T relaxation times

KINETIC RESULTS FOR DCP AND TCH. The portion of the 50.13 MHz 13C NMR spectra containing the methylene and methine carbon resonances of DCP and the resultant products of its (n-Bu)3SnH reduction are presented in Figure 2 at several degrees of reduction. Comparison of the intensities of resonances possessing similar T, relaxation times (see above) permits a quantitative accounting of the amounts of each species (D,M,P) present at any degree of reduction. 

Backbone dynamics are most commonly investigated by measurement of 15N T and T% relaxation times and the fyH -15N NOE in uniformly 15N-labeled protein. To circumvent problems associated with the limited dispersion of the NMR spectra of unfolded proteins, the relaxation and NOE data are generally measured using 2D HSQC-based methods (Farrow et al., 1994 Palmer et al., 1991). 

The 71Ga and 14N spectra of several of these films also showed partially-resolved shoulders shifted to higher frequency and having shorter T relaxation times that were attributed to Knight shifts in more heavily unintentionally doped regions of the film. These Knight shifts were observed in other GaN film samples [53] and will be discussed in more detail in Sects. 3.4.3 and 3.4.4, where MAS-NMR was used to improve the resolution in polycrystalline powders of h-GaN. Section 3.3.2 also shows 71Ga and 14N MAS-NMR spectra of GaN. 

To continue the investigation, carbon detected proton T relaxation data were also collected and were used to calculate proton T relaxation times. Similarly, 19F T measurements were also made. The calculated relaxation values are shown above each peak of interest in Fig. 10.25. A substantial difference is evident in the proton T relaxation times across the API peaks in both carbon spectra. Due to spin diffusion, the protons can exchange their signals with each other even when separated by as much as tens of nanometers. Since a potential API-excipient interaction would act on the molecular scale, spin diffusion occurs between the API and excipient molecules, and the protons therefore show a single, uniform relaxation time regardless of whether they are on the API or the excipients. On the other hand, in the case of a physical mixture, the molecules of API and excipients are well separated spatially, and so no bulk spin diffusion can occur. Two unique proton relaxation rates are then expected, one for the API and another for the excipients. This is evident in the carbon spectrum of the physical mixture shown on the bottom of Fig. 10.25. Comparing this reference to the relaxation data for the formulation, it is readily apparent that the formulation exhibits essentially one proton T1 relaxation time across the carbon spectrum. This therefore demonstrates that there is indeed an interaction between the drug substance and the excipients in the formulation. 

Sound waves provide a periodic oscillation of pressure and temperature. In water, the pressure perturbation is most important in non-aqueous solution, the temperature effect is paramount. If cu (= 2 nf, where/is the sound frequency in cps) is very much larger than t (t, relaxation time of the chemical system), then the chemical system will have no opportunity to respond to the very high frequency of the sound waves, and will remain sensibly unaffected. 

A NMR study of water adsorbed on silica gel has been made by Zimmerman el al. 18). Transverse (Ta) and longitudinal (Ti) relaxation times of various amounts of water adsorbed at 25° have been obtained with the use of the spin-echo technique and a two-phase behavior of both Ta and T relaxation times has been observed as illustrated in Figs. 10a and b. Generally only one T value is obtained, as for a single phase, except for x/m g HaO/g solid) values in the vicinity ol x/m = 0.126. Two values of Ta... 

Most relaxation measurements are conducted in such a way as to record the resulting magnetization after a variable delay, r, during which the initially created state is allowed to relax. In the spin-lattice relaxation experiment, the T relaxation time can be evaluated by non-linear three-parameter fitting of the following expression [31] to the intensities ... 

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. 

In Fig. 9, the relaxation times are plotted versus reciprocal temperature for the 180 °C cure of DGEBA and MDA. The T, relaxation time minima at —70 °C as well as the Tle relaxation time minima at —130 °C were attributed to methyl group reorientation. 

The C spectra of both compounds include all carbon multiplicities, i.e. CH CH, CH and C and are ideal to demonstrate methods for spectra editing. Furthermore they include carbon nuclei with rather different T, relaxation times. 

Table 6.2-. C chemical shifts, C T, relaxation times and H/ C J-connectivities of peracetylated P-D-Glucose...

The spin-lattice relaxation process is usually exponential. Theoretically, the effect of spin-diffusion, characterized by the coefficient D (order of 1(T12 cm2 s 1), has an influence on T, relaxation times when ix > L2/D, where Lis the diffusion path length. NMR studies of model systems f6r rubber networks, based on a styrene-butadiene-styrene block copolymer (SBSy, in which styrene blocks act as a crosslink for polybutadiene rubber segments of known and uniform length, indicate that spin diffusion operating between PS and PB phases causes a lowering of Tg for the PS component in SBS (as compared to the pure PS) and hindering of the motion of the PB component (as compared to the pure PB)51). 

Fig. 9. Plot of log T, relaxation time vs. reciprocal temperature for the diglycidyl ether of bisphenol-A uncured (A) and cured with methylenedianiline at 54 °C (B), 100 °C (C) and 180 °C (D) (adapted from Refs.53) and16y)...

It is very important that there be sufficient time between pulses in FT-NMR experiments so that the nuclei can return to the original equilibrium state. If the equilibrium state has not been reached before the next H pulse, the still-excited nuclei will not participate in the transition and thus will produce a decreased signal intensity relative to the previous signal. As the experiment proceeds and more pulses are applied, more nuclei will remain in the exited state until eventually none of the nuclei will be in the lower energy state when pulsed. At this point the sample is saturated and will not produce a signal. The length of time required for the nuclei to relax is called the spin-lattice or T relaxation time. 

It is often found that spin 3 nuclei have very long T, relaxation times (up to several hours in some cases) particularly in amorphous solids. Quadrupolar nuclei, however, generally relax quite fast, which makes them of special interest in the study of the solid state. 

For comparison, the T values for the DH groups in the MDj M oligomers are consistently shorter than the T values for the D groups in MD M oligomers. The Si—H groups present in the MDj M oligomers provide a dipolar contribution to the relaxation mechanism which shortens the 29 Si T relaxation times. 

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