Electron spin resonance single-line spectrum

Figure 2.5.4 (a) One porphyrin molecule or (b) two amphiphilic nucleic bases may be dissolved within one micelle. No more The electron spin resonance (esr) spectra at the bottom of (a) were taken from the modeled micellar SDS solution (left) and the aqueous solution without SDS (right). The multiline signal corresponds to a copper(II) monomer the single line spectrum is for an aggregate. The diameter of the micelle is about 5 nm.. 

The esr spectrum of the paramagnetic component coincident to the metal-metal axis was observed at 4 K through the coupling of a single-crystal dielectric resonator of K2Pt(CN)4Bro.3o(H20)3 with an electron spin resonance spectrometer. Preliminary results include a line at g w 2 with a complex hyperfine structure (208). An impurity center associated with the presence of a copper(II) complex of Z>4a symmetry has recently been detected by esr (466). 

Electron spin resonance spectroscopy The ESR spectrum of humic and fulvic acids consists of a single line identified by its position and width. In general, the ESR spectra are devoid of hyperfine splitting. The ESR spectra of humic substances result from only a small fraction of the total number of molecules that comprise the humic and fulvic acids. The number of free radicals per unit weight, the g value and the line width can be calculated. The predominant radicals are semiquinones. Comparison of the ESR properties among humic fractions reveals that fulvic acids contain greater quantities of free radicals than humic acids. 

Hyperfine splitting. As was discussed above, one consequence of placing a free electron onto a molecule is to alter its 0-value. Another is that the electron spin comes under the influence of any magnetic nuclei present in the radical, with the result that the spectrum is split into a number of lines centred on the position of the single resonance expected for the simple /transition discussed above. This hyperfine structure is the most useful characteristic ofepr spectra in the identification of an unknown radical species. 

Fig. 6 compares the lineshapes of the ( T>- Z transition for a single molecule and an ensemble of about 10 molecules. The ensemble spectrum was obtained with the laser in resonance with the fluorescence excitation line of the Oi ensemble. The transition shows an asymmetric hneshape with a steep decrease towards higher microwave frequencies for both the single molecule and the ensemble case. The line-shape results from the hyperfine interaction of the triplet electron spin with the pen-tacene proton spins (/ = 1/2). Each proton can exist in one of its two nuclear spin states which yields 2 nuclear spin configurations. The h3fperfine interaction of each of these nuclear configurations causes a slight shift of the resonance. As pointed out in Section 4.1. In zero-field the hyperfine interaction is a second-order effect which leads to the observed opposite asynunetric lineshapes for the ((T>- Z and the ( T>- Z transitions (see Fig. 5). 

Sometimes it is very difficult to determine unambiguously the precise structure of the spin adduct from the EPR signal obtained. Isotopic substitution EPR experiments are recommended in an attempt to identify the observed adducts. The strategy is that the unpaired electron in a radical interacts with the nucleus of the atom it orbits, and the spin of the nucleus determines the number of lines or peaks in the spectrum. For example, has a nuclear spin of j while has no spin. An unpaired electron, which is associated with atoms having no spin, will exhibit an EPR spectrum containing only a single line. The spin of the nucleus influences the resonance of the unpaired electron so that the EPR resonance splits into two or more lines. The number of EPR resonance observed is equal to 21 + 1, where I is the nuclear spin. A practical... 

A single substance can produce several values of Bo at which resonance occurs, because different nuclear spin states will be found in different molecules in the sample and because the coupling constants at different nuclei can be different from each other. In the hydrogen molecule ion, hJ, the electron couples equally with the two protons. The molecule could be in a state with both proton spins up, in either of two states with one proton spin up and one down, or in a state with both proton spins down. Since the sum of the Mj values can equal 1, 0, or -1, we obtain a spectrum with three lines, where each line is produced by a different set of molecules. The states are nearly equally populated and the middle line is twice as intense as the other two, because there are two states with one spin up and one spin down. 

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