Magnetic anisotropy solvents

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. 

This aromatic solvent induced shift (ASIS) is explained by the formation of preferential collision complexes or clusters of aromatic solvent molecules in the vicinity of the polar groups of the solute, so that the effect of the solvent magnetic anisotropy within the space occupied... 

There is some evidence that while mono adducts of the j8-diketonate shift reagents may show time-averaged uniaxial symmetry, bis adducts often do not. Thus the spectra of [M(dpm)3L2], where M = Pr, Sm, Dy, Ho, Er or Yb and L = 3-picolinc or 3,5-lutidine, have been interpreted in terms of biaxial magnetic anisotropy. One magnetic axis is defined by the molecular C2 axis present in the X-ray crystal structure and the other two axes lie in similar orientations in all the adducts. A variable-temperature study of this system shows restricted rotation of the aromatic rings with a very solvent-dependent conformational equilibrium resulting from this.558-560... 

Ion-pairing between the paramagnetic [Co(tdt)2] monoanion and nine different cations was examined by Tsao and Lim.105 The cations belong to either of two classes, quaternary ammonium or substituted V-octylpyridinium ions. By recording the H NMR spectra as a function of concentration (nitrobenzene, 307 K), the concentration association constants (Kas) were obtained. Substituent effects were found to influence the ion-pair geometry, as deduced from the isotropic shifts of the cationic protons and their shift ratios. In low dielectric constant solvents, speculation consistent with the magnetic anisotropy and the relation between the cationic proton shifts and concentration was tendered for cylindrically shaped aggregates. 

The combined strategy of calculating the 19F chemical shifts has been studied for fluorobenzenes [62] in several solvents. Here crw has been found to be the dominant contribution to the total solvent-induced change of chemical shift the authors have neglected the solvent magnetic anisotropy contribution cra which is related to the short-range interactions. To obtain the agreement with the experimental data, the term [Pg.137]

Molecules that possess magnetic anisotropy experience a slight tendency toward alignment with an imposed magnetic field, but in normal isotropic solvents random thermal motions dominate, and no effects of orientation are usually... 

Another illustrative example is that of phenylacetylene. Table 6-7 summarizes the H NMR chemical shifts of its alkyne H-atom in a variety of solvents [273], Most solvents (except aromatic solvents) decrease the shielding of the acetylenic hydrogen nuclei. The corresponding low-field shifts have been interpreted in terms of weak specific association between the alkyne as hydrogen-bond-donor and electron pair-donor groupings of the solvent [273], The high-field shifts in aromatic solvents arise from the magnetic anisotropy of the solvent molecules (see below). The order of effectiveness of the solvent... 

The pronounced magnetic anisotropy of benzene helps to reveal subtle solute-solvent interactions which otherwise could not be detected. For example, ASIS s can be used to differentiate between axial and equatorial H-atoms or methyl groups adjacent to carbonyl groups. A typical shift for an axial 2-methyl group in a cyclohexanone is 0.2... 0.3 ppm upheld, in benzene relative to tetraehloromethane as solvent, while that of the eorresponding equatorial 2-methyl group is 0.05. .. 0.10 ppm downfield [3]. This can be used to determine the configuration at the 2-position and to assess the position of the conformational equilibrium in 2-methylcyclohexanone. 

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