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1 Polish Journal of Chemistry

1 2003

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Low-Barrier Hydrogen Bonds in Pyridine-Dichloroacetic Acid Complexes

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Perrin C. , Lau J. S. , Ohta B. K.

Polish Journal of Chemistry

2003

tom Vol. 77 / nr 11

1693-1702

Amixture of Cl2CHCOOH-16O2 and Cl2CHCOOH-18O2 was combined with an excess of a pyridine and studied by NMR at 23_C, -43_C, and -81_C. The 1:1 complexes were analyzed by isotopic perturbation of equilibrium. Plots of OH chemical shift and of 13C isotope shift vs. pKa of the pyridinium ion both exhibit maxima ~ 5, when the basicities of the hydrogen-bond acceptors become matched, the hydrogen bond is strongest, and the equilibrium is maximally perturbed by the isotope. In contrast, for the complexes of pyridine and 3-picoline the isotopic perturbation seems to disappear, consistent with a low-barrier hydrogen bond in a single structure, rather than a hydrogen that can take either of two positions in a mixture of two tautomers.