The Shortest Symmetrical O−H···O Hydrogen Bond Has a Low-Barrier Double-Well Potential

Abstract

The strong hydrogen bond, in which a hydrogen atom is centered between two electronegative atoms in a single minimum potential has been widely discussed but rarely definitively observed. The compound 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione has the shortest symmetrical O-H‚‚‚O hydrogen bond yet reported, at 239.3 pm. Neutron crystallography reveals the hydrogen-bonded proton to be nearly centered between the two oxygens, with a highly elongated thermal ellipsoid. Inelastic neutron scattering measurements of the normal and isotope labeled molecule reveal a hydrogen-bond vibrational frequency at 46.0 meV (371 cm -1 ). This frequency is too low to be compatible with a single-minimum potential and indicates a low-barrier double minimum. The temperature dependence of the NMR properties confirms the existence of a thermally accessible vibrationally excited state for the bond. Because other short hydrogen bonds show similar NMR behavior, it is likely that no "strong" hydrogen bond of this sort has yet been discovered.

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

“…It is a widespread opinion that structural, 7-10 dynamical, 19 and magnetic 16,[20][21][22] properties of VSHBs in crystals are inconsistent with the relatively long-range attraction, predominantly electrostatic in nature, attributed to weak hydrogen bonds. Gilli and co-workers, [8][9][10] or Tian and Li 23 have suggested a resonance between covalent structures ͑e.g., O u H¯O ↔ O¯H u O͒, but Sørensen et al 24 have opposed a counterexample to this view.…”

“…From the dynamical viewpoint, one can distinguish "single-well hydrogen bonds" ͑SWHBs͒ and "low barrier hydrogen bonds" ͑LBHBs͒, [12][13][14]21,22 depending on the shape of the potential for protons moving along the O¯O direction. ͑In the present paper, this coordinate is x a and the eigenstate vector ͉x a ͘ n is at h an .͒ These potentials could explain different chemical reactivities, for example, isotopic fractionation factors, 21 but experimental or theoretical evidences are rather scarce.…”

“…This question is controversial since a new intramolecular VSHB ͓R OO = 2.388͑5͒ Å at 20 K͔ has been reported in the crystal of 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione ͑4CTH͒. 22 In contrast to KHM, the hydrogen bond is not linear ͑r OH Ϸ 1.22 Å, OHÔ Ϸ 157°͒ and, needless to say, noncentrosymmetric. An INS band at 371 cm −1 , tentatively assigned to ͉x a ͘ 1 , along with solid-state NMR measurements, were regarded as possible evidences of a LBHB.…”

The probability distribution function as a descriptor of hydrogen bond strength

“…(b) Dd H-D uncertainty: quadrupole broadening produces a large uncertainty in experimental measurements of Dd H-D and thus precludes its use as a reliable LBHB descriptor [15]. (c) The position of the zero point energy (ZPE): the closeness of the ZPE relative to the energy barrier in an LBHB, has been alternatively described as ''above'', ''similar to'' and ''below'' the barrier [3,18,19]. This ambiguity complicates the identification of an LBHB by looking at the features of its potential energy surface [18,20,21].…”

“…(c) The position of the zero point energy (ZPE): the closeness of the ZPE relative to the energy barrier in an LBHB, has been alternatively described as ''above'', ''similar to'' and ''below'' the barrier [3,18,19]. This ambiguity complicates the identification of an LBHB by looking at the features of its potential energy surface [18,20,21]. (d) Classification of LBHBs: the semantic distinction between LBHBs and strong HBs is vague.…”