Intermolecular Bonding Features in Solid Iodine Published as part of the Crystal Growth & Design Mikhail Antipin Memorial virtual special issue Federica Bertolotti,* ,† Anastasia V. Shishkina,* ,‡ Alessandra Forni, § Giuliana Gervasio, † Adam I. Stash, ∥ and Vladimir G. Tsirelson ‡ † Department of Chemistry, University of Turin, Via P. Giuria 7, 10125 Turin, Italy ‡ D.I. Mendeleev University of Chemical Technology, Miusskaya Square 9, 125047 Moscow, Russia § CNR-ISTM (Institute of Molecular Science and Technology), University of Milan, Via Golgi 19, 20133 Milan, Italy ∥ L.Ya. Karpov Institute of Physical Chemistry, ul. Vorontsovo Pole 10, 103064 Moscow, Russia * S Supporting Information ABSTRACT: A detailed description of the ability of halogen bonding to control recognition, self-organization, and self-assembly in I 2 crystal, combining low-temperature X-ray diffraction experiments and theoretical DFT-D and MP2 studies of charge density, is reported. The bond critical point features were analyzed using the bonding ellipsoids, in order to make them more evident and easier to compare. We showed that one-electron potential, in contrast to Laplacian of electron density, allows the electron concentration and depletion regions in the valence shell of the iodine atoms to be revealed. Thus, it was demonstrated as an effective tool for understanding the molecular recognition processes in iodine crystal, describing the mutually complementary areas of concentration and depletion of electron density in adjacent molecules. This finding was also confirmed in terms of electrostatic potential, especially using the concept of σ-hole. The tiny features of the electrostatic component of halogen−halogen interactions were also visualized through the superposition of the gradient fields of electron density and electrostatic potential. The general picture provided significant arguments supporting the distinction between Type-I (van der Waals) and Type-II (Lewis molecular recognition mechanism) I··I interactions. The energies of these interactions, evaluated on the basis of empirical relationships with bond critical points parameters, have allowed estimating the lattice energy for crystalline I 2 , which has been found in reasonable agreement with the experimental sublimation energy. ■ INTRODUCTION Intermolecular interactions are important in assembly of molecular and supramolecular systems. 1 Among them halogen bonding (XB), a widely occurring type of noncovalent interaction, is now the focus of many studies. 2 It is well established that a covalently bonded halogen atom is able to simultaneously interact with both the negative and positive sites of neighboring molecules due to its strong atomic electron density (ED) anisotropy. 3 Therefore, the XB interactions play an important role in molecular recognition processes, 4 supramolecular chemistry, 5 and crystal engineering. 6 The nature of XB has been discussed for several decades from different viewpoints. 7 Conventional X-ray crystallography, Raman and microwave spectroscopy might provide a lot of empirical information about XB interactions that could be completed through quantum chemical calculations. However, owing to the very wide range (5−180 kJ/mol) of XB energies, where the weak Cl···Cl interactions in chlorocarbons and the very strong I − ···I 2 ones in I 3− complexes are the extremes, 8 there are contradictory data about the nature of these interatomic interactions. Some works show its primarily electrostatic character, 9 with small contributions coming from second-order terms, such as polarization, dispersion, and charge transfer. 10 Other studies 11 highlighted that the electron density transfer, from the lone pair of the Lewis base to the σ* C−X antibonding orbital or to outer portions of the halogenated molecule, can be in some cases competitive with the electrostatic contribution. In the past decade, a lot of work has been done in order to analyze the features of ED in the XB intermolecular region in molecular complexes and crystals. 12 The quantum theory of atoms in molecules and crystals (QTAIMC) 13 plays a significant role in these studies because it offers a consistent way of reconstruction of the atomic interactions in many- electron multinuclear systems, whose accurate wave function is computed or high resolution ED is measured. The QTAIMC features, such as the bounded atoms separated by the zero-flux surfaces of the gradient of ED, the critical points (CPs), and the lines of maximum density between nuclei (the bond paths, BPs), 14 yield a crystal structure description at the level of the bonding details. 15 In the majority of cases, BPs and associated bond critical points (BCPs) occur between pairs of atoms that Received: April 14, 2014 Revised: May 24, 2014 Published: May 30, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 3587 dx.doi.org/10.1021/cg5005159 | Cryst. Growth Des. 2014, 14, 3587−3595