13C spin-lattice relaxation time

C Spin-Lattice Relaxation-Times and n.O.e. Values of Resonance of Poly(n-butyl methacrylate) as 50% (w/w) Solution in Toluene-2H8... 

C Spin-Lattice Relaxation-Times, Line Widths, and Nuclear Overhauser Enhancements (n.O.e.) of PS 13140... 

NMR 13C spin-lattice relaxation times are sensitive to the reorientational dynamics of 13C-1H vectors. The motion of the attached proton(s) causes fluctuations in the magnetic field at the 13C nuclei, which results in decay of their magnetization. Although the time scale for the experimentally measured decay of the magnetization of a 13C nucleus in a polymer melt is typically on the order of seconds, the corresponding decay of the 13C-1H vector autocorrelation function is on the order of nanoseconds, and, hence, is amenable to simulation. 

Goddard and Tomalia [1] investigated the encapsulation of 2,4-dich-lorophenoxyacetic acid and acetylsalicylic acid into methyl ester-terminated PAM AM dendrimers by measuring the 13C spin-lattice relaxation times (7 ) of the guest molecules. In the presence of dendrimer, the 7 values of these guest... 

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... 

The NMR spectra were taken on a JEOL JNM-MH-100 (CW) spectrometer using tetramethylsilane as an internal standard. 13C spin-lattice relaxation time of the polymer was measured by the inversion-recovery Fourier transform method on a JNM-FX100 FT NMR spectrometer operating at 25 MHz. 

In order to study the segmental mobility of the copolymer, the 13C spin-lattice relaxation times (Tx) were measured on the copolymer containing... 

C spin-lattice relaxation times of individual nuclei can also be measured by PFT 13C H experiments using a 90°, r, 90", r,... pulse train and noise modulation of the proton decoupling frequency. This method is known as progressive saturation [43] and is based on the following concept. 

Fig. 2.29. Measurement of 13C spin-lattice relaxation times by saturation-recovery ...

Fig. 2.30. Determination of 13C spin-lattice relaxation times by inversion-recovery (a), progressive saturation (b), and comparative evaluation of the intensities (c) values from (a) with full, from (b) with empty characters, abscissa on top for C-2, abscissa on bottom for C-1 and C-3) sample propynol, 75% by vol. in hexadeuterioacetone. 25 C, not degassed, 15.08 MHz, 10 scans/ experiment in (a),...

Table 3.17. 13C Spin-Lattice Relaxation Times 7, of Ribo-nuclease A in Aqueous solutions (cone. 0.019 mol/L 45 °C 15.08 MHz maximum deviation +30% [177]).

Indeed, 13C spin-lattice relaxation times can also reflect conformational changes of a protein, i.e. helix to random coil transitions. This was demonstrated with models of polyamino acids [178-180], in which definite conformations can be generated, e.g. by addition of chemicals or by changes in temperature. Thus effective molecular correlation times tc determined from spin-lattice relaxation times and the NOE factors were 24-32 ns/rad for the a carbons of poly-(/f-benzyl L-glutamate) in the more rigid helical form and about 0.8 ms/rad for the more flexible random coil form [180],... 

Determination of Quadrupole Relaxation Times and Coupling Constants from 13C Spin-Lattice Relaxation Times... 

Table 3.20. 13C Spin-Lattice Relaxation Times Ti (s) of Saccharose in H20 and D20 at 42PC [166], "ch2oh...

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). 

C Spin-lattice relaxation times T at 25 and 50 MHz have been determined for PMMA and CMIM20 in deuterated chloroform solution over a temperature range of - 50 to 50 °C. 

C spin-lattice relaxation time measurements (Table VIII) have been used in the assignment of quaternary, 3C signals in brucine [419] (121) (for details see discussion on codeine in Section V). 

Through measurements of 13C spin-lattice relaxation times, a number of noteworthy motional characteristics related to overall molecular tumbling, hydrogen... 

Table I. 13C Spin Lattice Relaxation Times of Poly(w-butyl...

Figure 13.1 13C spin-lattice relaxation times (nTl) for methylene carbons in bulk polyisoprene at two different field strengths (62.5 (O) and 25.15 ( ) MHz) as a function of inverse temperature. The lines are the best fits of the data to the theory of Dejean de la Batie and co-workers [24] (long dashes) and for the modified relaxation function described in reference [27] (dotted lines)... 

S nuclear quadrupole coupling constants have been determined from line width values in some 3- and 4-substituted sodium benzenesulphonates33 63 and in 2-substituted sodium ethanesulphonates.35 Reasonably, in sulphonates R — SO3, (i) t] is near zero due to the tetrahedral symmetry of the electronic distribution at the 33S nucleus, and (ii) qzz is the component of the electric field gradient along the C-S axis. In the benzenesulphonate anion, the correlation time has been obtained from 13C spin-lattice relaxation time and NOE measurements. In substituted benzenesulphonates, it has been obtained by the Debye-Stokes-Einstein relationship, corrected by an empirically determined microviscosity factor. In 2-substituted ethanesulphonates, the molecular correlation time of the sphere having a volume equal to the molecular volume has been considered. 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and 13C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

NMR spectrum depends on the type of starch (amylose-to-amylopectin ratio) and is associated with the numbers of carbon atoms in the branching points and thermal glucose units. Tables X and XI present 13C spin-lattice relaxation times (Tus) and nuclear Overhauser enhancement (n.O.e) for l3C nuclei of starches of various origins. Figure 21 shows H NMR spectra of amylose and a high-amylopectin waxy sorghum starch. 

Levy and coworkers97 have measured 13C spin-lattice relaxation times, 7), for 3- and 4-aminobiphenyls in a number of solvent systems, and of the corresponding ammonium ions in acidic and nonacidic media. The observed 7) values indicated that the molecular tumbling is anisotropic for these species. In addition, the known biphenyl geometry allowed indentification and semiquantitative evaluation of internal rotation-libration motion. The protonated amine function is motionally more restricted by solvent-solute and ion-pair interactions than the corresponding neutral amine. Thus, in the 3-biphenylammonium ion, the principal axis for molecular reorientation is aligned close to the C3—NHj-bond, whereas in the amine the principal axis lies closer to the biphenyl C2-symmetry axis. In both 3- and 4-aminobiphenyls, the unsubstituted phenyl rings are less restricted due to rapid phenyl rotation or libration. Table 14 presents 13C Tj-data for 4-aminobiphenyl 37 (NH2 on C4) and 4-biphenylammonium acetate 38 and trifluoroacetate 39. 

Figure 2.27. Sequence of measurements to determine the 13C spin-lattice relaxation times of 2-octanol (42) [(CDs CO, 75% v/v, 25 °C, 20 MHz, inversion-recovery sequence, stacked plot]. The times at which the signals pass through zero, t0, have been used to calculate, by equation 10, the 7", values shown above for the 13C nuclei of 2-octanol...

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