Saturation of spins

It also is possible to determine py values from a single, nonselective experiment for more than three spin systems, by combining nonselective relaxation-rates with n.O.e. experiments. According to Noggle and Shir-mer, Eq. 5 can be solved under conditions in which, at time f = 0, spin i is observed with simultaneous saturation of spin j, resulting in the expression... 

Three-spin effects arise when the nonequilibrium population of an enhanced spin itself acts to disturb the equilibrium of other spins nearby. For example, in a three-spin system, saturation of spin A alters the population of spin B from its equilibrium value by cross-relaxation with A. This change in turn disturbs the whole balance of relaxation at B, including its cross-relaxation with C, so that its population disturbance is ultimately transmitted also to C. This is the basic mechanism of indirect nOe, or the three-spin effect. 

Another popular way of measuring the cross-relaxation rate a relies upon the saturation (by continuous irradiation) of one of the two nuclei, say S. Referring again to the first of equations (13) with S2 = 0 (saturation of spin S)... 

Saturation of one of the spins in this system, in addition to rendering relaxation of the other spin exponential, also changes the equilibrium magnetization of the second spin, giving rise to the NOE. Thus, if Bj=0 by saturation of spin B, and A is observed at equilibrium [iAJAt=% equation 21 becomes ... 

Figure 8.16. Calculated steady-state NOEs for spin C on saturation of spin A, as a function of the A-B-C angle in a rapidly tumbling isolated three-spin system. The resulting NOE is a balance between direct and indirect A-C effects and the two curves illustrate the dependence on relative intemuclear separations.

The saturation of spin packets in highly doped polymers decreases significantly due to the increase in spin-spin and spin-lattice interactions. In the ESR spectra of such samples, the Dysonian term normally appears due to the formation of a skin layer with thickness 8. In contrast with the classic ESR signal, the Dyson-like spectrum shape feels both the spin polarons and the spinless bipolarons diffusing through a skin layer. It is possible to determine the intrinsic conductivity Oac of the sample directly from its Dysonian ESR spectrum. If the skin-layer is formed on the surface of a spherical powder particle with radius R, the coefficients A and D in Eq. 5 can be determined from Eqs. 16, ... 

ELDOR is tlie acronym for electron-electron double resonance. In an ELDOR experiment [28] one observes a rednction in the EPR signal intensity of one hyperfme transition that results from the saturation of another EPR transition within the spin system. ELDOR measurements are still relatively rare bnt the experiment is fimily established in the EPR repertoire. 

Bain A D and Duns G J 1994 Simultaneous determination of spin-lattioe (T1) and spin-spin (T2) relaxation times in NMR a robust and faoile method for measuring T2. Optimization and data analysis of the offset-saturation experiment J. Magn. Reson. A 109 56-64... 

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine couphng constant (ajj), can be observed in organic radicals, and CIDNP 3... 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

The crystal and molecular structure of the 44, 45, and C5-vinylferro-cenyl-thymidine show that the substituted cyclopentadienyl ring is essentially co-planar with the nucleobase (164). DFT calculations indicate that, irrespective of the extent of saturation in the bridging C2-unit in ethynyl-, vinyl- or ethyl-ferrocenyl-C5-thymidine, a similar amount of spin density is transferred to the nucleobase (Fig. 48). The reduction potentials for these compounds are shifted little compared to the parent ferrocenyl derivatives. 

The spin-echo successfully imaged water held in the cracks of cement paste. This technique can be used to resolve cracks much smaller than the nominal resolution by relying on water saturation of the crack, the connectivity of the crack structure and the fact that it is relatively easy to detect a high intensity structure on a low intensity background. 

Kusumi, A., W. K. Subczynski, and J. S. Hyde. 1982b. Oxygen transport parameter in membranes as deduced by saturation recovery measurements of spin-lattice relaxation times of spin labels. Proc. Natl. Acad. Sci. USA 79 1854-1858. 

Decoupling The saturation of a particular signal or signals in order to remove spin coupling from those signals. Also referred to as spin decoupling. 

Low temperatures are required to slow down paramagnetic relaxation in order to get sharp EPR spectra. However, when a paramagnet can relax back to the ground state only slowly, then it is easy to saturate the system with microwaves, and this will lead to deformed spectra. In this chapter we consider the two key experimental parameters power (intensity of the microwaves) and temperature (of the sample) in combination with the key system parameter the spin. For a given system of spin S at a temperature T there is a single optimal value of P, which must be determined experimentally. The combined set of P, T, and S determines the complexity and the costs of EPR spectroscopy. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

There are two main caveats to Equation 9.4 firstly, that this formula gives SM in units of joule per kilogram per kelvin, meaning no account is taken of the volume of the material under test. For this, the density is required and these two parameters combine (-p ASM) to give a better characterization, with units of millijoule per cubic centimetre per kelvin, acknowledging the composition of the material in bulk. Densities can readily be calculated from X-ray crystallographic data so that this presents no extra hardship. Secondly, one should be aware that the highest possible spin is not always attained, and, particularly in 3d systems, it may be impossible to saturate the spin system in moderate fields [15]. 

Fig. 4 (a) Pulse scheme for signal enhancement of the CT and schematic representations of the population distribution for an ensemble of spin-3/2 nuclei under (b) thermal equilibrium, (c) satellite saturation, and (d) satellite inversion... 

Madhu et al. proposed the use of two consecutive RAPT pulse trains to obtain signal enhancement in spin-5/2 nuclei, such as 27Al and 170 [72]. In their scheme, the saturation of population achieved for the outermost ( 5/2) — 3/2)) transitions was followed by saturation of the 3/2) —> l/2) populations before detection of the CT NMR signal. Kwak et al. used similar RAPT pulse trains to obtain significantly improved signal enhancement on 27Al and 93Nb [73],... 

---