Nitrogen hyperfine interaction

Large changes in nitrogen hyperfine interaction are brought about by attachment either of alkoxyl groups or of acyl groups to the nitroxide nitrogen. In an alkoxy alkyl nitroxide, aN rises to nearly 30 G, whilst a typical acyl alkyl nitroxide has an aN-value of ca. 8 G, and in diacyl nitroxides this may fall below 5 G (Lemaire and Rassat, 1964). 

TABLE 10.4 Theoretical Values of the Eigenvalues of the Anisotropic Part of the Nitrogen Hyperfine Interaction Tensors"... 

The most significant are the Zeeman (g tensor) and nitrogen hyperfine interactions. These depend somewhat on chemical structure, solvent polarity, and temperature. An assumption has been made almost universally that magnetic parameters that are found in frozen solutions may be used to describe liquid phase spectra. This assumption should be recognized and considered carefully in precise work. It may be invalid for several reasons, including (1) altered distribution of nitroxide molecular conformations,... 

A,B are metal, and C,D are nitrogen hyperfine interaction constants in wave numbers. Q is a quadrupole interaction constant in oo... 

Although the principal hyperfine values of the heme and imidazole nitrogens lie on the order of 9 to 15 MHz, the effective hyperfine values in the heme plane are a factor 3 higher. [This is analogous to the difference between the g principal values in the S = 5/2 description (g = 2, gi = 1.98) and those in the = 1/2 description (gy eff = 2, gie = 6) [52]]. Using standard ESEEM experiments, effective nitrogen hyperfine interactions of 30-45 MHz cannot be observed. 

A detailed structural study has also been made of oxygenated cobalt(II) heme model systems and oxygenated Co(II) corrin complexes. " These used a combination of X-, Q- and W-band CW and pulsed EPR, X- and Q-band ENDOR, X-band HYSCORE and S-band ESEEM to determine the g and A tensors and investigate the proton and nitrogen hyperfine interactions. Both studies are excellent examples of the power of multi-frequency CW and pulsed ESR and ENDOR in determining the full electronic structure of metallo-complexes. 

The location of free radical solubilizates within the micelle has been determined from electron spin resonance (e.s.r.) spectra. As discussed in Chapter 3, these solubilizates can provide information on the nature of the various microenvironments of the micelle. Several workers have used nitroxide spin probes, the spectra of which are characterized by three sharp lines produced by nitrogen hyperfine interaction (Fig. 5.14). The distance between the resonance lines is determined by the hyperfine coupling constant, and this provides a sensitive... 

The spectrum of radical 101 appears as a quintet (1 2 3 2 1) caused by the hyperfine interaction (HFI) with two equivalent nitroxide nitrogen nuclei (<2n = 0.74 mT), each line of the quintet being additionally split due to hyperfine... 

The example of nitrogen lines in the spectrum of cobalamin points to the necessity of also writing out resonance conditions for the presence of ligand hyperfine interaction. In general we have ... 

In contrast to the narrow signal of H(I), which exhibits a single Lorentzian profile, the feature observed for H(IV) and H(V) was more complex (Fig. 23 lb) with splitting into five lines indicating a superhyperfine structure (line denoted A in Fig. 23 2). This phenomenon is due to the hyperfine interaction between localized spin and two neighboring nitrogen nuclei (nuclear spin... 

A characteristic of the magnetic resonance spectra of nitrosyl complexes is the presence of hyperfine interactions between the unpaired electrons and the nuclear spin of the nitrogen. The strength and orientation dependence of these interactions are determined by the geometry of the complex and the localization of unpaired electrons in ligand and metal orbitals. 

Coordination causes electron-spin density redistribution in the N-O fragment the contribution of resonance structure II increase. The redistribution of spin density results in changes in the parallel component of the nitrogen hyperfine tensor. TEMPO and anthraquinone (AQ) have been used in this way to probe the Lewis acidity of alumina and Li and Mg doped alumina matrices.176 The differences in the Lewis acidic strength towards TEMPO and anthraquinone are discussed. An interesting study has appeared aimed to study the guest-host interaction between poly(amidoamine) dendrimers labelled with nitroxides and several porous solids including alumina.177... 

Fig. 4. Simulated ESR spectrum of a flavin semiquinone considering only the hyperfine interactions from the strongly coupled (N(5) and N(10) nitrogens (left) and and experimentally observed ESR spectrum of a deuterated flavodoxin neutral semiquinone in HjO (right)...

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.

If a molecule contains an atom with a nucleus having nuclear spin I > 1/2 (e.g., N or Cl), the observed micro-wave spectrum will be more complex due to the electron-nuclear hyperfine interaction as well as nuclear qua-drupole interactions (23,24). For each of the three electron-spin directions, there are a number of different nuclear spin quantization directions. For nitrogen,... 

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