7 , T2 relaxation times

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

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... 

The most difficult materials to study by NMR microscopy are those with short T2 or T2 relaxation times and/or with low concentrations of the nudear spins, which normally result in poor NMR signal intensities. One possibility for improving the image quality is to adapt the shape and size of the rf coils to the size of the objects in order to achieve the best possible filling factor and therefore the best sensitivity [1]. In addition, methods with short echo or detection times have been developed, such... 

Human skin is the largest organ in the human body. It is fundamentally important to health as the semi-permeable barrier - the first line of defence - between the body and the external world. However, it remains relatively inaccessible to conventional magnetic resonance imaging, firstly because it is thin and therefore requires high spatial resolution, and secondly because it is characterized by relatively short T2 relaxation times, particularly in the outermost stratum comeum. Conventional studies have not usually achieved a resolution better than 70-150 pm, with an echo time of the order of a millisecond or so. As a planar sample, skin has proved amenable to GARField study where it has been possible to use both a shorter echo time and achieve a better spatial resolution, albeit in one direction only. Such studies have attracted the interest of the pharmaceutical and cosmetic industries that are interested in skin hydration and the transport of creams and lotions across the skin. 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

Morriss et al. [14] correlated the log mean (LM) T2 relaxation times with viscosity for a number of crude oil samples and viscosity standards using Eq. (3.6.2), as is illustrated in Figure 3.6.1,... 

The last mentioned dynamical capability of lasers is in its infancy. It is possible in principle to make optical measurements which are analogous to coherent NMR measurements, and thereby to observe homogeneous linewidths in inhomogeneously broadened systems, to measure optical or vibrational Ti and T2 relaxation times directly, and to observe quantum recurrences. 

Relaxation is an inherent property of all nuclear spins. There are two predominant types of relaxation processes in NMR of liquids. These relaxation processes are denoted by the longitudinal (Ti) and transverse (T2) relaxation time constants. When a sample is excited from its thermal equihbrium with an RF pulse, its tendency is to relax back to its Boltzmann distribution. The amount of time to re-equilibrate is typically on the order of seconds to minutes. T, and T2 relaxation processes operate simultaneously. The recovery of magnetization to the equilibrium state along the z-axis is longitudinal or the 7 relaxation time. The loss of coherence of the ensemble of excited spins (uniform distribution) in the x-, y-plane following the completion of a pulse is transverse or T2... 

For heteronuclear dipolar relaxation, the dipole-dipole coupling between two unlike spin- nuclei / and S (e.g., 13C-H pair) separated by an internuclear distance rIS is considered. The Zeeman spin-lattice (7jz) and spin-spin (T2) relaxation times for the / spin are given, respectively, by... 

FIGURE 22. Best fits to the signal decays of the C signals at (a) 78 ppm and (b) 114 ppm. Filled symbols, experiment without Li jr-pulses, giving an estimate of the T2 relaxation time. Unfilled symbols, experiment with Li jr-pulses. Reprinted with permission from Reference 220. Copyright 1997 American Chemical Society... 

C. Thomsen, K. E. Jensen and O. Henriksen, P NMR measurements of T2 relaxation times of metabolites in human skeletal muscle in vivo. Magn. Reson. Imaging, 1989, 7, 557-559. 

C. Remy, J. P. Albrand, A. L. Benabid, M. Decorps, M. Jacrot, J. Riondel and M. F. Foray, In vivo P nuclear magnetic resonance studies of Tj and T2 relaxation times in rat brain and in rat brain tumors implanted to nude mice. Magn. Reson. Med., 1987,4,144—152. 

The T, relaxation time is dependent on molecular motion. T, can exhibit more than one minimum when measured as a function of temperature. This happens when several distinct motions occur simultaneously. The T1 relaxation time is dependent upon molecular motion and has more than one minimum as well. The T2 relaxation time is related to the inverse of the NMR linewidth. 

The T2 relaxation time began to increase at approximately —5 °C. The increase in the T2 relaxation time was also associated with molecular motion. T2 approaches T, at higher temperatures. 

Correlation frequencies determined from T1 Tle and T2 relaxation times were plotted against reciprocal temperature and activation energies calculated. The methyl group had a comparatively high activation energy (4.7 kcal/mole) which was attributed to steric hindrance from the reorientation of the two methyls bound to the same carbon and steric hindrance arising from the two phenyl groups on the carbon atom. 

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