Three-line ESR spectrum

FIGURE 1 The three-line ESR spectrum of the nitrogen donor in 3C-SiC taken at 20 K. The lineshape is... 

For one adjacent nucleus with spin = I, the number of lines seen in the ESR spectrum is 2/ + 1. (This equatiou should be compared with the NMR equivalent). For N nuclei, the number of lines = 2N7 + 1. For example, an adjacent nitrogen atom, 7=1, would give a three line ESR spectrum, while Mn, 7=5/2, would give a six line spectrum. Figure 3.84a shows the simulated spectra expected from N adjacent nuclei with spiu = 1/2. The iuteusity patterns follow the binomial distribution (Pascal s triangle) as in NMR. Figure 3.84b shows similar spectral patterns for 7=1, but the relative intensities differ. For hyperfine coupling to IN atom, for example, the pattern would be 1 1 1, 2N atoms, 1 2 3 2 1, 3N atoms, 1 3 6 7 6 3 1, aud so ou. 

A spin label is a stable paramagnetic molecule that contains an atom or group of atoms with an unpaired electron spin that can be bonded to another molecule. In this way, one can detect molecules that otherwise would not give an ESR spectrum. Most spin labels are nitroxide compounds. For example, 2,2,6,6-tetramethylpiperidine-A-oxyl (known as TEMPO) is a common spin label with a characteristic three-line ESR spectrum, seen in Figure 3.87. 

Figure 4 (A to C) shows changes in the ESR spectra obtained with D-mannose phenylhydrazone treated with 1% potassium terf-butoxide in methyl sulfoxide. The original three-line pattern (spectrum A), ascribed to the nitroxide radical, rearranges to a new pattern in about 15 minutes (spectrum B) this pattern remains steady for hours if the solution is kept in the absence of oxygen (spectrum C).

L of air in rural Germany was trapped in a polycrystalline matrix of DzO at 77 K (Mihelcic et al., 1993). Spectrum b shows the ESR spectrum of NOz it can be seen that most of the observed ESR signals are due to NOz, calculated from reference spectra to be present at a concentration of 0.68 ppb in this sample. Spectrum c is the difference between spectra a and b, magnified by a factor of five. Spectra d, e, and f are those of N03, H02, and R02, respectively, and their sum is shown by the heavy line through spectrum c. Clearly, the signals in spectrum c reflect contributions from these three radicals, at concentrations of 5.2 ppt N03, 10 ppt HOz, and 5 ppt R02 in this particular sample. Detection limits for this method are 5 ppt for HOz and ROz, respectively (Mihelcic et al., 1993). 

An ESR spectrum at 4.2 K shows two strongly coupled H nuclei (ah = 2.04 mT, p protons) and four less strongly coupled ones ( h = —117 mT, a protons) at 77 K, a single line was observed. The low temperature spectrum supports the Ai strucmre at higher temperature, three equivalent Ai structures are averaged by dynamic JT distortion. The 62 structures are transition states op (2.04 mT) and Ua (—1.17 mT) fortuitously average to Ugvg observed in CFiCl-CFCE matrices. " ... 

For the triphenylmethyl radical (C6H5)3C , the tt MO of the unpaired electron is delocalized over the entire molecule, and there are couplings to six ortho, six meta, and three para hydrogens. The ESR spectrum has 7x7x4 = 196 lines. 

In order to examine the new ESR spectrum, the glass containing a larger amount of nitroethylene (28 mole-%) was irradiated, which gives a signal shape as shown in Fig. 6b. When the temperature is raised, the seven-line spectrum due to the free radicals formed from 2-methyltetrahydrofuran disappears, leaving the spectrum of present interest as shown in Fig. 6c. The spectrum has the hyperfine structure due to three... 

For Re2(02CC Hs)4Cl2 the first reduction wave near —0.3 V is quasi-reversible. The product of this one-electron oxidation was not isolated as a solid, but ESR spectra were obtained on frozen dichloromethane solutions at 77°K at both X- and Q-band frequencies. Both naturally occurring rhenium isotopes have a nuclear spin of 5/2 with slightly different nuclear magnetic moments. For dimeric rhenium species there are three distinct isotopic isomers and each generates a unique ESR spectrum as a result of differing hyperfine interactions. The observed ESR spectrum is a superposition of these lines with the intensity... 

A typical example is seen for 1-hydroxyethyl radical trapped in a y-irra-diated ethanol matrix at 77 K [16]. As is shown in Fig. 2, the cw ESR spectrum of the radical is composed of five lines due to hyperfine interactions with one a proton and three P protons of a methyl group. The hyperfine interaction depends on the location of the P protons with respect to the p. orbital of the unpaired electron. However, the observed hyperfine coupling constant is the same for all the p protons because of the quick rotation of the methyl group in the time scale of the cw ESR measurement. On the other hand, the ESE-detected ESR spectrum is composed of four lines due to the hyperfine interactions with... 

In the present experiment, we are concerned with the hyperfine structure of the ben-zosemiquinone radical anions. The delocalized unpaired tt electron is of course distributed over the entire molecular frame of six C atoms and two O atoms. With R = H, by symmetry, it is clear that the four protons are all equivalent in the para species hence five hyperfine lines with relative intensities 1 4 6 4 1 are expected in the ESR spectrum of this radical. By contrast, when R is not a proton, the three ring protons are not related by symmetry, and thus each may be expected to possess a different splitting constant. A hyperfine structure pattern of eight unequally spaced lines of equal intensity is expected. The line splittings and relative intensities in ESR spectra thus convey information about the geometric arrangement of the atoms. 

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