Restricted spin-lattice relaxation times

The second difficulty is not encountered in proton spectroscopy, where proportionality between peak area and concentration of the respective sequence is virtually guaranteed, but is present in caibon spectroscopy where one works under heteronuclear broad-band decoupling conditions. Under such conditions, both the nuclear Oveihauser effect (NOE) and the differences in spin-lattice relaxation time T, can alter the intensity. In this connection, however, Schaefer showed that for the different C nuclei inside the polymer chain, because of the restricted molecular movement, there are no large differences in NOE (121). 

The Pti samples (182) were prepared as colloids, protected by a PVP polymer film. Layer statistics according to the NMR layer model (Eqs. 28-30) for samples with x = 0,0.2, and 0.8 are shown in Fig. 63. The metal/ polymer films were loaded into glass tubes and closed with simple stoppers. The NMR spectrum and spin lattice relaxation times of the pure platinum polymer-protected particles are practically the same as those in clean-surface oxide-supported catalysts of similar dispersion. This comparison implies that the interaction of the polymer with the surface platinums is weak and/or restricted to a small number of sites. The spectrum predicted by using the layer distribution from Fig. 63 and the Gaussians from Fig. 48 show s qualitative agreement w ith the observed spectrum for x = 0 (Fig. 64a). 

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. 

The dynamics of intact lime cuticle and its two major component polyesters, cutin and wax, have been studied by the MAS NMR experiment [134]. By the measurements of spin-lattice relaxation times and spin-lattice relaxation times in the rotating frame which characterize respectively the megahertz- and kilohertz-regime motions, it is indicated that motional restrictions are present at the crosslinks of the cutin polymer and along the alkyl chains of the wax. The values of relaxation times, which differ for analogous carbon sites of cutin and wax individually, approach common values for the two materials in the intact lime cuticle. These results are considered to provide evidence for hydrophobic association within the plant cuticle of the long aliphatic chains of cutin and wax. 

Both the resonance linewidth and the spin lattice relaxation time (T ) of the water protons have been studied as a function of temperature at different water contents (16). For samples with low water content (approximately 2.7%), tRe linewidth decreases with increasing temperature from 230 to 300°K. For samples of higher water content, the linewidth decreases up to about 250°K, then remains constant. The T,. data (Figure 4) exhibits a minimum that shifts to higher temperatures with decreasing water content. This indicates restricted water mobility at low water content. The curves for higher (3-20%) levels of water show two minima, suggesting a complex motion or possibly two components. 

As an introduction, our previous studies on the conformations of maleic acid copolymers with aromatic vinyl monomers are summarized. To characterize the compact form and the pH-indueed conformational transition of the maleic acid copolymer with styrene in aqueous NaCl, 400 MHg H-NMR spectra were measured. The spectral form depended on the molecular conformation. Because each of proton resonance peaks could not be separated, the spin-lattice relaxation time T was estimated by using the inversion recovery technique (tf-t-tf/2). The T s for both side chain and backbone protons reflected the transition, and the protons were considered to be in a more restricted motional state in the compact form than in the coil form. Also, from temperature dependence of each Tj, motion of the copolymer in the coil form was described in terms of the local segmental jump (D) combined with the isotropic rotational motion (O), when a ratio between both the correlation times tq and Tq was about 0.07. For the compact form, the ratio was found to be about 10. By referring to theoretical diagram of Tj vs. tq for the methylene protons on the backbone, value of Tn for the compact form was compared with that for the coil form at 35 C. 

Unlike small molecules, whose spin-lattice relaxation times are typically controlled by their tumbling motions, the relaxation times of polymers are governed by the local segmental motions of groups of atoms. The atoms of the chain ends and the branch ends, typically those attached to the last few carbons, are more mobile and exhibit longer relaxation times. In comparison, the motions of the atoms in the polymer backbone are more restricted and therefore have shorter Tj s. In this way, the spin-lattice relaxation experiment provides a way to distinguish the resonances of chain ends/branch ends from those of the backbone. 

The restricted reorientational freedom in the crystalline regions of PE results in spin-lattice relaxation times of ca. 10 sec. While CH2 carbons of PE chains in amorphous material have T s ranging from O.Ss to 3s.Advantage can be taken of this differential in relaxation rates to study only the non-crystalline regions of PE. VanderHart has shown... 

Experimental and Calculated P Spin-Lattice Relaxation Times for DNA Restriction Fragments... 

Fig. 6. Spin-lattice relaxation time (T,) versus temperature for four DNA restriction fragments (length in base pairs) , 43 A, 69 , 84 O, 180. Adopted from Hart et al. (1981).

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