KHURSHID AYUB AND TARIQ MAHMOOD J.Chem.Soc.Pak.,Vol. 35, No.3, 2013 617 DFT Studies of Halogen Bonding Abilities of Nitrobenzene with Halogens and Chlorofluorocarbons KHURSHID AYUB AND TARIQ MAHMOOD * Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, Pakistan. mahmood@ciit.net.pk* (Received on 27 th September 2012, accepted in revised form 14 th March 2013) Summary: Chlorofluorocarbons are being used as coolants since the last more than fifty years, and their increasing concentration in atmosphere causes the ozone layer depletion, therefore to develop a sensor for detection of chlorofluorocarbons is environmentally significant. In this article the halogen bonding abilities of nitrobenzene along with halogens and different chlorofluorocarbons have been investigated theoretically at the B3LYP /6-31+G(d) method of DFT. All physical properties such as binding energy, equilibrium distance and dipole moment showed halogen bonding formation. Our investigations revealed that that electron rich nitrobenzene can be used as good sensor for the sensing of halogens (Cl 2 , Br 2 and F 2 ) and different chlorofluorocarbons (CFCs). Key words: Halogen bonding, Chlorofluorocarbons, Nitrobenzene. Introduction Non-covalent interactions are fundamental to supramolecular chemistry [1-3], among all non- covalent interactions, hydrogen bonding is the one that has been studied extensively [4-7], but since last one decade halogen bonding has got considerable attention due to its very important role in biology [8- 9], crystal engineering [10-12] as well as in molecular recognition systems [13]. Halogen bonding is directly related to electron donation of lewis bases to halogen atoms and can be defined as the non- covalent interaction of halogens with halogens or lewis bases [14-16]. Halogen bonding is generally weaker than hydrogen bonding (5-30 kJ/mol) [17- 19]. Halogen bonding reduces internuclear distances up to twenty percent as compared to the all Van der waals distances [20]. Generally halogen bonding is of two types, (i) halogen halogen interaction X----X which is due to anisotropic distribution of electron density among halogen atoms, (ii) halogen lewis base interaction X----AX. An anisotropic distribution of electron density in chlorofluorocarbons (CFCs), make them suitable lewis base acceptors. Chlorofluorocarbons are strong candidate for halogen bonding since multiple halogen atoms are available. Chlorofluorocarbons are being used in refrigerators and in cooling industry since more than fifty years as coolants [21], the most common example is freon. Chlorofluorocarbons produce chlorine and other reactive species on photolysis, which cause the ozone layer depletion [22]. Due to ozone layer depletion intensity of UV radiations increases on the earth surface, which may cause some eye diseases (Pterygium, Cataract Ocular melanoma), skin diseases like melanoma skin cancer, UV induce skin allergy and UV-induced immunosuppression [23]. A quick method to sense the chlorofluorocarbons is the need of time; we thought that a sensor for the detection of chlorofluorocarbons (CFCs) can be designed based on halogen bonding ability of CFCs toward strong bases. Nitro group attached to a benzene ring acts as a very strong lewis base due to high electron density on oxygen and the electron withdrawing inductive and resonance effect of nitro group [24]. As a result of these properties it can act as a potential candidate for the halogen bonding and here the main focus of this study was to investigate the halogen bonding ability of different halogens (Cl 2 , Br 2 and F 2 ) and chlorofluorocarbons with nitrobenzene theoretically. To the best of our knowledge halogen bonding ability of chlorofluorocarbons and nitrobenzene is not reported yet (Fig. 1) Fig. 1: Optimized Structure of Nitrobenzene. Results and Discussion Optimized complexes of nitrobenzene with chlorine, bromine, fluorine and chlorofluorocarbons (2-7) are shown in Fig. 2. Complexation energies (uncorrected and BSSE corrected) along with equilibrium distance and dipole moment are given in the Table-1. Geometrical characters of all halogen nitrobenzene complexes (2-4) were found almost similar, and all three halogens (chlorine, bromine and fluorine) interact with nitrobenzene in the same fashion. Bond angle of O-N-O and C-N-O bond angle in nitrobenzene (before complexation) were 123-124 o and 117-118.5 o , respectively. Similarly N-O and C-N bond lengths in nitrobenzene were 1.23-1.25 Å and 1.46-1.48 Å, respectively. * To whom all correspondence should be addressed. J. Chem. Soc. Pak., Vol. 35, No.3, 2013