Hydrogen/Deuterium Isotope Effects on the NMR Chemical Shifts and Geometries of Intermolecular Low-Barrier Hydrogen-Bonded Complexes

In this paper we describe H/D isotope effects on the chemical shifts of intermolecular hydrogen-bonded complexes exhibiting low barriers for proton transfer, as a function of the position of the hydrogen bond proton. For this purpose, low-temperature (100−150 K) 1H, 2H, and 15N NMR experiments were performed on solutions of various protonated and deuterated acids AL (L = H, D) and pyridine-15N (B) dissolved in a 2:1 mixture of CDClF2/CDF3. In this temperature range, the regime of slow proton and hydrogen bond exchange is reached, leading to resolved NMR lines for each hydrogen-bonded species as well as for different isotopic modifications. The experiments reveal the formation of 1:1, 2:1, and 3:1 complexes between AH(D) and B. The heteronuclear scalar 1H−15N coupling constants between the hydrogen bond proton and the 15N nucleus of pyridine show that the proton is gradually shifted from the acid to pyridine-15N when the proton-donating power of the acid is increased. H/D isotope effects on the chemical shifts of the hydrogen-bonded hydrons (proton and deuteron) as well as on the 15N nuclei involved in the hydrogen bonds were measured for 1:1 and 2:1 complexes. A qualitative explanation concerning the origin of these low-barrier hydrogen bond isotope effects is proposed, from which interesting information concerning the hydron and heavy atom locations in single and coupled low-barrier hydrogen bonds can be derived. Several implications concerning the role of low-barrier hydrogen bonds in enzyme reactions are discussed.