Electron spin label hyperfine interactions

Consider for example the simplest possible system consisting of the muon, an electron, and a single spin nucleus labelled i = n. Take the muon and nuclear hyperfine interactions to be istoropic. The level crossing of interest occurs near the field... 

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. 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.

Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the exact cancellation limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state. 

FIGURE 9.2 Vector models (projections) illustrating pair substitution. The labels 1 and 2 denote the electron spin of the first and the second radical of the pairs. The observed proton is contained in the first radical. Its spin state, la) or IP), is displayed at the respective leftmost projection. The radical pairs are bom in the triplet state, and the product is formed from the singlet state c gives the singlet character. First radical pair RPi, positive g-value difference, zero hyperfine coupling constant second radical pair RP2, equal g values, positive hyperfine coupling constant. For the situations without pair substitution, the spin evolutions under the influence of the Zeeman and the hyperfine interaction have been separated for clarity. Further explanation, see text. 

Electron nuclear double resonance is a powerful tool for the study of the electronic structure of triplet states because of its high precision. ENDOR linewidths can be as narrow as 10 kHz, which represents an increase in resolution of better than six orders of magnitude over that which can be obtained optically. The technique is particularly useful when combined with hf methods owing to the first-order nature of the hyperfine interaction in the presence of a field. Although such experiments are difficult, the information obtained is unique. Accordingly, the hf EPR (or ODMR) spectrometer has been modified for ENDOR operation in several laboratories. In order to illustrate the power of the method, we discuss here some recent optically detected hf ENDOR experiments on (njr ) benzophenone and its iso-topically labeled derivatives (Brode and Pratt, 1977, 1978a,b). The results, although incomplete, show considerable promise for the ultimate determination of the complete spin distribution in this prototype triplet state. 

To study the hyperfine interaction of a single molecular spin with individual nuclear spins one has to create a situation in which the hyperfine interaction with one or two nuclear spins dominates. This can be achieved by using isotopically labeled molecules such as pentacene in which all the protons have been replaced by deuterium except for the central positions. The magnetic moment of deuterium (7 = 1) is more than a factor of 6 smaller than that of a proton (7 = 1/2). Since the electron spin... 

Consider an unpaired electron on a hydrogen atom, which has a nuclear spin I =. The nucleus (a single proton) has two possible orientations of its spin, labeled by ntj = + j and mj = —Each orientation of nuclear spin will interact, or couple, with the unpaired electron in such a way that the local magnetic field experienced by the electron is slightly different The magnitude of this difference is represented by a hyperfine coupling constant a, such that... 

In principle, the polarity profile could be determined by measuring the three-hyperfine splitting constants T, Tyy and for the -N -0 group at various positions along the lipid chain. In practice, none of these parameters can be measured directly because of the partial motion average of the electron-nuclear dipolar interaction. An indirect method of obtaining the polarity profile is to estimate and Tj with the spin labels in randomly oriented samples. The -N -0 group at the C-5 position in the lipid matrix has been reported to be in an environment that is more polar than the C-12 and C-16 positions which are in a more hydrocarbon-like environment (Scheme 1). 

Electron-nuclear double resonance (ENDOR) studies of PFL-AE complexed to specifically isotopically labeled AdoMets has revealed the details of the interaction between AdoMet and the cluster in this enzyme. Deuterium ENDOR spectra of PFL-AE in the [4Fe-4S]" state complexed with methyl-D2-AdoMet showed a pair of peaks centered at the deuteron Larmor frequency and split by the hyperfine coupling to the spin of the cluster. Examination of the field-dependence of the coupling showed that it was dipolar in nature, and gave an estimation of the... 

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