Paramagnetic species electron-spin relaxation

The interaction of the unpaired electron on the paramagnetic metal ion with that of the nuclear spin is analogous to the nuclear-nuclear spin complex commonly observed in nuclear mimetic resonance spectroscopy. Thus, the electron-spin relaxation (Te) affects the nuclear-spin relaxation-rates (T, and T2). Eqs. 3 and 4, describing the contribution to the nuclear relaxation rate from the component spin on the paramagnetic species were derived by Solomon and Bloembergen ... 

A knowledge of electron spin relaxation permits predictions concerning utility of a paramagnetic species for instance as an MRI (Magnetic Resonance Imaging) contrast. 

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... 

The electron spin echo (ESE) method is a kind of time-domain ESR using pulsed microwave radiation and it directly observes the relaxation behavior of electron spins. Since the construction of an ESE spectrometer was first reported by Kaplan early in 1962 [8], the ESE method has become more and more popular slowly but steadily. Owing to the progress in microwave technology and to the continual effort of pioneering workers such as Tsvetkov, Mims, Kevan and others [9, 10]. the ESE method has been extensively developed and improved. It is now considered to be requisite for studying the paramagnetic relaxation of radical species. 

Similarities exist between the chemical characteristics of the actinides and those of the lanthanides. The metal ions are generally considered to be relatively hard Lewis acids, susceptible to complexation by hard (i.e., first row donor atom) ligands and to hydrolysis. Both actinide and lanthanide ions are affected by the lanthanide contraction, resulting in a contraction of ionic radius and an increasing reluctance to exhibit higher oxidation states later in the series. Most species are paramagnetic, although the electron spin-nuclear spin relaxation times often permit observation of NMR spectra, and disfavor observation of ESR spectra except at low temperatures. The elements display more than one accessible oxidation state, and one-electron redox chemistry is common. 

The enhancement of the relaxation process in natural hydrated silicate minerals by paramagnetic species such as Fe may be due to their facilitating the interaction between the Si nucleus and the electron spin as a result of the relatively high mobility of the hydrated paramagnetic species. A similar effect has been reported in zeolites (Cook-son and Smith 1985, Klinowski etal. 1986), in which the relaxation rate is significantly shortened by the presence of paramagnetic oxygen molecules. 

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