Spin-lattice relaxation time measurements, carbon

The Carbon-13 spin-lattice relaxation-time measurements are related to the reorientation time of a C—H bond axis. For mesitylene the C—H bond axis observed was that of the ring carbons (not the methyl carbon). For symmetric top molecules such as benzene and mesitylene the tnmr is related to both r and r by (Huntress, 1968)... 

Alms, et al. (1973,1974) have performed depolarized Rayleigh scattering and Carbon-13 spin-lattice relaxation-time measurements on solutions of benzene and mesitylene as a function of solvent viscosity. The solvents used were isopentane, cyclooctane,... 

Pearson, H., Gust, D., Armitage, I. M., Huber, H., Roberts, J. D., Stark, R. E., Void, R. R., and Void, R. L. (1975). Proc. Nat. Acad. Sci. U.S.A. 72, 1599. Nuclear Magnetic Resonance Spectroscopy Reinvestigation of Carbon-13 Spin-Lattice Relaxation Time Measurements of Amino Acids. 

Two-component systems have received some attention. - Poly(vinyl-idene fluoride>-poly(methyl or ethyl methacrylate)s blends have been mentioned in the previous section. A study of compatibility in blends of poly(vinyl chloride) and ethylene-vinyl acetate copolymer (EVA) has been reported. Spin-lattice relaxation time measurements were bi-exponential corresponding to rigid [poly(vinyl chloride)] and mobile (EVA) phases. A precipitated mixture showed intermingling of EVA in the poly(vinyl chloride) phase, but phase separation occurred on heating. Cashell et a/. have contributed a short note on the effects of magnetic susceptibility inhomogeneities of lincwidths in carbon black-filled... 

Malterud KE, Anthonsen T (1987) Carbon-13-NMR Studies on Partially 0-Methylated Phenols Spin-Lattice Relaxation Time Measurements as a Tool for Signal Assignments. Acta Chem Scand B41 6... 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

FIGURE 31. Typical data set for measurement of the spin-lattice relaxation times of the sp2-hybridized carbon atoms of, 6-carotene at 11.7 T. The chemical shift values are shown across the bottom of the figure. The t-value for each spectrum is the delay time in the inversion-recovery pulse sequence. Reprinted with permission from Reference 49. Copyright (1995) American Chemical Society... 

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

Complementary NMR measurements, such as rises of carbon polarisation in a spin-lock experiment and determination of 13C spin-lattice relaxation times in the rotating frame, Tip(13C), support these conclusions about the correlation times of the side-ring CH and CH2 motions in the various poly(cycloalkyl methacrylates). 

The temperature dependencies of the ( 172)0/ 1/2 ratio, where ( 1/2)0 is the 1/2 value measured at room temperature, determined for the CHOH - CH2 - O and CH2 - N units of the hydroxylpropyl ether (HPE) sequence (Fig. 92) in the HMDA network [63] are shown in Fig. 97. It is worth noticing that the 1/2 values of these two types of carbons have the same temperature dependence. Up to 60 °C, the 1/2 values are constant and equal to the rigid-lattice values, indicating that the HPE sequence does not undergo any local motion at a frequency equal to or higher than 105 Hz in this temperature range. Above 60 °C, mobility develops, which leads at 100 °C to motions in the tens of kilohertz for the whole HPE sequence. These results are qualitatively confirmed by data on 13C spin-lattice relaxation time in the rotating frame, Tip(13C). 

Aso et al. (40) examined the molecular mobility of sucrose and polyvinylpyrrolidone in 1 1 lyophilized mixtures by measuring the spin-lattice relaxation times (7)) of individual carbon atoms by NMR for systems containing residual moisture at varying levels. 7) of the pyrrolidone ring carbon increased with residual moisture for lyophilized PVP alone. However, the mobility of these carbons did not increase with residual moisture when PYP was colyophilized with sucrose. Similarly, the mobility of sucrose did not increase with water activity as much in sucrose/PVP mixtures as much as in sucrose alone. Inhibition of sucrose crystallization by PVP in the presence of water appears to be linked to the effect of PVP on the molecular mobility of sucrose. 

The information obtained from the model studies was then applied to the spectra of sinomenine (38), codeine (39), and thebaine (40), the spectra of which are also recorded in Table VII. Except for the assignment of the quaternary carbon atoms in 39 and 40 the other assignments followed readily from model studies and from application of shift parameters. The quaternary carbon atoms of codeine had already been examined by Wehrli (55) using spin lattice relaxation time (7 ) measurements. In this way an unambiguous assignment of the signals at C-3, C-4, C-ll, and C-12 was achieved. 

The pulse width is an important factor in the measurement of pulsed spectra. The optimal pulse-width may be estimated21 from the equation cos a = exp(— TJT), in which a is the pulse width (in degrees), Tt the spin-lattice relaxation-time (in s), and T the pulse-repetition time (in s). For monosaccharides in 20% aqueous solution, values of the protonated carbon atoms are22 1 s at 30°. Using 8 k of computer memory for the acquisition, and a sweep width of 5-6 kHz, T becomes 0.6-0.8 s, and the equation gives an optimum pulse-width of 60°. In Fig. 1 is shown a series of spectra measured at different pulse-widths, all other variables being kept constant. The best s/n is seen to correspond to a 63° pulse. If, 3C-n.m.r. spectra are recorded for very concentrated solutions, or impure samples, the Tj values may become small, and, in such cases, a 90° sample pulse will be optimal. 

Selected spin-lattice relaxation times are shown in Fig. 1. Carbon nuclei bonded to protons in typical complexes of the type reviewed here frequently have short spin-lattice relaxation times (7 ) and hence relatively large tilt angles can be used in routine FT measurements. Tx is considerably longer for carbon nuclei not directly bonded to protons, e.g., a value of 56 sec has been measured for the meso-allyl carbon atom in 6, which accounts, in part, for the low intensity of the signals of such nuclei under standard measurement conditions. 

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