Specific Solvent Effects on NMR Chemical Shifts

Specific solvent effects on the resonance positions of the nuclei of dissolved compounds consist mainly of hydrogen-bonding effects and aromatic solvent-induced shifts (ASIS effects). The interactions between the solute and the surrounding solvent molecules lead, in these cases, to molecular species which are more or less definable entities. If the 

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 

Solvent-induced H chemical shifts of hydrogen-bonded H-atoms are not only found for hydrogen-bonded solutejsolvent complexes but also for hydrogen-bonded soluteffolute complexes. A well-studied example is the intermolecularly hydrogen-bonded 1 1 complex between trifluoroacetic acid (as HBD) and 2,4,6-trimethylpyridine (as HBA), as shown in Eq. (6-23) [408] cf. also Eq. (4-29) in Section 4.4.1. 

In this case, the solvent-induced H chemical shift of ca. 1 ppm is best explained by assuming a double-minimum potential model of the hydrogen-bond, ie. the existence of a rapid solvent-dependent proton-transfer equilibrium between (a) the covalent and (b) the ionic hydrogen-bonded complex. With increasing solvent polarity, the proton-transfer equilibrium is shifted in favour of the ionic complex (b). 

In conclusion, NMR spectroscopy has proven to be one of the most sensitive spectroscopic methods, both qualitatively and quantitatively, for studying hydrogen bonds [270-272, 277] cf. also Section 2.2.5. 

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