Molecular and supramolecular ionic aggregates HxOyz in organic and organometallic crystalline hydrates

Ionic aggregates of the form HxOyz (z not equal 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002). Acta Cryst. B58, 380-388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O center dot center dot center dot O = 2.6-3.0 angstrom), charge assisted (CAHB; O center dot center dot center dot O= 2.5 angstrom) and molecular (MHB; O center dot center dot center dot O = 2.4 angstrom). The three modes are consistent with previous reports, our experimental results, and quantumchemical-optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen-bonded networks, ignorance of which can lead to erroneous crystal structure models and other physicochemical data associated with composition and charge balance.