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-15
N (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-15
N 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.
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