Alkali metals relaxation rates

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

Figure 14 Measured relative molar shifts (a) 8 = the dielectric relaxation time, and (b) 8 = the intramolecular proton magnetic relaxation rate, of aqueous alkali-metal halide solutions these are plotted on the vertical axis against negative ion radii on the horizontal axis and against positive ion radii on the third axis...

The results of nuclear magnetic resonance (NMR) measurements on alkali fullerides K cC o reported. The NMR spectra demonstrate that material with 0 < X < 3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C o Ions rotate rapidly in the KsC q phase at 300 K, while 50 ions in the insulating KaC o phase are static on the time scale of the lineshape measurement. The temperature dependence of the spin-lattice relaxation rate in the normal state of is found to be characteristic of a metal, indicating the... 

Figure 17 Effect of alkali impregnation on Pt spin-lattice relaxation for Pt/Ti02. (a) Spin-lattice relaxation rates across the NMR spectrum for several clean-surface (open symbols) and alkali-impregnated (filled symbols) samples. The changes are important near l.lOG/kHz (the surface signal) and undetectable at 1.13 G/kHz. (b) Korringa relationship for the spin-lattice relaxation at the surface peak of the spectrum. The alkali impregnation does not change the metallic character, but increases the Ef-LDOS on the metal surface. The dash-dotted straight line is an extrapolation of earlier clean-surface data obtained at lower temperatures for comparison.

Above 450 K, the NMR spectrum of Rb3C6o contains 2 sharp resonances arising from mbidium in non-equivalent octahedral and tetrahedral sites (Walstedt etal. 1993). The octahedral peak appears at about 52 ppm and the tetrahedral peak is at about 195 ppm with an octahedraktetrahedral intensity ratio of 1 2, consistent with the known crystal structure. As the temperature is lowered these resonances broaden and shift slightly and a third tetrahedral resonance appears at 200 K the 3 resonances occur at 40 ppm (octahedral), 165 ppm (tetrahedral) and 270 ppm (new tetrahedral). The formation of the second tetrahedral site has been explained in terms of alkali-metal vacaneies which occur only in the tetrahedral positions (Apostol et al. 1996). Measurements of the Rb and Rb relaxation rates indicate a quadrupole relaxation mechanism involving phonons, and no change in either the NMR spectrum or the relaxation rates was found in the vicinity of Tc for this compound (Corti 1993). 

The problem of demonstrating the involvement of a metal ion in a biological reaction has always been a very difficult one, since the most important metals (such as the alkali metals, alkaline-earth metals, and zinc) possess very few convenient handles . The replacement of the native metal by a less retiring one has become a well-established technique, and the n.m.r. work of Cohn and her school on systems in which the paramagnetic Mn + ion has been substituted for the diamagnetic Mg + ion needs no introduction. The interaction of transfer-RNA and related compounds with Mn + has recently been demonstrated by the enhancement of the proton relaxation rate (p.r.r.) technique. 

Comparatively little work has been done on the kinetics of complex formation between the alkali metal ions and simple ligands in view of the high rate constants and low stability constants involved. Atkinson has recently studied the ultrasonic absorption of the five alkali metal sulfates in water in the frequency range 25-250 MHz, where he found only one relaxation for each salt. The results are analyzed in terms of the normal two-step mechanism (the fast formation of an outer-sphere complex followed by rapid conversion to the inner-sphere complex) in which the rates of the two steps approach each other as the concentration of the solution decreases. (The concentrations were in the range 0.3-1.0 mol dm". ) As expected, the reactions are nearly diffusion controlled the rate constants for inner-sphere complex formation at 0.5 mol dm and 25°C are 1.0 x 10 s for Li, Na, Rb, and Cs sulfates but 2.0 x 10 s for the potassium salt. 

Effects in NMR Chiroptical Spectroscopy, Orientated Molecules and Anisotropic Systems Heteronuclear NMR Applications (Ge, Sn, Pb) Heteronuclear NMR Applications (O, S, Se, Te) Liquid Crystals and Liquid Crystal Solutions Studied By NMR Magnetic Resonance, Historical Perspective NMR Data Processing NMR in Anisotropic Systems, Theory NMR Relaxation Rates NMR Spectroscopy of Alkali Metal Nuclei in Solution P NMR Parameters in NMR Spectroscopy, Theory of Product Operator Formalism in NMR Relaxometers Xenon NMR Spectroscopy. 

A review of nuclear magnetic properties of alkali metal nuclei is followed by a brief survey of solvation of alkali metal cations, their hydration especially. The chemistry of alkali metal anions is then evoked. Chemical shifts and relaxation rates will be described with emphasis on the predominant factors contributing to these observables. A brief history of alkali metal NMR will be followed by selected applications to ion pairing phenomena, inclusion complexes, preferential solvation, the chelate effect, and polyelectrolytes. 

The same conclusion follows from consideration of the IQ values (Table 4) in the relaxation equations, relaxation rates are governed by the magnitude of the product Q by the (21 + 3)/ I (2I-1) term. All the other alkali metal nuclei have moderate... 

Applications, Clinical MRI Applications, Clinical Flow Studies MRI Theory NMR Relaxation Rates NMR Spectroscopy of Alkali Metal Nuclei in Solution Nuclear Overhauser Effect. 

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