Density Functional Theory Study of Hydrogen Bonding in Ionic Molecular Materials Nicole A. Benedek, †,‡ Kay Latham, ‡ Ian K. Snook, † and Irene Yarovsky* ,† Departments of Applied Physics and Applied Chemistry, RMIT UniVersity, GPO Box 2476V, Melbourne 3001, Australia ReceiVed: April 10, 2006; In Final Form: May 22, 2006 Crystal structures are usually described in geometric terms. However, it is the energetics of intermolecular interactions that determine the chemical and physical properties of molecular materials. 1 In this paper, we use density functional theory (DFT) in combination with numerical basis sets to analyze the hydrogen bonding interactions in a family of novel ionic molecular materials. We find that the calculated binding energies are consistent with those of other ionic hydrogen bonded systems. We also examine electron density distributions for the systems of interest to gain insight into the nature of the hydrogen bonding interaction and investigate the effects of different aspects of the crystal field on the geometry of the hydrogen bond. 1. Introduction The discovery of electrically conductive polymers 2,3 in the early 1970s sparked an explosion of interest in the properties of molecular materials. Semiconducting materials consisting of small organic molecules are promising candidates for optoelec- tronic devices and have thus been the subject of numerous theoretical 4-6 and experimental 7,8 studies. In contrast to their more conventional organic counterparts, inorganic molecular materials may contain any element. A far more diverse range of intra- and intermolecular interactions is therefore possible. Many inorganic molecules are charged, and hence also have counterions in condensed phases. This has consequences for the diversity of interactions, the energies associated with them, and the material properties. 1 An interesting family of novel ionic molecular crystals, mixed inorganic/organic phosphonates, was recently synthesized and characterized. 9 These layered materials contain a copper ion (Cu 2+ ,d 9 ) coordinated to two phenanthroline ligands and a halogen substituent, either Cl - , Br - ,I - , or NCS - . The positively charged copper-halogen ions are interleaved between parallel sheets of negatively charged, hydrogen-bonded phenylphos- phonic acid dimers, which act as counterions. There are no covalent bonds between individual molecules, either within a layer or between layers. The compounds crystallize in a monoclinic space group, C2/c, and have their Cu-X bond lying along a 2-fold crystallographic axis. The molecular unit of the iodo form is shown in Figure 1, and a 1 × 2 × 1 supercell is shown in Figure 2. Our interest in these materials stems from the desire to better understand the complex interactions often present in hybrid organic/inorganic molecular materials. The properties of mo- lecular solids are dictated by features of both molecular and condensed matter physics: their physical properties depend on their crystalline structure, which in turn is governed by interactions between molecules. The diverse range of elements and bonding scenarios in organophosphonate molecular materi- als means they provide interesting “laboratories” for the study of intermolecular interactions. In this paper, we use theoretical methods to study both the geometry and energetics of the hydrogen bonding interaction between phenylphosphonic acid dimers in the organophospho- nate molecular materials. The phenylphosphonic acid dimers * Corresponding author. E-mail: irene.yarovsky@rmit.edu.au. † Department of Applied Physics. ‡ Department of Applied Chemistry. Figure 1. ORTEP perspective of the molecular unit of the iodo form, [Cu(C12H8N2)2I][(OH)2OPC6H5][(OH)O2PC6H5]. Figure 2. A1×2×1 supercell of the iodo form. Different atoms are distinguished by the following colors: gray, carbon; white, hydrogen; blue, nitrogen; orange-, copper; dark purple, iodine; light purple, phosphorus; red, oxygen. The same atom coloring scheme has been used for all figures. 19605 J. Phys. Chem. B 2006, 110, 19605-19610 10.1021/jp062239t CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006