Water: new aspect of hydrogen bonding in the solid state.

All water-water contacts in the crystal structures from the Cambridge Structural Database with ≤ 4.0 Å have been found. These contacts were analysed on the basis of their geometries and interaction energies from CCSD(T)/CBS calculations. The results show 6729 attractive water-water contacts, of which 4717 are classical hydrogen bonds ( ≤ 3.0 Å and α ≥ 120°) with most being stronger than -3.3 kcal mol. Beyond the region of these hydrogen bonds, there is a large number of attractive interactions (2062). The majority are antiparallel dipolar interactions, where the O-H bonds of two water molecules lying in parallel planes are oriented antiparallel to each other. Developing geometric criteria for these antiparallel dipoles (β, β ≥ 160°, 80 ≤ α ≤ 140° and > 40°) yielded 1282 attractive contacts. The interaction energies of these antiparallel oriented water molecules are up to -4.7 kcal mol, while most of the contacts have interaction energies in the range -0.9 to -2.1 kcal mol. This study suggests that the geometric criteria for defining attractive water-water interactions should be broader than the classical hydrogen-bonding criteria, a change that may reveal undiscovered and unappreciated interactions controlling molecular structure and chemistry.