COMMUNICATION pubs.acs.org/crystal
Water/Aromatic Parallel Alignment Interactions. Significant Interactions at Large Horizontal Displacements . Veljkovic,‡ and Snezana D. Zaric*,† Goran V. Janjic,‡ Dusan Z † ‡
Department of Chemistry, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia Institute of Chemistry, Technology, Metallurgy, Njegoseva 12, P.O. Box 473, 11001 Belgrade, Serbia ABSTRACT: Analysis of crystal structures from the Cambridge Structural Database (CSD) and high level ab initio calculations reveals that the water/aromatic parallel alignment interactions, where the water molecule or one of its O H bonds is parallel to the aromatic ring plane, can be significantly strong at large horizontal displacements. We found out that the strongest energies of the interactions are calculated for the water position with the large horizontal displacements, out of the aromatic ring and out of the C H bond region. For calculated systems, normal distances were decreasing with increasing the horizontal displacement, in accord with the data found in crystal structures. The calculated energies of the interactions are significant, up to ΔECCSD(T)(limit) = 10.25 kJ/mol (at a horizontal displacement of 2.6 Å), and comparable with the energy of the slipped-parallel benzene/benzene dimer. Both dispersion and electrostatic components of the interaction energy are important. The calculated interaction energies reveal that also at long horizontal displacements of 3.5 Å, interaction is substantially strong, up to ΔECCSD(T)(limit) = 6.23 kJ/mol. The data show the existence of the water/aromatic parallel alignment interactions in crystal structures and indicate their importance for various systems.
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nteraction of water molecule with aromatic systems is of large importance in many areas;1 water molecule is an essential molecule in nature, while aromatic molecules and groups appear in various systems, from biological molecules to materials. Water chains surrounded by π-systems attracted much interest because of rapid water flow through apolar pores. Water chains were experimentally observed in aquaporins (water-transporting proteins), 2 nanotubes,3 and nanoporous materials.4 In addition to strong hydrogen bonds,5 water molecules also have important weaker interactions that enable interactions of nonpolar groups with polar solvent. In OH 3 3 3 π and CH 3 3 3 O interactions, the water molecule interacts with aromatic rings.1 The water benzene complex has been the subject of extensive investigation.6 Most theoretical studies agree that the minimum energy structure of the water benzene dimer is a structure with OH 3 3 3 π interaction, with the water above the ring oriented to form a single hydrogen bond with the ring. Analysis of the data in crystal structures from the Cambridge Structural Database (CSD) enables study of noncovalent interactions.7,8,11 Recently, we recognized the parallel alignment interactions of water and aromatic rings.8 In the parallel alignment interactions, either the whole water molecule or one of its O H bonds lies parallel to the C6-aromatic ring. This important new interaction had been discovered by examining crystal structures from the CSD and analyzed by high level ab initio quantum mechanical calculations. The most stable conformation with one O H bond parallel to the C H bond of the aromatic ring had an interaction energy of 6.69 kJ/mol. The calculations and the positions observed in the crystal structures showed that the interaction is stronger for a water molecule above the C H bond region than for one above the ring.5 In spite of the stronger r 2011 American Chemical Society
OH 3 3 3 π interactions (13.35 kJ/mol), the parallel-alignment interactions occur quite frequently in the crystal structures. The additional stabilization is achieved as all atoms of the water molecule in the parallel alignment can form classical hydrogen bonds simultaneously. Here, we present our new result on the parallel alignment water/aromatic interactions, based on analyses of crystal structures from the CSD and high level ab initio quantum mechanical calculations. The results show surprisingly large interaction energies (up to 10.25 kJ/mol) at large offset (horizontal displacement) distances. To the best of our knowledge, this is the first study describing these significant energies for parallelalignment interactions. The statistical study is based on the crystal structures archived in the Cambridge Structural Database (November 2009 release, version 5.31).9 The crystal structures involving a noncoordinated water molecule and a C6-aromatic group were screened for intermolecular contacts. The CSD search program ConQuest 1.1010 was used to retrieve structures satisfying the following criteria: (a) the crystallographic R factor
