig. 1) and because of the very small s character of their SOMO, the magnitude of their phosphorus hyperfine coupling constants flp is below 15 mT [1]. 

The electron spin resonance of the nitroxalkylcorrinoids can be readily observed in aqueous solution at room temperature. Both the cobalamin and cobinamide show nitrogen hyperfine coupling constants of 17.2 gauss. A typical spectrum is shown in Fig. 20. The line widths for the low, intermediate, and high field peaks are 1.87, 1.87, and 2.20... 

E° vs. NHE for the couple Ni /Ni measured by cyclic voltammetry carbon paste electrode, scan rate 100 mV s, pH — 9.6 in aqueous 0.1 M NaCI04. Values in italics are estimated values. Nitrogen hyperfine coupling constant in gauss. c E° measured with [NH3] = 1M. 

Table 2.2 Atomic spin densities (in a.u.) and nitrogen isotropic hyperfine coupling constants (in gauss) computed for DM NO (1 in Figure 2.1) by different QM methodsa and the EPR-II basis set...

Table 2 Isotropic hyperfine coupling constant for the ground states of the boron atom (2PU), carbon atom (3Pg) and nitrogen atom (4S U), using different levels of the Cl treatment. NO s were used as one-particle basis.

Fig. 3. First derivative electron spin resonance spectra. (A) ESR spectrum of an unpaired electron. (B) ESR spectrum of an unpaired electron interacting with a nitroxide resulting in a nitrogen hyperfine coupling constant aN. (C) ESR spectrum of an unpaired electron interacting with a H nucleus and a l4N nucleus as is typical for PBN radical adducts. (D) ESR spectrum of an unpaired electron interacting with the l3C nucleus, the H nucleus and the 14N nucleus of the trichloromethyl radical adduct of PBN, where the carbon tetrachloride was labeled with 13C.

Wood et al. (141) were able to estimate that the unpaired spin-density is mainly localized in the p-orbital of the nitrogen, while the nitrogen lone pair of electrons is largely in the 2s orbital. The ESR hyperfine coupling constants of some alkylamino and alkylimino radicals are given in Table 10. 

The thiatriazinyl system (43), prepared by the reduction of the corresponding S-chloro derivative, is also a In-electron radical. In the solid state, it exists as a cofacial dimer with an S-S distance of ca. 2.67 A. In solution, the ESR spectrum of (43) gives a seven-line pattern with equal hyperfine coupling constants to all three nitrogen atoms. 

Cr(NH3)5(NO)] + (103), introduce nonzero overlap between the unpaired electron and the nitrogen cr-orbitals, which also makes a contribution to the hyperfine coupling constant. 

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