ENDOR induced ESR

As pointed out above, the most accurate and reliable information obtained by ENDOR is the resonance frequency that directly gives the hyperfine coupling con.stant. In this respect, the firequency-derivative ENDOR spectra obtained by cw-ENDOR detection by employing the frequency modulation scheme, are particularly important because of high-frequency resolution. Combined with ENDOR-induced ESR, the maximum ENDOR frequency, important in identifying the peak value of the spin density distribution, can be determined very accurately [7]. Specific examples are discussed in the cases of solitons in polyacetylene and polarons in poly(paraphenylene vinylene) in Sections 3.2 and 3.3. 

ENDOR frequency determination with the aid of ENDOR-induced ESR in cis-rich samples gives the half-width of the spin distribution of 18 carbon sites and the ratio of the peak value of the negative density to that of the positive density of p(l)/p(0)%0.44 [104,105], Similar results to those in cis-rich samples, that is similar spectra with resolved structures for the stretch direction and similar spectral frequencies, have been reported for stretch-oriented trans-polyacetylene prepared by the Durham route using pulsed ENDOR techniques [99,106,107], The unpaired electrons observed in Durham samples were conjectured to be trapped solitons from nearly temperature-independent ENDOR spectra. In the studies of this system the first successful application of TRIPLE resonance in polyacetylene has been reported [99,107], Readers can find reviews on the results of Durham samples [2,99] as... 

Figure 6.20. Anisopfropy of the frequency-derivative ENDOR spectra of a sfretch-onented ci j-rich sample for v. branch [104], The external field is parallel and perpendicular to the stretch direction of the sample for the thick- and thin-line curves, respectively. Positions A-C show the frequencies at which ENDOR-induced ESR are shown in Figure 6.22. Dashed lines show the frequencies corresponding to the hyperfine tensor components as indicated, (after ref [104])...

Overlapping spectra due to different species can be differentiated by ENDOR-induced ESR (EIE). 

The improved resolution in ENDOR spectra makes it possible not only to measure the hyperfine couplings with high accuracy, but also to observe splittings that are not resolved by ESR. An example is shown in Fig. 2.5 for the malonic acid radical, HCc (COOH)2 [24]. The ESR spectrum is a main doublet due to hyperfine coupling with the H at the Ca position. The resolution is limited by the line-width and the occurrence of forbidden and so-called spin flip lines discussed in Chapter 4. The ENDOR lines denoted V(t and Va are narrower than the ESR lines by more than an order of magnitude. As in the liquid state the intensities between the pair differ due to hyperfine enhancement and relaxation factors. The additional lines in the ENDOR spectrum were examined using the ENDOR Induced ESR (EIE) method described... 

Fig. 2.11 (a) ESR, (b) ENDOR, and (c) ENDOR-induced ESR (EIE) spectra of a single crystal of HC02Li-H20, X-irradiated to a dose of about 25 kGy and measured with the external magnetic field oriented along crystal axis . The ENDOR spectra are recorded with the external magnetic field locked at the positions indicated by the arrows and the labels in the EPR spectrum. The EIE spectra are recorded with the ENDOR frequency locked to the ENDOR transitions li2 and Li5, as indicated by the labels. The figure is reproduced from [29(c)] with permission from Dr. T.A. Vestad... 

Figure 1 First-derivative spectra obtained by continuous-wave techniques for the phenalenyl radical (A) ESR, (B) ENDOR, (C) special TRIPLE resonance, (D) general TRIPLE resonance, and (E) ENDOR-induced ESR. The arrows indicate the positions of the RF pumping frequencies there are several possibilities in (D). The RF pumping frequency in (E) is set in the same position as shown in (B). (Reprinted from Kurreck H, Kirste B, and Lubitz W (1988) Electron Nuclear Double Resonance Spectroscopy of Radicals In Solution. Weinheim VCH Publishers.)...

The ENDOR technique refers to electron-nuclear double resonance. This consists of the effect on a partially saturated ESR line of simultaneously irradiating the sample with a radiofrequency to induce nuclear resonance transitions of hyperfine coupled nuclei. It may enable one to obtain information about signs of coupling constants. ELDOR is the technique corresponding to electron-electron double resonance. Such techniques, coupled with TRIPLE resonance, have been utilized and well described in a discussion of pyridine and 4,4-bipyridyl radical anion ESR spectra measured in sodium/liquid ammonia (80JMR<41)17). 

These assignments are consistent with PES data ° and supported by theoretical calculations " in C2v symmetry, IS " " has two low-lying radical cationic states, and Ai. " The state is the ground state of IS " " the calculated hyperfine coupling constants (B3LYP/6-31G //MP2/6-31G ) are compatible with chemically induced dynamic nuclear polarization (CIDNP) and ESR/ENDOR results. No spin... 

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

Box HC (1977) Radiation effects ESR and ENDOR Analysis. Academic Press, New York Box HC, Budzinski EE (1975) Primary radiation damage in thymidine. J Chem Phys 62 197-199 Box HC, Potter WR, Budzinski EE (1975) The reduction of nucleotides by ionizing radiation Uridine 5 phosphate and cytidine 3 phosphate. J Chem Phys 62 3476-3478 Box HC, Budzinski EE, Freund HG, Evans MS, Patrzyc HB, Wallace JC, Maccubbin AE (1993) Vicinal lesions in X-irradiated DNA Int J Radiat Biol 64 261-263 Box HC, Freund HG, Budzinski EE, Wallace JC, Maccubbin AE (1995) Free radical-induced double base lesions. Radiat Res 141 91-94... 

The basis of the concept of a dimer of chlorophyll a in P-700 rests on evidence obtained with optical or ESR spectroscopy. In the P-700 redox difference spectrum, although very similar to that obtainable upon chemical oxidation of chlorophyll a, there are significant differences in the red region, which presents a splitting of the peak (at 685 and 700 nm) which is absent in chlorophyll a [53]. Moreover, the ESR and ENDOR spectra also present characteristics that have been interpreted as due to a dimeric arrangement [16]. Alternative interpretations have been offered suggesting that the spectral distortions are caused by a modification of the chemical environment of chlorophyll a in the RC complex. Definitively not in line with the dimer hypothesis is the spectrum of the light-induced triplet state of P-700, that can be observed when the intermediate acceptor is prereduced chemically in this spectrum the zero field parameters are the same as those of chlorophyll a monomers [54]. It is not clear, however, whether the triplet state resides on P-700 or on other chlorophylls of the RC complex. 

EMX)R measurements. The ESR spectrum of the radiation induced free radicals in polycrystalline lithium formate, proposed as an ESR-dosimeter (see Section 9.3, in Chapter 9), consists of a singlet with a line width of 0.92 mT. The g- and C-hyperfine tensors obtained from ESR, g = (2.0037, 1.9975, 2.0017), and A(i C) = (465.5, 447.5, 581.3) MHz identify the species as CO2 in agreement with an assignment made by single crystal ESR and ENDOR studies of samples with natural isotopic composition, discussed in Chapter 2. The observed stability of the CO2 radicals formally formed by H atom detachment from HCO2 is attributed to the solvation structure of surrounding molecules and Li" -ions. 

There are a variety of techniques for the determination of the various parameters of the spin-Hamiltonian. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR), Electron Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occassionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detection of Magnetic Resonance (ODMR), Atomic Beam Spectroscopy and Optical Spectroscopy. The extraction of the magnetic parameters from the spectra obtained by application of these and related techniques follows procedures which may in detail depend on the technique, the state of the sample (gaseous, liquid, unordered solid, ordered solid) and on spectral resolution. For particulars, the reader is referred to the general references (D). 

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