Electron spin resonance relaxation times

The NMR spectra of these complexes under ambient conditions exhibit sharp, paramagnetically shifted features that can span up to 400 ppm (90, 134). Electron exchange is fast on the NMR time scale, so there is an effective twofold symmetry, which approximately halves the number of distinct features observed. The sharpness of the resonances is due to the short electron spin-lattice relaxation time of the Fe(II) center, which allows even the CH2 protons adjacent to the coordinated nitrogen atoms to be observed. 

The conditions necessary for observation of proton magnetic resonance spectra in paramagnetic systems are well established (1). Either the electronic spin-lattice relaxation time, T, or a characteristic electronic exchange time, Te, must be short compared with the isotropic hyperfine contact interaction constant, in order for resonances to be observed. Proton resonances in paramagnetic systems are often shifted hundreds of cps from their values in the diamagnetic substances. These isotropic resonance shifts may arise from two causes, the hyperfine contact and pseudocontact interactions. The contact shift arises from the existence of unpaired spin-density at the resonating nucleus and is described by 1 (2) for systems obeying the Curie law. 

The line broadening in spectra of paramagnetic compounds is caused by short electronic spin-lattice relaxation times and/or hyperfine electron-nuclear coupling. Consequently, it is usually the case that materials giving useful EPR spectra have nuclear magnetic resonances so broad as to be unobservable. The two methods are therefore to a large extent complementary. 

The effectiveness of a crude oil demulsifier is correlated with the lowering of the shear viscosity and the dynamic tension gradient of the oil-water interface. The interfacial tension relaxation occurs faster with an effective demulsifier [1714]. Short relaxation times imply that interfacial tension gradients at slow film thinning are suppressed. Electron spin resonance experiments with labeled demulsifiers indicate that the demulsifiers form reverse micellelike clusters in the bulk oil [1275]. The slow unclustering of the demulsifier at the interface appears to be the rate-determining step in the tension relaxation process. 

The electron spin resonance (ESR) technique has been extensively used to study paramagnetic species that exist on various solid surfaces. These species may be supported metal ions, surface defects, or adsorbed molecules, ions, etc. Of course, each surface entity must have one or more unpaired electrons. In addition, other factors such as spin-spin interactions, the crystal field interaction, and the relaxation time will have a significant effect upon the spectrum. The extent of information obtainable from ESR data varies from a simple confirmation that an unknown paramagnetic species is present to a detailed description of the bonding and orientation of the surface complex. Of particular importance to the catalytic chemist... 

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

The 19F NMR resonance has been measured from 115 to 245 K at different field strengths of solid OsF6 and spin-lattice relaxation times determined.785 The electron affinity of OsF6 has been compared with those of other metal hexafluorides.762 786 787 The Mossbauer spectrum has been recorded.184... 

Titanium h.f.s. are resolved (500) at 77°K in Ti + doped Al(acac)8. The symmetry here is and the large trigonal distortion (S = 2000-4000 cm ) increases the spin-lattice relaxation time so that resonance is observed at 77°K. The electron is in the a- d 2) orbital in contrast with most other cl ions. Titanium h.f.s. are observed on the F-center line in H2-reduced BaTiOs 664). 

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