Ab initio studies of the properties of some halogen-bonded complexes of ammonia, water, phosphine and hydrogen sulphide Ponnadurai Ramasami a , Thomas A. Ford b,⇑ a Department of Chemistry, University of Mauritius, Reduit, Mauritius b School of Chemistry, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa article info Article history: Received 13 October 2011 Accepted 11 November 2011 Available online 29 November 2011 Keywords: Ab initio calculations Halogen-bonded complexes Interaction energies Molecular structures Vibrational spectra abstract Ab initio calculations, at the second order level of Møller–Plesset perturbation theory and with Dunning’s augmented correlation-consistent polarised valence triple-zeta basis set, have been carried out on two series of molecular complexes. One set consists of the common bases ammonia, water, phosphine and hydrogen sulphide and the electron-acceptor halogens difluorine, dichlorine and dibromine, and the other comprises the interhalogens bromine fluoride, chlorine fluoride and bromine chloride as electron acceptors, with the same set of four bases. The interaction energies, molecular structures, vibrational spectra and charge distribution properties of the complexes have been computed, and the adducts have been characterised as halogen-bonded complexes. The trends in the values of these properties correlate with systematic changes in the base and the halogen species. The computed physical data are in quite good agreement with such experimental results as are available, while the anomalous features of some of the properties of the complexes with phosphine have been rationalised. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The term ‘‘halogen bond’’ was apparently coined by Dumas et al. in 1978 in a report of the detection of the electron donor–acceptor complexes formed between carbon tetrachloride and tetrabromide and some oxygen and nitrogen bases, in which the halogen atom interacted directly with the oxygen or nitrogen atom [1]. Many other reports have since appeared of the observation of similar complex species, including some containing NH 3 [2–11],H 2 O [11– 17], PH 3 [4,5,11,18,19] and H 2 S [4,8,10,11,20–23] as electron do- nors, of relevance to the present work. These complexes are known generically as halogen-bonded complexes, and the properties of these unusual non-covalent adducts have been extensively re- viewed [10,24–27]. The source of the interaction is interpreted in terms of electrostatic potentials. The lone pairs of electrons on atoms such as N, O, P and S represent sites of high electron density, and the peripheral surface of a halogen-containing molecule at the halogen ‘‘end’’ of the molecule is a region of positive electrostatic potential, the so-called r-hole [24–27]. The attraction of these elec- tropositive and electronegative regions confers some stability on the adduct and, since the positive electrostatic potential acts along the direction of the r bond to the halogen atom, this leads to a high degree of directionality in the halogen bond. This feature draws attention to the fundamental similarity between the halogen bond and the hydrogen bond (in which, in the complex ClHÁNH 3 , for example, the region of positive electrostatic potential of HCl, lo- cated at the hydrogen atom ‘‘end’’ of the HCl molecule, interacts collinearly with the lone pair associated with the nitrogen atom of NH 3 ). Thus we would expect the complexes formed between NH 3 ,H 2 O, PH 3 and H 2 S as electron donors and halogen or interhal- ogen molecules as electron acceptors to have a range of properties in common with the analogous hydrogen-bonded complexes of acids such as HF, HCl and HBr with the same set of bases. In this pa- per we explore a series of such halogen-bonded complexes with a view to establishing whether those properties governed by the nat- ures of the electron donor and acceptor, namely interaction ener- gies, molecular geometries, vibrational spectra and charge transfer, show the same trends as those of some corresponding sim- ple hydrogen-bonded complexes as the acid and base molecules are varied systematically. The microwave spectroscopic work of Legon and co-workers [2–23] has built up an extensive library of physical data, including rotational constants, centrifugal distortion constants and nuclear quadrupole coupling constants, on those intermolecular complexes formed between the bases listed above and the halogen (F 2 , Cl 2 and Br 2 ) and interhalogen (BrF, ClF and BrCl) molecules with which to compare the results of our computational studies. Thus we were able to initiate this project with a good understanding of the trends in the complex properties to be expected. Ab initio studies of some of the complexes which make up the subject of the present inves- tigation have also appeared [14–17,23,28–33], so there are also some theoretical data against which our results may be measured. 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.11.024 ⇑ Corresponding author. Tel.: +27 312601096; fax: +27 312603091. E-mail address: ford@ukzn.ac.za (T.A. Ford). Computational and Theoretical Chemistry 990 (2012) 227–235 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc