NMR detection of oriented association via dilution shifts In tetramethylsilane solvent 2. Aliphatic esters

Solute interactions are disrupted by the usual NMR solvents. Tetramethylsilane, a moderately good symmetrical non-polar solvent having low polarizability, tends not to interfere with solute self-association. A.s a result, changes in 1H, (13)C (and 19F) deshielding values may be seen upon dilution, and are attributable to dissociation of dimers/ Solutes show sizable localized dilution shifts near the presumed dimerization site, hut near-zero shifts for more distant nuclei, thus providing an ʹintramolecular internal reference.ʹ Considerable information about the nature and orientation of dimerzation, and the weak forces involved, can be derived from routine two-tube NMR data using the measuring capability of modern equipment. The infinite-dilution deshielding values, generated simultaneously, characterize the solute molecule exactly, even in mixtures with other unknown species. Aliphatic esters ar found to self-associate by weak non-conventional hydrogen bonds and by polar interactions, by the latter exclusively when no acyl alpha-hydrogen is present. Copyright © 1999 John Wiley & Sons, Ltd.