The question of the strength of hydrogen bonds has been a subject of interest and contention for most of the 20th century (1). By the time of the publication of Pauling's book The Nature of the Chemical Bond in 1939 (2), weak hydrogen bonds were generally accepted. Evidence for strong hydrogen bonding in HF Ϫ 2 also appeared in the decades of the 1920s through the 1950s, and this strongly hydrogen bonded ion was accepted as a special case. HF Ϫ 2 is one of a handful of species for which there is compelling evidence for symmetrical hydrogen bonding; that is, the proton is equally shared between the fluoride ions, and its gas phase strength is estimated to be 37 kcal mol Ϫ1 (3). Other strongly hydrogen bonded species have been documented. In addition to fluoride containing species, the hydrated hydronium ion H 5 O ϩ 2 is regarded as a case of symmetrical hydrogen bonding. In general, the strongest hydrogen bonds are found in ionic compounds and are regarded as partially covalent (1,3,4). Weak hydrogen bonds are regarded as arising from weak dipolar electrostatic attractions.
In this article we consider two questions about strong hydrogen bonding. Do strong hydrogen bonds occur in organic compounds in aqueous solutions? Should the strongly basic properties of proton sponge molecules be attributed to strong hydrogen bonding or to relief of steric strain upon protonation? These questions are debatable, and the purpose of this article is to consider the currently available evidence bearing on them and to define the terms of the debate.
CHARACTERIZATION OF STRONG HYDROGEN BONDS
Physicochemical characterization of strong hydrogen bonds includes the application of X-ray crystallography, neutron diffraction, infrared spectroscopy (IR or FTIR), nuclear magnetic resonance spectroscopy (NMR), and calorimetry. The interpretation of information provided by these techniques has been reviewed (1, 3). X-ray crystallography and neutron diffraction give the distances separating heteroatoms participating in hydrogen bonding, and in small molecules they give the distances separating each heteroatom from hydrogen. Very short inter-heteroatom distances imply strong hydrogen bonding (3). However, care must be exercised in the interpretation of these distances because hydrogen bonds are not necessarily linear, and knowledge of the position of the proton is necessary to determine