Relaxation time 284 measuring

Sohaublin S, FIdhener A and Ernst R R 1974 Fourier speotrosoopy of non-equilibrium states. Applioation to CIDNP, Overhauser experiments and relaxation time measurements J. Magn. Reson. 13 196-216... 

H longitudinal relaxation time, measured by the inversion-recovery technique, at 293 K. 

The Fourier transform of a pure Lorentzian line shape, such as the function equation (4-60b), is a simple exponential function of time, the rate constant being l/Tj. This is the basis of relaxation time measurements by pulse NMR. There is one more critical piece of information, which is that in the NMR spectrometer only magnetization in the xy plane is detected. Experimental design for both Ti and T2 measurements must accommodate to this requirement. 

Turning from chemical exchange to nuclear relaxation time measurements, the field of NMR offers many good examples of chemical information from T, measurements. Recall from Fig. 4-7 that Ti is reciprocally related to Tc, the correlation time, for high-frequency relaxation modes. For small- to medium-size molecules in the liquid phase, T, lies to the left side of the minimum in Fig. 4-7. A larger value of T, is, therefore, associated with a smaller Tc, hence, with a more rapid rate of molecular motion. It is possible to measure Ti for individual carbon atoms in a molecule, and such results provide detailed information on the local motion of atoms or groups of atoms. Levy and Nelson " have reviewed these observations. A few examples are shown here. T, values (in seconds) are noted for individual carbon atoms. 

A Relaxation time measurement in the solid (Al) in solution (A2). B Mechanical spectroscopy. C Variable-temperature NMR spectroscopy (coalescence temperature measurement). D Variable-temperature EPR spectroscopy... 

Fig. 7. A C-13 relaxation time measurement of solid state wetted cellulose acetate (6% by weight water) using the inversion recovery (IR) method at 50.1 MHz and spinning at 3.2 kHz at the magic angle (54.7 deg) with strong proton decoupling during the aquisition time (136.3 ms), (upper part of the Figure). Tau represents the intervals between the 180 deg (12.2 us) inverting and 90 deg (6.1 us) measuring pulse. 2200 scans were collected and the pulse delay time was 10 s, Cf. Table 3 and Ref.281...

Maryott A. A., Farrar T. C., Malmberg M. S. 35C1 and 19F NMR spin-lattice relaxation time measurements and rotational diffusion in liquid CIO3F. J. Chem. Phys. 54, 64-71 (1971). 

Equation 24.20 gives -y = 1.4 0.1. The relaxation times measured for this material at P = 0.1 MPa [82] in combination with the EOS for the mbber [81] enable the value of 7V associated with each... 

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

The measurement of viscosity is important for many food products as the flow properties of the material relate directly to how the product will perform or be perceived by the consumer. Measurements of fluid viscosity were based on a correlation between relaxation times and fluid viscosity. The dependence of relaxation times on fluid viscosity was predicted and demonstrated in the late 1940 s [29]. This type of correlation has been found to hold for a large number of simple fluid foods including molten hard candies, concentrated coffee and concentrated milk. Shown in Figure 4.7.6 are the relaxation times measured at 10 MHz for solutions of rehydrated instant coffee compared with measured Newtonian viscosities of the solution. The correlations and the measurement provide an accurate estimate of viscosity at a specific shear rate. 

Several commercial companies now offer process compatible NMR systems. These systems are either low resolution based on relaxation time measurements or high resolution Fourier Transform spectral measurements. The low resolution systems are manufactured by Process Control Technologies (www.pctnmr.com) and Progression, Inc. (www.progression-systems.com). Progression s systems... 

Chemical engineers should be aware of the existence of relaxation techniques for studies of very fast reactions. However, since relaxation time measurements call for sophisticated experimental equipment and techniques, they are seldom made outside of basic research laboratories. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

NMR relaxation time measurements (7) and T2) can provide valuable information for investigating the molecular dynamics of water in food systems. However, a number of factors can seriously complicate the analysis... 

Any NMR field-cycling (FC) relaxometry experiment presumes that the sample is subject to a magnetic field of various intensities for time intervals of varying durations. More specifically, between the various intervals of a relaxation-time measurement, the external magnetic field induction... 

Reif also made intensity and relaxation time measurements on the Br and Br lines from AgBr samples which show the effects of defects in the solids. The effect of atomic motion has been measured in many other solids, such as the alkali metals 98), hydrogen in palladium (99), and KH2PO4 (100). 

From nuclear relaxation time measurements, Alexandre and Rigny (3) were able to determine the chemical shift difference between the equatorial and the 2 axial fluorine atoms as 50 2 ppm. They also obtained a value of 195 Hz for the mean 01—F coupling constant and values for the exchange time between the fluorine atoms. 

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et al. (Macromolecules, 1983. 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, it appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. 

In solution, the quadrupolar coupling constant is not measured directly. Instead, the quadrupolar splitting constant is obtained from relaxation time measurements. The QSC is related to x according to equation 10 ... 

A unique feature of NMR is its sensitivity for dynamic processes. Using different techniques, from the well-known lineshape analysis to the application of relaxation time measurements, the correlation times of dynamic processes which can be studied span a... 

Investigations on solid samples have been reported for a rather large variety of compounds generally using phosphorus resonance but in some cases proton, fluorine, boron and even 23Na resonances (1969 217) and relaxation time measurements have been used. Phosphides, and especially simple or mixed paramagnetic phosphides, have often been... 

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