Carbon-13 spin-lattice relaxation times

Figure 2 Schematic representation of carbon spin-lattice relaxation time, T,c, and spin-spin relaxation time under CPMAS or DDMAS condition, T2C, as a function of correlation times.

C NMR studies on [3-13C]Ala-labeled bR reveal that the C terminal residues, 226-235, participate in the formation of the C terminal a-helix as manifested from the peak position of 15.91 ppm with reference to the conformation-dependent 13C chemical shifts.68 69 81 82 103 The presence of the a-helix was also proved in view of the corresponding conformation-dependent displacement of peaks from [2-13C] and [l-13C]Ala-bR.81 Only part of this a-helix was visible by X-ray diffraction,25 owing to the presence of motions with correlation times of the order of 10-6s, as judged from the carbon spin-lattice relaxation times, 7jc, and spin-spin relaxation times, T2C, under CP-MAS conditions.81... 

HiU AJ, Pas SJ, Bastow TJ, Burgar MI, Nagai K, Toy LG, Freeman BD. Influence of methanol conditioning and physical aging on carbon spin-lattice relaxation times of poly(l-trimethylsilyl-l-propyne). J Membr Sci 2004 243(l-2) 37-44. 

The lack of hysteresis shows the intimacy to the amorphous phase (Fig. 5). Both chain mobiUty and fast carbon spin-lattice relaxation times (Ti( C) 1 s) are promoted by the random distribution of the propylene... 

In general, no large energy barriers are to be crossed for the motional phenomenon. The activation energy was evaluated by the relaxation values to be as small as 4.5 kj mol in PHTP, lower than a gauche/trans interconversion barrier [39]. Large librations of methylenes account for the short carbon spin-lattice relaxation times at room temperature. 

As illustrated in Fig. 15B, more detailed characterization of molecular motions in membrane proteins is feasible by the following sets of relaxation parameters, carbon spin-lattice relaxation times, T,, carbon spin-spin relaxation times under CP-MAS condition, and proton spin-lattice relaxation times... 

Figures 14 and 15 show the relationship between PTB A content and carbon spin-lattice relaxation time (Ti) of the PTMSP side-chain and backbone carbon atoms, respectively. The effect of PTBA content on Ch is also shown in these figures. Ti of the methyl group carbon atoms (a) in the side-chain of PTMSP became longer with increases in PTBA content. This result indicates that the molecular motion of the trimethylsilyl groups became slower. On the other hand, Ti values for the carbon atoms of the other methyl group (b) and the backbone chain show a maximum at 20 vol % PTBA, which suggests perfect mixing of PTMSP and PTBA. These results coincide with the maximum stability of the gas permeability coefficient (c/. Figure 10).

In contrast, motion at high frequencies in solids is usually of small amplitude thereby resulting in a substantially reduced spectral density in the Larmor frequency region.As such, the values for carbons in solids may vary between a few seconds to minutes and even hours, e g, the carbon spin-lattice relaxation time for crystalline polyethylene is ca. 700 seconds at ambient temperature Clearly, obtaining a spectrum can be time consuming if indeed not prohibitive in the case of multi-line spectra The situation is further aggravated by the fact that linewidths in the solid spectrum may be... 

In principal, resolution of Individual carbon resonances in bulk polymers, allows relaxation experiments to be performed which can be Interpreted in terms of main chain and side chain motions in the solid. In addition to the spin-lattice relaxation time in the Zeeman field, the spin-spin relaxation time and nuclear Overhauser enhancement, other parameters providing data on polymer dynamics include the proton and carbon spin-lattice relaxation times in the rotating-frame, T p, the cross-relaxation time Tqr, and proton relaxation in the dipolar field. Schaefer and Stejskal have carried out pioneering work in exploring polymer dynamics using solid-state NMR techniques. Measurement of T values in glassy PMMA at ambient temperature reveals that the a-CH3 carbon relaxes in <0.1s, the ester methyJL and methylene carbons in ca. Is and the two non-protonated (carbonyl and quaternary) carbons in ca. 10s. These results are consistent with the onset of internal reorientation of a-CH3 at this temperature relatively... 

Tipc Carbon spin-lattice relaxation time in the rotating frame... 

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