Crystallographic evidence for oxygen acceptor directionality in oxyanion hydrogen bonds

A survey of 2632 D-H⋯O-A hydrogen bonds in crystal structures (where D is any atom and A is the central atom of a trigonal planar (A = C, N) or tetrahedral (A = P, S, Cl, As, Se, Cr, Mo) oxyanion, has established the existence of a distinct directionalities at the oxygen atom acceptors. The directionality depends primarily on the geometry of the oxyanion. With the trigonal planar oxyanions NO₃^-, HCO₃^-, and CO₃^2-, the average H⋯O-A angle is 115 ± 12° and there is a clear preference for the hydrogen to lie within the plane of the anion. With the tetrahedral oxyanions H₂PO₄^-, HPO₄^2-, HSO₄^-, SO₄^2-, ClO₄^-, H₂AsO₄^-, HAsO₄^2-, AsO₄^3-, HSeO₄^-, SeO4^2-, CrO₄^2-, and MoO₄^2-, the average H⋯O-A angle is 122 ± 12°, and there is a weak preference for eclipsed H⋯O-X-O dihedral angles. The observed directionality closely coincides with minima on electrostatic potential surfaces calculated for the anions.