Analysis of the less common hydrogen bonds involving ester oxygen sp3 atoms as acceptors in the crystal structures of small organic molecules.

An analysis of hydrogen bonds involving ester Osp3 atoms as acceptors has been performed based on the data extracted from the Cambridge Structural Database [Allen (2002). Acta Cryst. B58, 380-388; version 5.25, November 2003], using the ConQuest package to evaluate the stereochemical and electronic properties of the acceptors. Evidence for the existence of this particular type of hydrogen bond and its structural function in crystal packing is presented. Using a cut-off limit on residual indices of R < 0.05 (for the structures with hydrogen bonds involving an oxygen as part of the donor group) and R < 0.085 (for nitrogen as part of the donor group), 230 structures out of the total CSD entries of 298,100 were found to contain hydrogen bonds with the ester Osp3 atoms as acceptors. The hydrogen-bond donors include water molecules, hydroxyl groups, primary and secondary amines and, in a few cases, imino groups. Four modes of the participation of the ester Osp3 atoms in hydrogen bonding are detected: as a single acceptor, as a double acceptor, as a single acceptor of a H atom involved in an intermolecular bifurcated hydrogen bond, and as a shared acceptor function with the ester Osp2 atom in a bifurcated hydrogen bond. The role of such directed noncovalent interactions in crystal packing is demonstrated by a small gallery of selected structures.