Anistropy

Figure 4.9 illustrates time-gated imaging of rotational correlation time. Briefly, excitation by linearly polarized radiation will excite fluorophores with dipole components parallel to the excitation polarization axis and so the fluorescence emission will be anisotropically polarized immediately after excitation, with more emission polarized parallel than perpendicular to the polarization axis (r0). Subsequently, however, collisions with solvent molecules will tend to randomize the fluorophore orientations and the emission anistropy will decrease with time (r(t)). The characteristic timescale over which the fluorescence anisotropy decreases can be described (in the simplest case of a spherical molecule) by an exponential decay with a time constant, 6, which is the rotational correlation time and is approximately proportional to the local solvent viscosity and to the size of the fluorophore. Provided that... 

The specific heat for uncoupled spins does not depend on the orientations of the anisotropy axes however, the corrections due to the dipolar coupling do, as can be seen in Figure 3.4. As for the linear susceptibility, the effect of dipolar interaction is stronger in the case of parallel anistropy than for random anistropy. 

Further details may be found in the above quoted references. In particular, de Josselin de Jong (D14) and Saffman (SI, S2, S3) give relatively rigorous developments that take into account the anistropy caused by the flow. Thus different estimates are obtained for the dispersion coefficients depending on whether or not the direction considered is perpendicular or parallel to the mean flow. 

James Bayer. Would the authors define what they mean by ordered arrangement of micelles or degree of anistropy in semicokes, as related to rank and fluidity ... 

The first combined 13C- H MAS/CP experiment was performed by Schaefer and Stejskal (23). The double-resonance procedure decouples the strong heteronuclear dipolar interactions and indirect J couplings, while the weak 13C signal is enhanced by proton polarization transfer. The residual spectral width of several kHz arises from 13C chemical shift anistropy and weak 13C- 3C dipolar interactions between 1.1 % abundant 13C nuclei. Both of these interactions are removed by MAS. 

Anistropy may cause differences in the magnitude of indentations or result in deformed indentations (Fig. 7.1), from which the true hardness in a given crystallographic direction should be calculated (Fig. 7.2). 

Thus recognition of the characteristic coupling constants for an a-proton, as well as establishing the radical as of the general type R2CH, also serves to fix the direction of the C—H bond and of the carbon 2p orbital. This not only shows how the radicals are located in the crystal, but also provides a basis to which any other anisotropic pattern can be anchored. Thus, for example, the small anistropy of /3-protons can be fitted to the radical. This is not particularly informative for rotating... 

C. Bertucci, E. Domenici, and P. Salvadori, Circular dichroism detection in HPLC Evaluation of the anistropy factor, J. Pharm. Biomed. Anal., 5 843-846 (1990). 

Magnetic anistropy. (S. Mitra, Transition Metal Chem., 1972, 7, 183). 

The anistropy of a quasi-one-dimensional conductor provides considerable insight into the directional dependence of the electronic structure of the compound. It is, however, difficult to measure in (TMTSF)2X salts and in some of the (ET) X salts. Because of the low crystallographic symmetry, crystal faces are not perpendicular to high-symmetry directions. Nevertheless, adopting a simple tight-binding form for the electronic energy band ... 

Kundu et al. (2007) employed the citric acid sol-gel method to prepare a nanoscale ordered perovskite cobaltite, which consists of 90° ordered domains of fhe layered "112 LaBaCoiOg. This perovskife exhibifs a high magnefic anisofropy in contrast to the disordered and ordered phases. The authors explained that the locking of fhe cobalf spins due fo the existence of 90° oriented nanostructure domains is responsible for the observed anistropy. 

Also rather few solid state metal NMR spectra of clusters have been reported. This type of experiment is very promising for the metal NMR of clusters, basically for two reasons. First, solid state NMR gives tensorial values of the chemical shifts, and the anistropy of coupling constants reflecting more precisely the electronic structure. The measurement of dipole-dipole interactions, where possible, should allow the determination of distances involving light nuclei such as H, Li, Be which are not easily accessible by X-ray diffraction. The second reason for interest in solid state studies is that the fluxional behaviour is often frozen out and that consequently NMR results are more amenable to correlation with X-ray studies. 

J. Stumpe, L. Muller, D. Kreisig, G. Hauck, H. D. Koswig, R. Ruhmann, and J. Rtibner, Photoreaction in mesogenic media, 5. Photoinduced optical anistropy of liquid-crystalline side-chain polymers with azodiromophores by linearly polarized light of low intensity, Makromol. 

J. Stumpe, L. Lasker, T. Fischer and T. Geue, Photoinduced optical anistropy in LB multilayers, liquid crystalline and amorphous polymers,. /. Inf. Rec. Mater. 21,449-455 (1994). 

Figure 1. Emission anistropy (r) of excimer for solutions of polystyrene of varying concentration and molar mass.

Fluorescence anisotropy of N-acetyl-tryptophananide has been measured (18) in various environments. This experiment was particularly difficult because it required excitation at the fourth harmonic (266 nm) with limited energy (2yJ) and detection in the UV at 340 nm. Fig. 3 shows the result of increasing viscosity on the decay time of the anistropy calculated using Eq. l. In order to obtain these results, 400 traces of I(t) and Ij.(t) were averaged. 

Figure 3. Examples of UV-fhtorescence anisotropy decay. Anistropy decay of fluorescence oflN-aceiyl-iryptophanamide with excitation at 266 nm and detection at 340 nm. Solvent viscosity dependence is investigated with 0% glycerol (a), 50% glycerol (b), and 97% gfycerol (c). The scattered excitation pulse defines t = 0. To obtain this signal-to-noise ratio, 400 shots were averaged.

Recent esr measurements of low-spin Fe111 porphyrins at liquid helium temperature show sizable in-plane magnetic anistropy produced by the porphyrin skeleton (gi=2.9, g2 =2.3, = 1.5) and the contact... 

Figures are parts by weight and all vulcanizates are heated in the mould for 10 mins at lOO c before the temperature is raised to the value shown in order to minimise anistropy.

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