Halogen bonding: A lump–hole interaction Keiamars Eskandari * , Homam Zariny School of Chemistry, Damghan University of Basic Science, Damghan 36715-364, Iran article info Article history: Received 23 January 2010 In final form 8 April 2010 Available online 10 April 2010 abstract The Laplacian of electron density is employed in the description of halogen bonds between some halogen containing molecules, A–X, and ammonia. We show that a halogen bond is an interaction between a region of charge depletion (a hole) on the halogen atom and a region of charge concentration (a lump) on another molecule. The existence of the hole on the Valence Shell Charge Concentration (VSCC) of hal- ogen atom is a necessary condition in the formation of halogen bonds. In addition, for some chlorine con- taining molecules, it is shown that the position and properties of the hole determines the geometry and strength of the halogen bond. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction It is well known that halogen atoms can act as electron accep- tors and interact with electron donor species [1–5]. This type of noncovalent interaction is usually called ‘halogen bonding’, in or- der to emphasize the similarity with hydrogen bonding [3,6–8]. A halogen bond, XB, is indicated by A—X  B in which the halogen bond acceptor, B, is often a Lewis base; that is it has an available pair of electrons (n or p) [9–13]. X is typically iodine, bromine, chlorine and rarely fluorine, whose strength of the interaction in- creases in order (F<) < Cl < Br < I [14–16]. A halogen bond is rela- tively strong and highly directional; the A–X–B angle is always close to 180° [17]. In spite of the importance of halogen bonding in the fields of molecular recognition [7,18,19], crystal engineering [20–23], supramolecular chemistry [24,25] and drug design [23,26–28], the fact that both the halogen atom (X) and the halogen bond acceptor (B) are usually negatively charged, make them more interesting for theoretical studies [29–36]. The different varieties of terms and phrases which has been used to describe this type of interactions, illustrate the struggle to understand the phenom- ena. ‘bumps-in hallow’, ‘pairs-in-pocket’, ‘exaltation of valency’, ‘donor–acceptor interactions’, ‘charge transfer interactions’ and ‘filling of antibonding orbitals’ are some examples of these descrip- tive phrases [3,17]. Recently Politzer and co-workers [37,38] and Clark et al. [39] explained the problem by introducing the concept of r-hole bond- ing; the lone pairs of halogen atom form a belt of negative electro- static potential around its central part leaving the outermost region positive, the positive r-hole, which can interact with nega- tive sites on other molecules to form XB. Politzer and co-workers [25,37,38] showed that the positive r-hole accounts for existence, directionality and strength of halogen bonds. However, it seems that the r-hole concept is useful when the electrostatic forces play the major role in stabilizing the halogen-bonded complexes, and this concept fails when other energy components are also signifi- cant. The CH 3 Cl molecule is a good example to explain this prob- lem. The chlorine atom in this molecule does not have a positive electrostatic potential, a positive r-hole, on its surface and conse- quently should not be able to form a halogen bond with bases [26,37,40]. But Hobza and co-worker [35] showed that this mole- cule is able to form a halogen bond with OCH 2 . Meanwhile, they indicated that the largest interaction energy component for this system is dispersion, while electrostatic forces, which have been widely believed to be responsible for halogen bonding interactions, play a smaller role in stabilizing this complex. On the other hand, the distribution of the Laplacian of the elec- tron density provides a useful tool to describe the acid–base interactions [41]. Very recently, Espinosa and co-workers used experimental electron density and its Laplacian to study halogen– halogen interactions [42]. They showed that a halogen–halogen interaction may be understood as an interaction between charge concentration and charge depletion regions in adjacent halogens. In this Letter we perform a topological analysis of the Laplacian of electron density, on a series of halogen-bonded complexes formed between ammonia and several halogen containing mole- cules including CH 3 F, CH 3 Cl, CH 3 Br and some A–Cl molecules. Since in the selected A—Cl  B complexes, B is always NH 3 , we will focus on the properties of the chlorine atom in the A–Cl molecules. 2. Computational details Molecular geometries and their electronic wave function have been optimized with GAUSSIAN 03 program [43], at MP2/6-311++G- (d,p)6d level of theory. The AIMPAC suite of programs [44,45] was 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.021 * Corresponding author. E-mail address: eskandari@dubs.ac.ir (K. Eskandari). Chemical Physics Letters 492 (2010) 9–13 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett