Spin-lattice relaxation time Subject

The second role of the chemical exchange phenomena can be seen in Eq. (2) the exchange lifetime competes with the in-complex nuclear spin-lattice relaxation time and can become a limiting factor in the attainable PRE. This aspect of the problem is highly relevant in practical consideration in the case of Gd(III) complexes as a potential contrast agent, because the water exchange in these systems is not too fast. This issue is considered to be outside of the scope of this article and we refer to recent literature on the subject 5,160) and to other contributions in this volume. 

Silicon only has one naturally occurring isotope ( Si) with a nonzero nuclear spin and Si NMR spectroscopy has become one of the most widely used techniques for the identification of silicon compounds. Unfortunately, the natural abundance of Si is only 4.7%, which combined with its long spin-lattice relaxation times, means that relatively long acquisition times may be needed to obtain high quality spectra. Si NMR spectroscopy has been the subject of a number of reviews and tables of chemical shift data and coupling constants are available. " ... 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

The dynamic nature of the gramicidin channel has been the subject of much interest. The N spin-lattice relaxation time (Tj) of the nitrogen atom at the Leu-4 position has been used to investigate the local djmamics about the Ala-3/Leu-4 linkage. Evidence was obtained that suggests a correlation between the local dynamics and ion transport through the channel. The... 

However, the spin system is subject to other interactions, the very interactions that bring about thermal equilibrium. These interactions can be collectively called spin-lattice interactions. They comprise radiationless interactions between the spin system and the thermal motion of the lattice or surroundings. The inverse of the rate of spin-lattice induced transitions is described by a characteristic time called the spin-lattice relaxation time and is denoted by the symbol T. ... 

In this case, the K s are the intersystem crossing rate constants. A detailed description of the different experimental techniques used at the present time, together with equations derived for molecules whose emission results from two zf levels, is given in a recent review of the subject (87). In this section we will give only a brief description of these PMDR methods. In all but the pulsed-excitation method, these assumptions are made (a) the spin-lattice relaxation is absent and (b) the microwave radiation either saturates or inverts the population of... 

At very high frequencies, domain walls are unable to follow the field and the only remaining magnetisation mechanism is spin rotation within domains. This mechanism eventually also shows a dispersion, which always takes the form of a resonance. Spins are subjected to the anisotropy field, representing spin-lattice coupling as an external field is applied (out of the spins easy direction), spins experience a torque. However, the response of spins is not instantaneous spins precess around the field direction for a certain time (the relaxation time, r) before adopting the new orientation. Fig. 4.62. The frequency of this precession is given by the Larmor frequency ... 

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