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 CO32-, 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-, HPO42-, HSO4-, SO42-, ClO4-, H2AsO4-, HAsO42-, AsO43-, HSeO4-, SeO42-, CrO42-, and MoO42-, 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. Copyright © 2002 American Chemical Society.