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The measurement of very short spin-lattice, or longitudinal, relaxation (SLR) times (, 10 < < 10 s) is of great importance today for the study of relaxation processes. Recent case studies include, for example, glasses doped with paramagnetic ions (; ), amorphous Si (dangling bonds) and copper-chromium-tin spinel (Cr) (), and polymer resins doped with rare-earth ions (; ). The ability to measure such fast SLR data on amorphous Si and copper-chromium-tin spinel led to an understanding of the role of exchange interaction in affecting spin-lattice relaxation, while the data on polymer resins doped with rare-earth ions provided evidence of spin-fracton relaxation (, ). But such fast SLR times are not measurable by the most commonly used techniques of saturation- and inversion-recovery (; ), which only measure spin-lattice relaxation times longer than 10 s. A summary of relevant experimental data is presented in Table 1.

The spin-lattice, or longitudinal, relaxation time plays an important role in magnetic resonance because it provides significant information about the coupling of a paramagnetic ion with its environment via its dependence on such factors as temperature, frequency (; ), spin concentration (), and magnetic field (; ). But the measurement of electronic spin-lattice relaxation times is problematic because the times span the range from the very short (10 s) to the very long (1 s; ). The one microsecond spin-lattice relaxation time demarcates “short” from “long” relaxation times, which traditionally have each required their own methods of measurement. For example, long relaxation times are measured by using cw-EPR spectrometers to record spectra at multiple power levels near and under the condition of saturation; the spin-spin and spin-lattice relaxation times are then calculated from lineshape parameters. But the so-called short relaxation times are not measurable on the time scale of common cw-EPR instrumental detection methods. Short spin-lattice relaxation times are therefore measured by resorting to different (, transient) magnetic resonance techniques such as pulsed saturation, spin echo ( Poole & Farach, 1971), and amplitude modulation (,).

The physical principles that underlie organic reactions were established by a systematic study of chemical reaction dynamics that employed correlated measurements of reaction rates and a physical parameter that could be related of the electronic properties of the molecules in question (). Today, molecular science emphasizes the concept of molecular device, which connotes a supramolecular structure (the term “supramolecule” loosely means a molecule that has multiple functionalities associated with it; for example, an enzyme might be regarded as a supramolecule in the sense that it features a supported metal catalyst and a receptor site that recognizes a specific substrate upon which the catalyst acts) that acts in some specific fashion. A molecular device may be biological (, enzymes, contractile proteins; ), or it may be produced by synthetic means (, molecular wires, switches, machines, etc.; ; ). Current synthetic chemistry provides the technical means that enable one to create and modify molecular devices so that structure may elicit some specific function, and so physical organic chemists are interested in reactions that involve engineered and structurally complex systems such as supported catalysts, protein active sites, or nanostructures (. amilton, 1996; ).

Biological systems exhibit properties of amorphous materials. The Mn(II) ion in amorphous materials is characterized by distributions of spin-Hamiltonian parameters around mean values. It has a certain advantage over other ions, being one of the most abundant elements on the earth. The extent to which living organisms utilize manganese varies from one organism to the other. There is a fairly high concentration of the Mn(II) ion in green plants, which use it in the O evolution reaction of photosynthesis (Sauer, 1980). Structure-reactivity relationships in Mn(II)-O complexes are given in a review article by Coleman and Taylor (1980). Manganese is a trace requirement in animal nutrition; highly elevated levels of manganese in the diet can be toxic, probably because of an interference with iron homeostasis (). On the other hand, animals raised with a dietary deficiency of manganese exhibit severe abnormalities in connective tissue; these problems have been attributed to the obligatory role of Mn(II) in mucopolysaccharide metabolism (). Mn(II) has been detected unequivocally in living organisms.

The pulse Fourier transform approach to magnetic resonance spectroscopy has been extensively developed and successfully applied to systems of one-half spin and their mutual interactions. But resonance spectroscopy of spin systems with the higher half- and integer spin quantum numbers is commonplace, for example, in the case of alkali metal nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) of transition metal compounds involving multi-quantum transitions. Similarly, magnetic resonance at zero field entails the observation of multi-quantum transitions.