Stabilities and properties of O 3 –HOCl complexes: A computational study Mohammad Solimannejad a, * , Ibon Alkorta b, * , Jose Elguero b a Quantum Chemistry Group, Department of Chemistry, Arak University, 38156-879 Arak, Iran b Instituto de Quı ´mica Me ´dica (CSIC), Juan de la Cierva, 3 28006 Madrid, Spain Received 18 August 2007; in final form 8 October 2007 Available online 12 October 2007 Abstract MP2/6-311++G(2d,2p) and MP2/aug-cc-pVTZ calculations are used to analyze the interaction between hypochlorous acid (HOCl) and ozone. Six and five minima are located on the potential energy surface of HOClO 3 complexes at MP2/6-311++G(2d,2p) and MP2/aug-cc-pVTZ computational levels, respectively. The more strongly bound contains OHO along with OCl bond. The rest of the minima present in addition to the OHO hydrogen bond, OO and OCl interactions. Binding energies of the complexes cor- rected with BSSE lies in the range of 6–12 and 9–16 kJ mol 1 at MP2/6-311++G(2d,2p) and MP2/aug-cc-pVTZ levels, respectively. The atom in molecules (AIM) theory was also applied to explain the nature of the complexes. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction The importance of non-covalent intermolecular interac- tions in many areas of contemporary chemical physics has been demonstrated in numerous studies of such systems [1]. From a fundamental point of view, the complexes formed by these non-covalent interactions are significant per se, as they bridge the gap between the free molecular systems and the corresponding condensed phases. Non-covalently bonded molecular clusters are of practical importance in many areas, such as atmospheric chemistry and catalysis, as well as in biochemical processes. As an illustrative exam- ple related to the significance of non-covalent systems in atmospheric chemistry, the proposed mechanisms aiming to explain ozone layer depletion involve formation of cer- tain intermolecular complexes (or clusters) [2–5]. To under- stand the details of the reactions occurring in atmospheric conditions, it is necessary to study the structure, stability, and certain other properties of the intermolecular clusters taking part in these reactions. Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4% per decade in the total amount of ozone in Earth’s stratosphere since around 1980; and a much larger, but seasonal, decrease in stratospheric ozone over earth’s polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole [6–9]. Halogen-containing species are involved in ozone degradation, not only chloro- fluorocarbons, but also hypochlorous acid [HOCl] and chlorine monoxide radical [ClO] in particular in what con- cerns sudden depletion events [10–12]. HOCl has been detected in the upper stratosphere by mass spectrometry in clusters with nitric and sulfuric acids and their anions [13,14]. There is an experimental study concerning the observation of a heterogeneous source of OClO from the reaction of ClO radicals on ice, relevant for the mechanism of ozone depletion [15]. The ClO dimerization cycle is thought to be responsible for approximately 70% of the total ozone loss in the polar vortex [16]. Although the title complexes of the present work are of interest in the field of atmospheric chemistry, so far studies of non-covalent com- plexes of HOCl are limited to a number of systems combin- ing HOCl with Cl [17], Br [18], [H 2 O] n=14 [19], SO 3 [20] and the HO 2 radical [21]. 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.10.024 * Corresponding authors. Fax: +98 861 2774031 (M. Solimannejad). E-mail addresses: m-solimannejad@araku.ac.ir (M. Solimannejad), ibon@iqm.csic.es (I. Alkorta). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 449 (2007) 23–27