Anisotropic rotation, effect

Finally, since the long-range transfer probabilities are all, to some degree, dependent on mutual orientation of donor and acceptor, it is necessary to retain the facility to average over all orientations. Further attention is given in Chap. 5 to rotational effects during reactions between anisotropic molecules in solution. In Chap. 5, Sect. 4.2, the experimental evidence currently available indicates that molecules re-orient more rapidly than they react. 

Zhang J, Jonas J. NMR Study of the geometric confinement effects of the anisotropic rotational diffusion of acetonitrile-dj. J Phys Chem 1993 97 8812-8815. 

Schurr, J. M., Babcock, H. R, Fujimoto, B. S. A test of the model free formulas effects of anisotropic rotational diffusion and dimerization. J. Magn. Reson. Ser. B 1994,105, 211-224. 

It is well-established that solutes incorporated in liquid crystalline phases are oriented in a manner and to an extent that depends primarily on structural similarities between the solute and the mesogen (2a-b,3-8). The rotational and diffusive mobility of both the solute and the mesogen is rendered strongly anisotropic, an effect that also correlates (for the former) with solute/mesogen structural similarities. 

The ESR studies of local segmental dynamics of polymers benefited from the progress achieved in theoretical description of the effects of dynamics on line shape of nitroxide ESR spectra and in resulting software. The Schneider-Freed set of programs made possible the analysis of slow-motional spectra of nitroxides subjected to anisotropic rotational diffusion with rotation symmetry axis oriented quite arbitrarily with respect to the nitroxide axis system. The simulation of one such ESR spectrum with computers available in the 1980s required 20 min of computer time and the best fits to experimental spectra were found by visual comparison simulated spectra with experimental ones. 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

Even if the peculiarities of net-formation of nonspherical particles are not taken into account, at least two fundamentally new effects arise during the flow of dispersion. First, this is the possibility to be oriented in the flow, as a consequence of which the medium becomes anisotropic. And second, this is the possibility to rotate the spherical particles in the flow (spherical particles can, of course, rotato too, but their rotation does not affect the structure of the system as a whole). 

This is obvious for the simplest case of nondeformable anisotropic particles. Even if such particles do not change the form, i.e. they are rigid, a new in principle effect in comparison to spherical particles, is their turn upon the flow of dispersion. For suspensions of anisodiametrical particles we can introduce a new characteristic time parameter Dr-1, equal to an inverse value of the coefficient of rotational diffusion and, correspondingly, a dimensionless parameter C = yDr 1. The value of Dr is expressed via the ratio of semiaxes of ellipsoid to the viscosity of a dispersion medium. 

---