Anisotropic reorientation

Woessner D E 1962 Spin relaxation processes in a two-proton system undergoing anisotropic reorientation J. Chem. Rhys. 36 1-4... 

The main objectives of this article are (i) to give an account of the simple theory related to spin-lattice relaxation-rates, in a language that is directed, as far as possible, to the practising chemist rather than to the theoretician (ii) to caution against uncritical use of this simple theory for systems that are strongly coupled, or undergoing anisotropic reorientation, or both (hi) to introduce the pulse n.m.r. experiments that are used to measure spin-lattice relaxation-rates, and to stress the precautions necessary for accurate... 

A variety of relaxation time studies have been performed on toluene. The choice of deuterated toluene avoids certain complicating factors which affect proton NMR studies, such as, dipolar or spin-spin couplings. The dominant relaxation mechanism is quadrupolar and the relaxation times are determined by the reorientation of the C-D bond vector. Relaxation times such as T, are sensitive to the motions of the solvent around the larmor frequency, which is on the order of 14 MHz in this study. T2 measurements may probe slower motions if the molecule undergoes slow and/or anisotropic motion. The relaxation time results presented in Figure 3 are significantly shorter than those found in bulk toluene solutions (18.19). In bulk toluene, the T and T2 values are equal above the melting temperature (1.2.). In this polymer system T2 < T indicative of slow and/or anisotropic reorientation. 

Woessner47 has also treated the case of a methyl group attached to an axially symmetric ellipsoid, whereas Levy et al.66 derived equations for the methyl internal rotation superposed on a fully anisotropic motion. The effect of anisotropic reorientation can dramatically alter the relationship between rigidly held methine, methylene, and methyl C—H vectors. Deviation from the ratio T,(CH)/ T,(CH3) = 3 can be considerable, depending on the relative orientation of C—H vectors with respect to the principal diffusion axes. 

Contrary to carbon-black-filled conventional rubbers, which form a semi-rigid interface at the carbon black surface, PDMS chain units at the silica surface are not rigidly linked to the silica surface. Two types of dynamic processes are thought to occur at the interface relatively fast anisotropic reorientation of chain units in the interfacial layer and slow adsorption-desorption of chain units (Figure 10.13) [108, 113]. 

The practical independence of Li-0 and Li-D distances with temperature shows the strong orientational correlation of water molecules around Li it is most likely that an average orientation of the coordinated water molecules is such that the four atoms in a Li -D20 unit is pyramidal. The strong orientational correlation of the bound water molecules rationalizes the anisotropic reorientational motion of the water molecules in the hydration shell found by nuclear magnetic relaxation data of supercooled LiCl solutions. The evolution of the secondary hydration shell of Li may be a hint of nucleation of ice, glass transition, and partial recovery of hydrogen bonds in the supercooled state. 

Interpretation of relaxation data may be complicated by the presence of hydrogen bonding, ion-pairing effects, anisotropic reorientation, chemical exchange and intramolecular motion. In contrast to the findings of Eaton and co-workers,Kirby et al. reported that the Co relaxation rates measured for a series of Co(III) complexes with high and low symmetries do not show any dependence on the Larmor frequency, hence ruling out any predominant... 

CALCULATION OF ROTATIONAL DIFFUSION CONSTANTS FOR ANISOTROPIC REORIENTATION BY THE MODIFIED HILL METHOD. 

Anisotropic reorientation has also been demonstrated using the quadrupole relaxation of two nuclei for some other symmetric-top molecules, e.g. VOCl and Cl NMR Ref. [127]), CCl CN and... 

As a first example we consider the anisotropic reorientation of acetonitrile, the first system for which anisotropic reorientation was detected experimentally 19,101. Besideninr relaxation a number of other spectroscopic methods have been used since then to investigate the reorientational dynamics of this liquid 11. ... 

Nuclear magnetic resonance (NMR), in particular, deuterium NMR, has proven to be a valuable technique for determining the nature of molecular organization in liquid crystals. The utility of the NMR technique derives from the fact that the relevant NMR interactions are entirely intramolecular, i.e. the dominant interaction is that between the nuclear quadrupole moment of the deuteron and the local electric-field gradient (EFG) at the deuterium nucleus. The EFG tensor is a traceless, axially symmetric, second-rank tensor with its principal component along the C—D bond. In a nematic fluid rapid anisotropic reorientation incompletely averages the quadrupolar interaction tensor q, resulting in a nonzero projection similar to the result in Eq. (5.6) ... 

II.5 " H NMR background Fast, anisotropic reorientations of a C-D bond lead to a quadrupolar interaction which is no longer averaged to zero. ° When motions are uniaxial around a macroscopic symmetry axis, such a residual interaction splits the liquid-like NMR line into a doublet whose spacing is in frequency units ... 

The effect of pressure on the anisotropic reorientation of acetonitrile-c/3 in the liquid state can serve as in illustrative example of high pressure NMR studies of molecular reorientation in liquids. The deuteron and nitrogen spin-lattice relaxation times of acetonitrile-( 3 have been measured as a function of pressure up to 200 MPa at 23°C, using the NMR Tj relaxation technique. Since the quadrupole coupling constants for nitrogen and deuterons in acetonitrile-( 3 are known, the experimental Ti data are interpretable in terms of the rotational diffusion constants for motion perpendicular and parallel D to the symmetry axis. 

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