Water: new aspect of hydrogen bonding in the solid state

All water–water contacts in the crystal structures from the Cambridge Structural Database with d _OO ≤ 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 ( d _OH ≤ 3.0 Å and α ≥ 120°) with most being stronger than −3.3 kcal mol ⁻¹ . 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 ₁ , β ₂ ≥ 160°, 80 ≤ α ≤ 140° and T _HOHO > 40°) yielded 1282 attractive contacts. The interaction energies of these antiparallel oriented water molecules are up to −4.7 kcal mol ⁻¹ , while most of the contacts have interaction energies in the range −0.9 to −2.1 kcal mol ⁻¹ . 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.