Cation-anion hydrogen bonds: a new class of hydrogen bonds that extends their strength beyond the covalent limit. A theoretical characterization.

The existence of O-H···O hydrogen bonds having a strength within the -80 to -210 kcal/mol range, that is, in the range of strength of covalent bonds and well beyond the so-called covalent limit (-50 kcal/mol), is reported on complexes where the O-H proton donor and O acceptor groups are located in ions of opposite sign. A complete analysis of short distance O-H···O hydrogen bonds between charged fragments was performed for cases where the OH and O groups are both located on charged molecules. It shows that these interactions (a) are nonsymmetrical for the O-H and H···O distances, (b) have a noncovalent H···O bond critical point, and (c) have a strong and energetically stable electrostatic component when the OH and O groups are located in oppositely charged molecules. These cation-anion O-H···O interactions are energetically stable, satisfy the usual topology for hydrogen bonds, HBs, and also have the same directionality found in other HBs. Therefore, they should be considered as a new class of HBs, the cation-anion hydrogen bonds.

A computationally inspired investigation of the solid forms of (R)-1-phenylethylammonium-(S)-2-phenylbutyrate.

Following the computation of a lattice energy landscape which predicted that there should be more stable, denser forms of (R)-1-phenylethylammonium-(S)-2-phenylbutyrate, crystallizations from a range of solvents were performed to search for other polymorphs and investigate the possibility that the known P4(1) structure could be a hydrate. Extensive crystallization experiments from a wide range of solvents gave fine needles or microcrystalline samples. A redetermination of the P4(1) structure by powder X-ray diffraction located all protons, and in conjunction with other experimental and computational evidence showed that the structure was anhydrous. Evidence for two additional forms was found as mixtures with form I. These include an orthorhombic form, possibly a Z' = 3 polymorph, and another as yet unidentified form obtained as a minor component from dichloromethane solution. However, both these forms appear to be metastable with respect to form I (P4(1)), which is therefore probably the most thermodynamically stable form that can be crystallized from solution under ambient conditions. This determination of the solid state behavior of the less readily crystallized member of the diastereomeric salt system (R)-1-phenylethylammonium-(R/S)-2-phenylbutyrate provides a challenge to the theoretical modeling to explain its ideal resolution behavior.

O-H...O interactions involving doubly charged anions: charge compression in carbonate-bicarbonate crystals.

The O-H...O interaction formed by the anions HCO(3)(-) and CO(3)(2-) has been investigated on the basis of data retrieved from the Inorganic Crystal Structure Database (ICSD) and by means of ab initio computations. It has been shown that the O-H...O separations associated with HCO(3)(-)...(3)(2-) interactions are shorter than those found in crystals containing hydrogen carbonate monoanions such as HCO(3)(-)...HCO(3)(-). Ab initio MP2/6-311G++(2d,2p) computations on the crystal Na(3)(HCO(3))(CO(3)).2H(2)O have shown that the interaction between the monoanion donor and the dianion acceptor, for example HCO(3)(-)...CO(3)(2-), is more repulsive than that between singly charged ions, for example HCO(3)(-)...HCO(3)(-), but is largely overcompensated for by anion-cation electrostatic attractions. The shortening of the (-)O-H...O(2-) interaction relative to the (-)O-H...O(-) interaction has been explained as a consequence of the increased charge compression, that is of the stronger cation-anion interactions established by the CO(3)(2-) dianions with respect to those established by monoanions, and does not reflect an increase in the strength of the (-)O-H ...O(-) interaction. To expand the structural sample in the crystal packing analysis, the structure of the novel mixed salt K(2)Na(HCO(3))(CO(3)).2H(2)O has been determined by single-crystal X-ray diffraction and compared with the structure of the salt Na(3)(HCO(3))(CO(3)).2H(2)O used in the computations.