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

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-HO 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.