Hyperfine couplings HFC constant

In the anion-radicals of nitro compounds, an unpaired electron is localized on the nitro group and this localization depends on the nature of the core molecule bearing this nitro substituent. The value of the hyperfine coupling (HFC) constant in the electron-spin resonance (ESR) spectrum reflects the extent of localization of the unpaired electron values of several nitro compounds are given in Table 1.1. 

Electron spin resonance spectra of ion radicals reveal a quantitative distribution of the spin density. The ESR spectrum determines the hyperfine coupling (HFC) constant for the ith... 

As a second point of the comparison, the selectivity factors mentioned are in accord with values of hyperfine coupling (HFC) constants in ESR spectra of the corresponding cation radicals. Thus, for the cation radical of 1,2,3,5-tetramethylbenzene, the HFC constant = 0.3 mT, whereas a V-Mc = 1.7 mT (Dessau et al. 1970). For the p-methy-lanisole cation radical, a= 0.02 mT and aMe = 1.5 mT for the anisole cation radical,... 

For the calculations of the hyperfine coupling (HFC) constants the unrestricted Becke s UB3LYP hybrid functional in combination with the triple-zeta EPR-llI basis set [50] implemented in Gaussian 98 program [45] was used. Solvent effects on the hyperfine constants were estimated using the polarizable continuum model (PCM) [51] implemented in Gaussian 98 [45]. The isotropic HFC constant, a, is related to the spin density at the corresponding nucleus by ... 

Here, the spin-spin interaction between an electron spin and a nuclear spin can be represented by the isotropic term (AS I ) when the radical rotates very fast in solution. This term is called the hyperfine coupling (HFC) and the coefficient (A) the (isotropic) HFC constant. When it is fixed in a crystal or solid matrix, however, the interaction becomes anisotropic, but the latter case is not considered in this section. 

We have also interpreted the zero-field splitting (ZFS) and hyperfine coupling (HFC) parameters of the triplet state. The lowest spin sublevel has the spin quantization axis perpendicular to molecular plane in agreement with ODMR and EPR experiments. Our interpretation of the HFC constants for protons accords in the... 

As mentioned for the relationship between the PE spectrum of a parent molecule and the electronic spectrum of its radical cation, any close correspondence between the electronic spectra of anions and cations or their hyperfine coupling patterns holds only for alternant hydrocarbons. The anions and cations of nonalternant hydrocarbons (e.g., azulene) have significantly different hyperfine patterns. Azulene radical anion has major hyperfine splitting constants (hfcs) on carbons 6, and 4,8 (flH = 0-91 mT, H-6 ah = 0-65 mT, H-4,8 ah = 0-38 mT, H-2) in contrast, the radical cation has major hfcs on carbons 1 and 3 (ah = 1.065 mT, H-1,3 Ah = 0.152 mT, H-2 ah = 0.415 mT, H-5,7 ah = 0.112 mT, H-6). °°... 

When ZnTPP (TPP2- = teteraphenylporphyrin dianion 5.0 X 10-4 M) is oxidized by exactly 1 equiv. of Ru(bpy)3+ (bpy = 2,2 -bipyridine 5.0 X 10 4 M), ZnTPP+ is produced without leaving any neutral species. In such a case, no self-exchange electron transfer of ZnTPP+ with ZnTPP occurs when the ESR spectrum of ZnTPP+ detected at 233 K exhibits well-resolved hyperfine structures as shown in Fig. 13.2a. The hyperfine coupling constants (hfc) are determined by comparison of the observed spectrum with the computer simulation spectrum as shown in Fig. 13.2b. 

EPR spectra of organic radicals recorded in fluid solution present two principal parameters The g factor and the isotropic hyperfine coupling constant, hfc. The g factor characterizes the center of the EPR signal and spin orbit coupling leads to specific deviations from the value of the free electron, 2.0023. 

The same picture emerges from an electronic structure derived from interaction of the u and g combinations of the HOMOs of two molecules of 5 dianion and suitable f- and d orbitals of U+. Accordingly, we expect a correlation between the contact shifts and the hyperfine coupling constant (hfcs)06 of the radical anion. The comparisons in Table 2 show an excellent parallelism between the two sets of numbers. 

Hyperfine coupling of the unpaired electron with fluorine 3-atoms is also anisotropic with the minimum and maximum values observed at a = 0° and a = 90°, respectively. The isotropic HFC constants for fluorine atoms can be assessed by the formula... 

To determine how the electronic structure of D is influenced by the mutation we studied the spin density distribution in the cation radical by ENDOR and Special TRIPLE [14-16] spectroscopy. These techniques were used earlier to measure the spin density distribution of D in RCs of Rb. sphaeroides R-26 [7-9]. It was established that the unpaired electron in is delocalized over both BChl a molecules. The spectral characteristics of this delocalization have been explained by a model in which the unpaired electron is equally shared between the two halves of the dimer [17] this results in a halving of the isotropic hyperfine coupling constants (hfc s) of the dimer with respect to the monomer. According to this model the width of the EPR spectrum of the dimer should be reduced by V2. More refined models for the delocalization of the unpaired electron have now been proposed to account, for example, for the fact that the reduction factors of the isotropic hfc s deviate from 2, (see, for example, [9]). Irrespective of the details of the model, a delocalization over both halves of the dimer should manifest itself in a reduction of the hfc s and a reduction of the EPR linewidth compared with the monomer, whereas when the unpaired electron (hole) is localized on one half of the dimer, the hfc s and linewidth should be that of the monomer. 

Table 1. Methyl proton hyperfine coupling constants (hfc s) [MHz] and ratios of hfc s in BChl a and in D+ of Rb. sphaewides wild type and mutants...

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