Geometries and Tautomerism of OHN Hydrogen Bonds in Aprotic Solution Probed by H/D Isotope Effects on 13C NMR Chemical Shifts

The H-1 and C-13 NMR spectra of 17 OHN hydrogen-bonded complexes formed by (CH3COOH)-C-13(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF3/CDF(2)C1 mixture as solvent. The slow proton and hydrogen bond exchange regime was reached. and the H/D) isotope effects on the C-13 chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding H-1 chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the (13) chemical shifts of the carboxyl group with the OHN hydrogen bond geometries.