Electron spin resonance spectra dependence

The electron spin resonance spectrum of a free radical or coordination complex with one unpaired electron is the simplest of all forms of spectroscopy. The degeneracy of the electron spin states characterized by the quantum number, ms = 1/2, is lifted by the application of a magnetic field, and transitions between the spin levels are induced by radiation of the appropriate frequency (Figure 1.1). If unpaired electrons in radicals were indistinguishable from free electrons, the only information content of an ESR spectrum would be the integrated intensity, proportional to the radical concentration. Fortunately, an unpaired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The arrow in Figure 1.1 shows the transitions induced by 0.315 cm-1 radiation. 

Figure 6.2 Electron spin resonance, (a) Dependence of electron energy on magnetic field, (b) ERS spectrum of a simple free radical, (c) Idealized ESR spectrum of the methyl radical...

The most probable order of d levels in copper phthalocyanine is illustrated in Fig. 19 (131, 145). The calculated energies (131) are appended. The relative order of the ea and b2g orbitals is the inverse of that found for copper acetylacetonate (238). Although the data for cobalt phthalocyanine (139) cannot be assigned unambiguously, the orbital levels are probably in the same relative order. The hole would then be in the d orbital rather than in the dxt-yt (as in copper phthalocyanine) and this is in accord with the observation that the electron-spin resonance spectrum of cobalt phthalocyanine is solvent dependent, while that of copper phthalocyanine is not (131, 139). The alternative assignment of the cobalt data places the hole in the dxy orbital lying some 16,000 cm-1 above the dxz,dyX pair, which seems unlikely. 

To simplify terminology of axial systems, gzz is defined to be g(l (the g-value observed with the symmetry axis of Cu + parallel to the applied field), and gxx (= gyy) is defined to be gA (the g-value observed with the symmetry axis perpendicular to the applied field). An elongated z-axis (depicted in Figure 11 for Cu(H20)5 +) results in gjj > gj. For axially symmetric Cu + rigidly bound in a crystal, the g-value can then vary between the minimum (gj.) and maximum (g(,), depending on orientation of the crystal within the magnetic field. However, for axial Cu + bound in a powdered clay sample, all possible orientations, and therefore all g-values between gA and gj are represented in the "powder" spectrum. Therefore, electron spin resonance occurs only for field values, H, between Hjj and H, where ... 

In the anion-radicals of nitro compounds, an unpaired electron is localized on the nitro group and this localization depends on the nature of the core molecule bearing this nitro substituent. The value of the hyperfine coupling (HFC) constant in the electron-spin resonance (ESR) spectrum reflects the extent of localization of the unpaired electron values of several nitro compounds are given in Table 1.1. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

The present work deals with soUd CT complexes of phenazines which, in the most favorable cases, include the crystal structure, single-crystal electron spin resonance (epr) data, the temperature dependence of the magnetic susceptibility or of the powder conductivity, and a CT absorption spectrum. We have not pursued complexes that could not be crystallized, nor have we investigated completely similar materials. 

Electron spin resonance spectroscopy (ESR), also known as electron paramagnetic resonance (EPR), is based on the property that an unpaired electron placed in a magnetic field shows a typical resonance energy absorption spectrum sensitive to its environment. Recently, this technique, which was primarily developed for biological studies of membrane properties, has been adapted for the study of adsorbed polymer/surfactant layers. The mobility of the ESR probe (stable free radical incorporated into the polymer or surfactant molecule) depends of orientation of the surfactant or polymer and the viscosity of the local environment around the probe. 

An underlying question in many of the ESR measurements of the lineshape of the N-donor and its temperature dependence is the possible existence of an additional broad line at a similar g-value to the three-line spectrum. Most authors agree that the three lines in the isolated N-donor spectrum should have symmetric lineshapes and, since there is a slight asymmetry to the full spectrum, this leads to the conclusion that there is another relatively broad ESR line shifted slightly from the three-line spectrum. This could be due to a second donor, a conduction electron spin resonance or a structural defect. A weak signal, possibly due to a Si vacancy, is also observed at higher temperatures (T > 50 K) in some samples. 

Line-width effects in electron spin resonance spectra have been the subject of three recent reviews " and so there seems little point in further detailed repetition of the principles at this time. Basically, line-width effects will be observed when the Hamiltonian describing the spin systems contains time-dependent elements having frequency components comparable to frequency separations in the spectrum. The mechanisms... 

It should be noted that there is an important distinction to be made between the hexamethylacetone-sodium ion-quartet and the situation described by the original G.F.F. theory. In the G.F.F. theory it was assumed that the solvent dependence of hyperfine splitting constants is to be attributed to modifications in spin density distributions, whereas for the ion-quartet the spin density distribution is the same in tetrahydro-furan and in methyltetrahydrofuran. The variation in with solvent must be due to variation in the geometry of the ion-quartet, which will in turn vary the efficiency of the mechanism whereby spin is transferred to the alkali metal nucleus. Thus, in this case, the solvent dependence is to be attributed to variations in Q rather than p. The situation is common in the study of ionic association through electron spin resonance spectroscopy and has thwarted many attempts at quantitative descriptions of the effect of solvation upon such association until the geometry of the ionic associate in solution is firmly established it is not too rewarding to discuss how the spectrum varies with change in solvent. 

The analysis of the diffusion-eontrolled features might be simplified by identifying the two types of free radieals the active and the trapped ones. Electron spin resonance speetroseopy shows that active (mobile) radicals give a 13-line spectrum and trapped (statie) radicals give a nine-line spectrum. Also, photopolymerization of a number of neat acrylate monomers used in polymer coatings for optical fiber was studied with photo DSC and with a cure monitor using a fluorescent probe. The acrylates had a functionality of one to six. It was found that conversion of monomers ranges from 40% to 100%. This, however, is depended upon functionality and structure of particular monomers. It can also be a function of the type and amount of the photoinitiator used. 

Electron paramagnetic resonance (EPR) spectroscopy (also called electron spin resonance (ESR) spectroscopy), is used to study paramagnetic species with one or more unpaired electrons, e.g. free radicals, diradicals, metal complexes containing paramagnetic metal centres, defects in semiconductors and irradiation effects in solids. While diamagnetic materials are EPR silent, paramagnetic species always exhibit an EPR spectrum. This consists of one or more lines, depending on the interactions between the unpaired electron (which acts as a probe ) and the molecular framework in which it is located. Analysis of the shape of the EPR spectrum (the number and positions of EPR lines, their intensities and line widths) provides information... 

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