Relaxation time studies

AppHcations of soHd-state nmr include measuring degrees of crystallinity, estimates of domain sizes and compatibiHty in mixed systems from relaxation time studies in the rotating frame, preferred orientation in Hquid crystalline domains, as weU as the opportunity to characterize samples for which suitable solvents are not available. This method is a primary tool in the study of high polymers, zeoHtes (see Molecular sieves), and other insoluble materials. 

A variety of relaxation time studies have been performed on toluene. The choice of deuterated toluene avoids certain complicating factors which affect proton NMR studies, such as, dipolar or spin-spin couplings. The dominant relaxation mechanism is quadrupolar and the relaxation times are determined by the reorientation of the C-D bond vector. Relaxation times such as T, are sensitive to the motions of the solvent around the larmor frequency, which is on the order of 14 MHz in this study. T2 measurements may probe slower motions if the molecule undergoes slow and/or anisotropic motion. The relaxation time results presented in Figure 3 are significantly shorter than those found in bulk toluene solutions (18.19). In bulk toluene, the T and T2 values are equal above the melting temperature (1.2.). In this polymer system T2 < T indicative of slow and/or anisotropic reorientation. 

Association between lanthanide ions and azide or thiocyanate ions has been studied in solution by electronic, Raman and NMR spectroscopy.186,187 The complexation constant in water between Nd3+ and N3 is approximately 2.5. Longitudinal relaxation time studies for Gd-Dy indicate that the M—N—NN angle is bent (135° approximately). 

Slade, R.C.T., Forano, C.R.M., Peraio, A. and Alberti, G. (1993) A JH NMR relaxation-time study of dynamic processes in zirconium phosphates of differing crystallinities and in related compounds. Solid State Ionics, 6, 23-31. 

Recent fast and quantitative relaxation time studies led to new observations regarding relaxivity parameters after MCAO within minutes after occlusion a decrease in T2 and an increase in T1 were observed at 8.5 T by Calamante and colleagues (1999), bringing up other causes of relax-... 

Finally, higher-rate motions (106—109 Hz) can be examined by spin-lattice relaxation time studies. Spin-lattice relaxation (as other relaxation processes) relies on fluctuations in nuclear spin interactions induced by molecular motion. Thus, in cases where relaxation is dominated by one particular nuclear spin interaction, the spin-lattice relaxation times can be calculated for different motions and compared with experimentally derived values. 

A spatially unresolved experiment is usually taken to imply a spectroscopy experiment, although relaxation time studies (Section II.B), transport (Section... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

Kuntz, G. P. P., and Kotowycz, G. (1975). Biochemistry 14, 4144. A Nuclear Magnetic Resonance Relaxation Time Study of the Manganese(II)-Inosine 5 -Triphosphate Complex in Solution. 

Deslauriers, R., Levy, G. C., McGregor, W. II., Sarantakis, D., and Smith, I. . P. (1975). Biochemistry 14, 4335. Conformational Flexibility of Luteinzing Hormone-Releasing Hormone in Aqueous Solution. A Carbon-13 Spin-Lattice Relaxation Time Study. 

Relaxation time studies of the filling process of porous silica with water and cyclohexane have been used to establish whether the adsorption is homogeneous. It was found that water initially collected in small puddles at interstices in the structure, and then formed a surface layer over the silica surface before the remaining pore volume was filled. On the other hand, cyclohexane appeared to fill the smaller pores completely before spreading to the larger pores. A similar effect was observed for water adsorbing in a silica that had been chemically treated to make the surface hydrophobic. Thus, the fluid location in mesoporous materials at low loadings depends critically on the wettability of the surface. 

The structure of liquid alcohols and of alcohol-water mixtures has been reviewed by Franks and Ives. The pure liquids are associated by hydrogen bonding and dielectric relaxation time studies suggest rapid equilibrium between short-lived polymeric chains of various lengths. 

Structures that are more representative of typical DHBs (e.g., 6-7) came from the author s and Morris groups in the form of an extensive series of metal hydrides in which a DHB is formed with OH or NH protons (= A-H) as the weak acid partners, both intramolecularly and intermolecularly. The characteristics of the DHB proved to be a short H... H distance (ca. 1.8 A versus the sum of the van der Waals radii of 2.4 A) found from x-ray and neutron diffraction work in the solid state and by NMR relaxation time studies in solution. Theoretical and experimental studies led to a typical A-H... H-M interaction strength of ca. 4-8 kcal/mol. 

Our belief that dynamics and flexibility are both retained in A-pivot lariat ether complexes derives from two sources. First, we have studied the relaxation times (Ti) for a variety of A-pivot lariat ethers using C-NMR [6]. The C-NMR results show essentially no change in the mobility of detectable nuclei whether or not the lariat ether is complexed by sodium or potassium cations. Second, ultrasound relaxation time studies conducted collaboratively with Petrucci and Eyring [13] have confirmed high binding rates for a two-step complexation mechanism. 

This Is especially justified In presence of Hg Ions but was recently questioned for metal-free solutions (except ATP) on the basis of relaxation time studies [29]. 

Torrell, L. M., and Angell, C. A., Ion-matrix coupling in polymer electrolytes from relaxation time studies, Br. Polym. J., 20, 173-179 (1988). 

Dielectric spectroscopy and scattering studies on the structural relaxation in many different materials have assumed that the normal T-dependence of the relaxation time of a liquid will closely resemble that of propylene glycol (PG), that is, both bulk water and confined PG relax in the same manner, and with an apparent continuity. The main relaxation time of PG exhibits a thermal behavior that differs from that proposed for bulk and confined water. Confined water relaxation times seem substantiaiiy altered when compared to bulk water (which evidently is not the case in confined EG). It also shows an apparent FSC. In addition, an even more dramatic change in the T-dependence of water confined in nanoporous MCM-41 is clearly evident. These results are not unique in that they simply exhibit the typical behavior of supercooled water in biological materials and in other confined environments. Thus, we consider both bulk and confined ethylene glycol (EG, OHCH2CH2OH). Figure 17 shows the EG dielectric relaxation times studied. 

Linewidth effects and relaxation time studies have been very extensive in this field. These are, of course, primarily studies of rates, which are not my present... 

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