title: Ab initio study of the complexes of halogen-containing molecules RX (X=Cl, Br, and I) and NH3: towards understanding the nature of halogen bonding and the electron-accepting propensities of covalently bonded halogen atoms.
abstract: Ab initio calculations have been performed on a series of complexes formed between halogen-containing molecules and ammonia to gain a deeper insight into the nature of halogen bonding. It appears that the dihalogen molecules form the strongest halogen-bonded complexes with ammonia, followed by HOX; the charge-transfer-type contribution has been demonstrated to dominate the halogen bonding in these complexes. For the complexes involving carbon-bound halogen molecules, our calculations clearly indicate that electrostatic interactions are mainly responsible for their binding energies. Whereas the halogen-bond strength is significantly enhanced by progressive fluorine substitution, the substitution of a hydrogen atom by a methyl group in the CH(3)X...NH(3) complex weakened the halogen bonding. Moreover, remote substituent effects have also been noted in the complexes of halobenzenes with different para substituents. The influence of the hybridization state of the carbon atom bonded to the halogen atom has also been examined and the results reveal that halogen-bond strengths decrease in the order HC triple bond CX > H(2)C=CHX approximately O=CHX approximately C(6)H(5)X > CH(3)X. In addition, several excellent linear correlations have been established between the interaction energies and both the amount of charge transfer and the electrostatic potentials corresponding to an electron density of 0.002 au along the R-X axis; these correlations provide good models with which to evaluate the electron-accepting abilities of the covalently bonded halogen atoms. Finally, some positively charged halogen-bonded systems have been investigated and the effect of the charge has been discussed.
authors.affiliation: Key Laboratory for Molecular Design and Nutrition Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China. jwzou@css.zju.edu.cn
authors.forename: Jian-Wei
authors.initials: JW
authors.lastname: Zou
authors.forename: Yong-Jun
authors.initials: YJ
authors.lastname: Jiang
authors.forename: Ming
authors.initials: M
authors.lastname: Guo
authors.forename: Gui-Xiang
authors.initials: GX
authors.lastname: Hu
authors.forename: Bing
authors.initials: B
authors.lastname: Zhang
authors.forename: Hai-Chun
authors.initials: HC
authors.lastname: Liu
authors.forename: Qing-Sen
authors.initials: QS
authors.lastname: Yu
mesh_terms: D000465:Algorithms; D000641:Ammonia / Q000737:chemistry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004563:Electrochemistry; D004583:Electrons; D006219:Halogens / Q000737:chemistry*
chemical_list: D006219:Halogens; D000641:Ammonia