Spin lattice relaxation molecular size

Three parameters are readily obtainable from FiMR spectra which may be useful in studying binding interactions the chemical shift [jS], the linewidth (Av) or the apparent or effective spin-spin relaxation time (T2 ), and the spin-lattice relaxation time (Ti). C chemical shifts can reflect steric strain and change in the electronic environment within a molecule when it hinds to another species. Spin-lattice and spin-spin relaxation times can yield information on the lifetimes, sizes and conformations of molecular complexes. 

In contrast to chemical shifts and spin-spin coupling constants, spin-lattice relaxation times provide no direct structural information. Spin-lattice relaxation is a function of molecular motion, and thus, to the extent to which it is dependent on molecular size and shape, spin-lattice relaxation can provide important information not obtainable... 

A number of mechanisms are known to contribute to spin-lattice relaxation in a molecule dipole-di-pole, spin-rotation, quadrupolar, scalar, and CSA, each associated with a corresponding relaxation rate. Analytical interest focuses only on the intemu-clear C-H dipole-dipole relaxation with a rate I IDD = 1/Tidd = (f//1.988)(l/Ti), which is proportional to the inverse sixth power of the carbon-proton distance rca, and depends on the rotational and internal molecular motion. For medium-sized molecules of nearly spherical shape (isotropic molecular reorientation) in solvents of low viscosity the condition of extreme narrowing holds, i.e. ct)cTc l for all Larmor frequencies ft)c, and therefore Ri=ANtc. The correlation time %c is a measure of the velocity of the molecule s rotational diffusion jumps. For N nearest protons within a distance rca the quantity A takes the form A = jrca... 

Xu et al. investigated the interaction of water with NAFION under acid, sodium, and potassium forms using NMR relaxometry techniques in terms of dispersion R co) reveahng two types of bound water, the expected bound water and water with considerably reduced mobility in the driest NAFION [105]. Lee et al. applied NMR spin-lattice relaxation techniques to characterize the molecular motion of H20 in NAFION 117, AQUIVION E87-05, and sulfonated-RADEL proton exchange membranes [106]. It was concluded that the motion of H20 is affected by the acidity and mobihty of the sulfonic acid groups to which the water molecules are coordinated at low hydration level. At higher levels of hydration, the molecular motion of H20 is affected by the phase separation of the hydrophihc/hydrophobic domains and the size of the hydrophilic domains. 

Spin diffusion to paramagnetic impurity is an important mechanism, not only in solids but in some large molecules which exist in solution. In fact, there can be spin diffusion not only to impurities but to any center which has good coupling to the lattice and this could be a sub-molecular unit with its own motion conducive to rapid relaxation or a quad-rupolar nucleus. Because of the sheer size of the electron moment, spin diffusion to paramagnetic impurities is more common than to these other heat sinks. See the review by Noack (1971) for references to this mechanism. 

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