Hydrogen bond geometries and 1 H NMR chemical shifts of OHO hydrogen-bonded systems have been analyzed using an improved valence bond order model. This model predicts that the heavy atom hydrogen bond coordinate q 2 = r 1 + r 2 is a function of the proton coordinate q 1 = ½(r 1r 2 ), where r 1 and r 2 represent the OH and the HO distances.In the first part, it is shown that this correlation reproduces published equilibrium geometries of the Zundel cation H 5 O 2 + as well as those of water clusters in the gas phase and embedded in the fullerene C180. Using the example of the water hexamer, it is shown that changing the level of calculation shifts the calculated geometries along the correlation curve, but not away from the curve. In order to take quantum zero-point vibrational effects (QZPVE) into account, an empirical correction is proposed. It is shown that this correction properly describes the calculated classical and quantum hydrogen bond geometries of compressed ice as well as calculated geometric H/D isotope effects. The improved valence bond order model is used to analyze a large number of OHO hydrogen bond geometries contained in the Cambridge Structural Database.In the second part, a relation between the geometries and the 1 H NMR chemical shieldings of OHO hydrogen bonded systems is established using the valence bond order model. GIAO calculations of the isolated symmetric Zundel cation where H is located in the hydrogen bond center show only a small dependence of the chemical shifts on the O...O distance. This result is rationalized in terms of neighbor group effects and deshielding in the naked proton. The consequence is that the 1 H NMR chemical shifts are not much affected by QZPVE. Calculations on water clusters indicate that the influence of the chemical environment of the OHO hydrogen bonds on their 1 H NMR chemical shifts is smaller for the strong hydrogen bond regime but large for the weak hydrogen bond regime. A simple chemical shift vs. q 1 relation is then used to calculate the average chemical shifts of water clusters in the regime of fast hydrogen bond exchange between hydrogen bonded and free OH groups. It is shown that average chemical shifts of about 6 ppm are possible as the clusters considered exhibit a broad distribution of stronger and weaker hydrogen bonds. The implications for water in organic solvents and for liquid water are discussed, based on published data on the 1 H chemical shift distribution in the latter.
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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