Electron spin resonance spectra interpretation

Suppose that we seek a detailed theoretical interpretation of the electron spin resonance spectrum of a paramagnetic molecule having one unpaired electron. We need a realistic, one-electron wave function representing the orbital in which the odd electron resides. MO theory will... 

Fig. 2. First derivative of the electron spin resonance spectrum of Me2C OH radicals, together with an integrated spectrum and interpretation.

M. A. J. Wilks and M. R. Willis, Interpretation of the high-resolution electron spin resonance spectrum of the 2,4,5-triphenylimidazolyl radical. J. Phys. Chem. 1968, 72, 4717. 

The electron spin resonance spectrum of La2Cu04 (vQ = 9.464GHz, room temperature) is shown in Figure 3(a). The spectrum can be interpreted with... 

The effect of temperature on the association of vanadium compounds in asphaltenes was investigated by Tynan and Yen (1969). Using electron spin resonance (ESR), they observed both anisotropic and isotropic hyperfine structures of vanadium, interpreted as bound or associated and free vanadium, from asphaltenes precipitated for a Venezuelan petroleum and reintroduced to various solvents. Higher temperatures and more polar solvents resulted in a transition from bound to free vanadium, as shown in Fig. 12. At 282°C, only 1% of the anisotropic spectrum was observed. An activation energy of 14.3 kcal/mole was observed for the transition. 

The applicability of spectroscopic methods (other than NMR) for determining functionality in humic substances is reviewed. Spectroscopic methods, like all other investigational techniques, are severely limited when applied to humic substances. This is because humic substances are comprised of complicated, ill-defined mixtures of polyelectrolytic molecules, and their spectra represent the summation of the responses of many different species. In some cases only a small fraction of the total number of molecules contributes to the measured spectrum, further complicating the interpretation of spectra. The applicability and limitations of infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, spectrofiuorimetry, and electron spin resonance spectroscopy to the study of humic substances are considered in this chapter. Infrared spectroscopy, while still very limited when applied to humic substances, is by far the most useful of the methods listed above for determining functionality in these materials. Very little information on the functionality of humic substances has been obtained by any of the other spectroscopic methods. 

INDO and McLachlan-modified HMO calculations have been conducted to help interpret the electron spin resonance (ESR) spectrum of cation radical (120) <78JPC1181> which is generated by protonation of (98) in TFA followed by reduction with zinc of the resultant dication (119) (Scheme 1) <63TL95>. The calculated energy level of the HOMO for (120) is of lower energy than that of the isoelectronic fluorene anion radical in both the INDO and HMO approximations. Similar calculations have been conducted to explain the ENDOR spectrum of (120) <84CPL398>. 

The radical anions of (43) and (44) have been examined by electron spin resonance spectroscopy <86JCS(P2)185i>. The ESR spectrum of the radical anion of (43) has been interpreted in terms of the presence of both (E,E)- and (Z, )-conformers and the activation parameters for the interconversion have been determined. 

The NH2 radical trapped in various matrices was extensively studied by electron spin resonance (ESR) spectroscopy see Table 13, p. 196. The ESR spectrum of NH2 in an Ar matrix at 4.2 K consists of the expected nine lines [1, 2] assigned to the threefold hyperfine splittings of 67 and 29 MHz due to the two protons and "" N, respectively. The peak Intensities within each triplet are roughly 1 1 1 instead of the expected 1 2 1. An Interpretation is given In [3]. Spectral lines of deuterated NH2 in Ar formerly assigned to ND2 [1] were reassigned to NHD [2]. 

More advanced experiments, such as ENDOR, electron spin echo envelope modulation (ESEEM), or relaxation measurements by pulsed ESR rely on a selective excitation of spins close to the resonance field. Usually, the powder ESR spectrum is much broader than the excitation bandwidth of the pulses, which is in the range between 2 and 10 G. In cases where one anisotropic interaction dominates the spectrum, the experiments thus select contributions only from certain orientations of the molecule with respect to the external magnetic field. Such orientation selection is more efficient and easier to interpret at a field that is high enough for the g anisotropy to dominate. Finally, the size of mw resonators scales with wavelength and thus scales inversely with frequency. At higher frequency, spectra can thus be measured with much smaller sample volumes, yet the concentration does not need to be significantly increased. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

On the other hand, if the excited state is strongly localized, i.e. if the electron stays during the emission process on the same atom with the same spin, a resonance line can be emitted. The existence of an exciton-type state could be taken into consideration to explain the presence of the resonance lines but, except for thorium, the ratio of Mjy and My R line intensities is different from the ratio of the corresponding absorption line heights. In particular, the Mjy R line is absent in the spectrum of plutonium. As a consequence, the emission spectra cannot be interpreted suitably by ex-citonic transitions. 

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