Selected ESR Parameters

Selected ESR Parameters 991 9.3.5 Zero-field Splitting and Exchange Coupling... 

The esr parameters for a large number of alkyl aryl nitroxides have been collected recently, in part as a source of reference for spin trapping studies (Rockenbauer et al., 1978) the conformations of these nitroxides are also discussed. A selection of spectral parameters for spin adducts of the more important nitroso traps which have been discussed in this Section is presented in Table 2. 

A selection of esr parameters for nitrone spin adducts is collected in Table 3. 

AS = 13 e.u. for the Cu-template resin, and AH = -0.8, AS = 9,8 (K - 540) for the resin synthesized without any template ion. The larger change in entropy observed in the complexation of the Cu-template resin indicated that die Cu-template resin selectively adsorbed Cu ions by entropic effect. Furthermore, the absorption spectrum of the Cu complex of the Cu template resin was located at a wavelength 10—20 nm shorter than those of the other resins70 and the ESR parameters of the Cu complex of the Cu-template resin were similar to those of the non-distorted planar Cu complex71. From these results, it was suggested that the conformation of the polymer-ligand chain in the Cu template resin remained the best one for the Cu ion. 

Metal- and proton-exchanged zeolites have been recently attracted much attention because of their selective catalytic activity to efficiently reduce nitrogen monoxide (NO) by hydrocarbon in an 02-rich atmosphere [1]. The formation of nitrogen dioxide (NO2) from NO and O2 has been suggested as an important step in the selective reduction [2, 3] NO2 is one of rare stable paramagnetic gaseous molecules and has been subjected to electron spin resonance (ESR) studies [4-7]. The ESR parameters and their relation/to the electronic structure have been well established [4] and NO2 can be used as a "spin probe" for the study of molecular dynamics at the gas-solid interface by ESR. 

Also, various spectroscopic quantities can be calculated in order to test experimental assumptions Once a structure of a supramolecular assembly has been assumed, optimized or propagated in time, properties like vibrational frequencies, infrared, Raman [93], or Resonance Raman [159] intensities, NMR or EPR parameters can be calculated with first-principles methods to be compared with the experimentally measured spectra in order to confirm or reject the structural basis assumed in the interpretation of the experimental spectra. It is impossible to review the work and achievements of theoretical chemistry in this respect. Therefore, we concentrate on selected examples in the following. The interested reader is referred to the book by Kaupp, Biihl and Malkin [160] for the calculation of NMR and ESR parameters and to Refs. [161, 162] for more general discussions of molecular property calculations. NMR parameters are molecular properties probed at atomic nuclei and thus ideal for linear-scaling or empirical approaches. An efficient linear-scaling method for supramolecular systems has been presented recently [163]. 

Table XII gives representative results for the spin Hamiltonian parameters as determined for d5 ions in different crystal fields. This table is not a complete listing of ESR results on d5 ions, since such a list would be much larger, but is rather a selection of results for various types of crystalline fields. Several facts become evident when examining the results listed in this table. The g factors are always near the free spin value of 2.0023, as would be expected for an S state. The hyperfine constant is isotropic, as would also be expected for an S-state ion. The hyperfine constant also...

ESR observations were conducted at 9.5 GHz, using a Varian E-line ESR spectrometer with variable temperature capabilities from 90K-300K. The g value was determined using a Varian pitch standard with a g-value of 2.00302 + 0.00005. The integrated intensity was also calibrated to a Varian pitch standard. The parameters g, AH u, AH-p and the radical density were determined for each sample. Saturation measurements were made on a selected subset of samples. Low temperature runs at 125 K were made for all inertinite samples, as well as for selected samples of the other maceral types. Little temperature variation in g value, linewidth, or lineshape, was seen in any sample. The integrated intensity varied approximately as 1/T, suggesting Boltzmann polarization of the spins at lower temperatures. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

The following discussion summarizes important criteria and parameters for the application of ESR in inorganic and organometallic chemistry, illustrated by selected examples. 

We have investigated in detail the parameters affecting the continuous y-radiolysis of concentrated solutions (intensity, pH, 02, scavengers, etc.). Transients were investigated by pulsed electron beam radiolysis and kinetic spectroscopy, and the reactions of the optically accessible excited states of nitrate were investigated by conventional photolysis. This paper represents a survey of our recent results which, taken in conjunction with the work of others, allows the construction of a model whereby the main features of this system may be understood and may even be predicted. Literature review is necessarily selective for the present purpose (because of doubtful relevance to liquid state processes, low temperature radiolysis, and hence ESR work has been omitted from... 

Fig. 3.30 Simulated powder ENDOR spectrum (in absorption) of NO-ligated ferrocytochrome c heme a3, at the field setting (g = 2.079) marked in the X-band (v = 9.32 GHz) ESR spectrum. The parameters g = (2.082, 1.979, 1.979) A( N-His) = (16.5, 16.1, 19.3) MHz, Q(> N-His) = (+0.67, -1.12, + 0.45 ) MHz, A( N-NO) = (30.56, 30.56 59.90) MHz, Q( N-NO) = (+1.03, -0.51, -0.52) MHz were employed for the simulation, using a method teiking angular selection into account. For experimenUil spectra see [R. LoBrutto et aL, J. Biol. Chem. 258 (1983) 7437], for simulation with an exact method see [49]. The spectrum is adapted from [R. Erickson, Chem. Phys. 202, 263 (1996)] with permission from Elsevier...

The characterization is based on the assumption that selected orientations for the major axes of magnetic resonance tensors exist. Another assumption is the random scattering of major axes with respect to this direction according to the Gauss law. The orientation and disordering parameters are optimised by simulation of theoretical ESR spectra and comparison with experimental spectra. The mathematical algorithm developed for the simulation program is also discussed. Results of this approach were applied to experimental data for films of copper phthalocyanine and dipivaloil methanate, obtained by sedimentation on quartz plates. 

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