Certain alkyl-, aryl-, and arene-substituted alcohols form linear, hydrogen bonded complexes with bromide and other halide ions in tetrachloromethane. The hydroxyl proton shifts ca. 4 p.p.m. downfield in the complex, regardless of the degree or type of substitution. The interaction is enhanced by inductive electron withdrawal by substituents on the C-0-H moiety. A comparison of the present results with infrared spectroscopy shows that the absorption at ca. 3300 cm-I is entirely due to 1 :I alcohol-anion complexes. The a coefficient in Buckingham's equation for electrostatic interaction of a molecule with an ion, 8csE = aE, -bE2, is found to be 5.5 for alcohol 0-H bonds.Canadian Journal of Chemistry, 47, 1639 (1969) Nuclear magnetic resonance (n.m.r.) and infrared (i.r.) spectroscopy have been used extensively in the detection and characterization of the hydrogen bonding of alcohols and other hydroxyl bearing compounds to a variety of bases (1-3). Characteristically, hydrogen bond formation results in a decrease in shielding of the hydrogen nucleus. The extent of this deshielding has been regarded as a measure of hydrogen bond strength, in much the same way as have hydrogen bond shifts and intensities in the i.r. (1,3,4). Both the i.r. frequency shift and the n.m.r. shielding change have been correlated with the enthalpy of hydrogen bond formation (5).Pursuant to our studies of specific complexes to anions in weakly interacting solvents, we have observed deshielding of hydroxyl protons in methanol, pjimary, secondary, and tertiary alcohols, and a phenol, of magnitude up to 4.5 p.p.m., on addition of a tetrabutylammonium salt to a dilute solution of the alcohol in tetrachloromethane. We interpret the monotonic salt concentration dependence of the hydroxyl proton shifts as arising from specific hydrogen bonded interaction of the alcohols with the anions. alcohol to alcohol hydrogen bonding, and the proton shielding observed as a function of the halide ion concentration.
Experimental
MaterialsMetllartol This was Shawinigan reagent grade, and contained less than 0.2% water when received. This material was dried by storing over 4A molecular sieves (m.s.) for several days; tlie initial vigorous effervescence ceased after about 24 h. The tightly stoppered vial was kept in tlie dry-box. Samples containing nietlianol and tetrabutylammonium bromide decomposed slowly, the n.ni.r. spectrum showing a change after 1 day, but no observable change within 1 11 of sample preparation. All speclra were obtained within 1 11 of preparation of the sample.
E t l~a~~o lThis was 98 %, as received, and showed no benzene in the n.m.r. and, at most, traces of water. This niaterial was distilled, and a center fraction totalling ca. 75% collected on n1.s. The fraction collected was refluxed over tlie m.s. for 1 11. The material was then redistilled with a center fraction totalling ca. 50% being collected on m.s. (boiling range 0.4 "C). The tightly-stoppered flask was immediately transferred to the dry-box, where the ethanol was placed in a ...
