Articles you may be interested inHydration in polymer studied through magic angle spinning nuclear magnetic resonance and heteronuclear 13C{1H} Overhauser enhancement spectroscopy: Crossrelaxation and location of water in poly(acrylamide)The effect of proton decoupling upon carbon-13 magnetic resonance spectra is treated in detail with the Solomon formulation for multiply irradiated spin systems. The factors affecting the nuclear Overhauser enhancement are discussed in terms of competition between the dipole-dipole relaxation mechanism essential to the Overhauser effect and other relaxation processes. Using a density matrix formulation, it is exhibited that the maximum Overhauser effect (achieved when the dipolar mechanism dominates) is independent of the number of hydrogen atoms interacting with a relaxing carbon-13 nucleus. On the other hand, if the molecular tumbling motion is isotropic and the dipole-dipole mechanism dominates the relaxation process, then to a first approximation Tl can be shown to depend inversely upon the number of directly bonded hydrogen atoms. Expressions for treating an AMX three-spin system are given also, and the effect of the third spin is discussed. The highly symmetric adamantane molecule provides an excellent test case for the validity of the theoretical conclusions as it is an isotropic tumbler because of the rigidity and its tetrahedral symmetry. Furthermore, it has two nonequivalent carbon positions having different numbers of hydrogen atoms. The experimental Overhauser enhancement was found to be the same for both carbons and the T1's for these two carbons, measured with the adiabatic rapid passage technique, reflected the two-to-one factor expected for CH and CH. groups, respectively. Minor deviations are explained with small contributions from vicinal hydrogens. Having exhibited the validity of the theoretical treatment for adarnantane, it becomes evident that exceptions may be interpreted in terms of deviations from isotropic tumbling motion. As most molecules lack the symmetry to be isotropic in rotational diffusional processes, this experimental technique offers an important way for studying anisotropic molecular motion in liquids.
The B11-F19 spin-spin coupling in BF4-has been found to vary between 1 and 5 c.p.s. in yarious aqueous solutions. This value is dependent upon both a concentration and specific cationic effect which is explained by ion-pair formation. The equilibrium constant for the association in S a B F 4 solutions is about 0.2 and the infinite dilution coupling constant for the BF4-ion in water is 1.13 * 0.07 c.p.s. The average coupling constant in the ion-pair species depends markedly upon the cation involved in the association.
The spin‐lattice relaxation time and the nuclear Overhauser enhancement (NOE) of 19F nuclei in dilute solutions of poly(p‐fluorostyrene) and poly(m‐fluorostyrene) were measured as a function of molecular weight, concentration, temperature, solvent, and field strength. Models for local motion of the chain backbone based on independent internal rotations or on one‐dimensional defect diffusion failed to provide a quantitative description of both T1 and the NOE. A model based on three‐bond crankshaft motions and a cutoff of coupling along the backbone gives a consistent account of these results as well as 13C nuclear magnetic resonance (NMR) relaxation data on polystyrene from the literature. The model provides a finite set of exponential correlation times to describe local motion, the center of the distribution lying in the nanosecond region for dilute solutions of polystyrenes in normal solvents. The apparent activation energy for the backbone motion obtained from the temperature dependence of the average correlation time based on NMR data is about 20 kJ mole−1. The local correlation times based on NMR are appreciably shorter than those determined by dielectric relaxation. This discrepancy points to the existence of several different local motions which make different contributions depending upon the experimental technique employed.
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