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

✍ Scribed by George V. D. Tiers; John Stevens; William L. Stebbings

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 85 KB
• Volume: 37
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199909)37:9<613::aid-mrc521>3.0.co;2-z

✦ Synopsis

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. As a result, changes in 1 H, 13 C (and 19 F) 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, but near-zero shifts for more distant nuclei, thus providing an 'intramolecular internal reference.' Considerable information about the nature and orientation of dimerization, 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 are found to self-associate by weak non-conventional hydrogen bonds and by polar interactions, by the latter exclusively when no acyl ˛-hydrogen is present.