Nuclear magnetic relaxation and molecular reorientation

Antony, J. H., Mertens, D., Dolle, A., Wasserscheid, R, and Carper, W. R., Molecular reorientational dynamics of the neat ionic liquid l-butyl-3-methyl-imidazolium hexafluorophosphate by measurement of C nuclear magnetic relaxation data., Chem. Phys. Chem., 4, 588-594, 2003. 

A nuclear magnetic relaxation study85 of lactose (17), which is a basic constituent disaccharide unit of all gangliosides, demonstrates that this molecule reorients anisotropically. The favored axis of molecular reorientation appears to lie along the axis of the molecule and therefore is reflected in the shorter T, value at C-4 of the galactose residue. The apparent differences between the relaxation times observed for the C-l resonances of the a (17a) and /3 (17b) isomers may also be reflected in their differing C—H orientations relative to the anisotropic axis. Similar anisotropic motion was observed for methyl /3-lactoside,85 methyl /3-cellobioside,84 and other disaccharide derivatives in solution. 

Nuclear magnetic relaxation represents In memy respects the most completely developed method to Investigate molecular reorientation In liquids. Experimental and theoretical progress has been made for many years and.the method Is well documented In textbooks 1-3 and review articles 4,5. ... 

In general, fluctuations in any electron Hamiltonian terms, due to Brownian motions, can induce relaxation. Fluctuations of anisotropic g, ZFS, or anisotropic A tensors may provide relaxation mechanisms. The g tensor is in fact introduced to describe the interaction energy between the magnetic field and the electron spin, in the presence of spin orbit coupling, which also causes static ZFS in S > 1/2 systems. The A tensor describes the hyperfine coupling of the unpaired electron(s) with the metal nuclear-spin. Stochastic fluctuations can arise from molecular reorientation (with correlation time Tji) and/or from molecular distortions, e.g., due to collisions (with correlation time t ) (18), the latter mechanism being usually dominant. The electron relaxation time is obtained (15) as a function of the squared anisotropies of the tensors and of the correlation time, with a field dependence due to the term x /(l + x ). 

A second important mechanism for fluorine spin-lattice and spin-spin relaxation is produced by the chemical shielding anisotropy (CSA) [13, 14, 21, 71]. The magnetic field experienced by a nuclear spin depends on both the electronic structure of the molecule and how easily the electrons can move in the molecular orientations. In addition, the CSA depends on how the molecule is oriented in the magnetic field. Like spin-spin and dipole-dipole interactions, the CSA of small, rapidly tumbling molecules will be an averaged value (the chemical shift). However, these tumbling motions cause fluctuations of the local magnetic field that lead to relaxation. Also slower reorientation, or an environment that restricts the molecular motion, will result in broader lines due to CSA. 

The parameters characterizing the nuclear magnetic resonance in a solid, in particular the linewidth, second moment, and the spin-spin and spin-lattice relaxation times, are strongly affected by molecular reorientations and atomic diffusive motions. Application of NMR methods to the study of hydrogen (and deuterium) diffusion in the non-magnetic rare-earth hydrides has been extensive. 

There are a large number of studies concerned with nuclear spin relaxation [7.45] in liquid crystals. The majority of these involve observation of the total proton magnetization arising from all protons in a mesogen. The experiments usually yield only one relaxation time, which is difficult, if not impossible, to relate to details of motion in the liquid crystalline phase. Deuteron and carbon-13 NMR may be used to study nuclear spin relaxation at several sites in a mesogen. In particular, direct measurement of spectral densities using deuterium resonant lines has made testing of motional models possible in liquid crystals. As yet, there is no report on systematic comparison between the different models of molecular reorientation. 

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