Solvent anisotropy effects

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

Selected entries from Methods in Enzymology [vol, page(s)] Anisotropy effects, 261, 427-430 determination by dynamic laser light scattering (quasi-elastic light scattering), 261, 432-433 determination for nucleic acids by NMR [accuracy, 261, 432-433 algorithms, 261, 11-13, 425, 430 carbon-13 relaxation, 261, 11-12, 422-426, 431, 434-435 cross-relaxation rates, 261,419-422, 435 error sources, 261, 430-432 phosphorus-31 relaxation, 261, 426-427, 431 proton relaxation, 261,51,418-422 relaxation matrix calculations, 261,12] deuterium solvent viscosity effects, 261,433 effect... 

Other difficulties are owing to the influence of the solvent. With stiff and bulky chains the so-called micro-form-effect becomes of importance, when the refractive index increment differs considerably from zero (7). In this case the random link approximately acts like a cylinder of length A and with a refractive index different from that of the solvent. Another effect occurs in good solvents which consist of anisotropic molecules. These molecules become oriented along the polymer chain, considerably contributing to its anisotropy [Frisman, Dadivanyan and Dyuzhev (752)]. In this way, the determination of the eigen anisotropy of weekly anisotropic polymer chains becomes rather doubtful. 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

The contribution neighbor anisotropy effect. This term accounts for the fields arising from electronic circulations around the atoms surrounding the observed nucleus. It depends on the nature of the neighbor atoms and on molecular geometry. Sometimes, a medium term must be added in eq. (3.2), correcting for solvent and pH effects. 

The most important result however, is related to the effective interpretation of the factors influencing the magnetic parameters. Thus, the solvent anisotropy has a very... 

Table 1 also lists the <5(OHO) values of AA in the various solvents. There is little correlation with either e or the hydrogen bonding potential of the medium. The signal furthest downfield occurs in benzene solution and probably arises due to anisotropy effects of the benzene ring which is in close association with the enol tautomer14. ... 

Nevertheless, it is possible to convert a racemic sample with chiral reagents into diastereomers or simply to dissolve it in an enantiomericaUy pure solvent R or S following this process, solvation diasteromers arise from the racemate (RP + SP) of the sample P, e.g. R RP and R SP, in which the enantiomers are recognisable because of their different shifts. Compounds with groups which influence the chemical shift because of their anisotropy effect (see Sections 2.5.1 and 2.5.2) are suitable for use as chiral solvents, e.g. 1-phenylethylamine and 2,2,2-trifluoro-l-phenylelhanol. ... 

Several authors have calculated the shifts due to anisotropy effects in cylindrically symmetrical solvent molecules,and some have succeeded in obtaining values comparable with experimental ones. Watts and Goldstein have shown that the shift produced in a-chloroacrylonitrile by benzene in excess of that due to the reaction field (see Section VI) is almost identical with the value obtained by the use of Buckingham s equation for the anisotropy effect of cylindrically symmetric solvents. 

The term has previously been applied to the effect on chemical shift of specific solute—solvent interactions in solution, and it has been pointed out that these should cause a small paramagnetic solvent shift in this usage, shifts due to anisotropy effects associated with specific interactions are included in the term. In this discussion, however, we shall take to represent the effect of solvent anisotropy in a geometrically specific solute—solvent orientation which has been brought... 

Aromatic solvents and related systems bringing about the ASIS (aromatic solvent induced shift) phenomenon [1,2] - a diamagnetic anisotropy effect. 

Substances dissolved in aromatic solvents generally give signals at higher fields than when dissolved in aliphatic solvents. These effects are attributed to the diamagnetic anisotropy of aromatic rings, and are more significant when... 

Multidimensionality may also manifest itself in the rate coefficient as a consequence of anisotropy of the friction coefficient [M]- Weak friction transverse to the minimum energy reaction path causes a significant reduction of the effective friction and leads to a much weaker dependence of the rate constant on solvent viscosity. These conclusions based on two-dimensional models also have been shown to hold for the general multidimensional case [M, 59, and 61]. 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

D Biren, BG Kabra, SH Gehrke. Effect of initial sample anisotropy on the solvent sorption kinetics of glassy poly(2-hydroxyethyl methacrylate). Polymer 33 554-561, 1992. 

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