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.

On the nature of halogen bonded complexes between carbonyl bases and chlorotrifluoromethane

“…The reported interaction energy in the NH 3 ···ClCF 3 system is equal to −2.37 kcal mol −1 (MP2/aug-cc-pVDZ) [21,22]. The PA of NH 3 is 204 kcal mol −1 , about the same as that…”

“…As outlined in several works [21,24,25,28] when CClF 3 interacts with electron donors of medium strength such as NH 3 , H 2 O or H 2 S, the C-Cl bond is contracted by moderate amounts. In the F 3 CCl···NH 3 system, the C-Cl bond is contracted by 6.6 mÅ [24] or 6.3 mÅ [21].…”

“…In the F 3 CCl···NH 3 system, the C-Cl bond is contracted by 6.6 mÅ [24] or 6.3 mÅ [21]. In the F 3 CCl···OH − system, the C-Cl bond is contracted by 7 mÅ [24], and in the weaker F 3 CCl···SH 2 complex, the C-Cl bond is contracted by only 2.9 mÅ [25].…”

“…Let us notice that the complexes between trifluoromethyl halides (CF 3 X) and guest molecules have been the subject of several theoretical investigations, as for example the interaction between CClF 3 and NH 3 [21,22], H 2 O [23,24], H 2 S [25], organic bases of different nature [26][27][28][29] or halide anions [30]. The influence of angular distortions on the energies of halogen bonds and hydrogen bonds has been recently discussed [31].…”

Halogen Bonding in Crystal Engineering

“…[20] The bond strength can be increased further by substituting the second halogen atom in dihalogens with fluorine, which polarizes the other halogen even more strongly. [56,57] The lighter dihalogens are volatile and so the crystal structures of such systems are rather rare. However, some of these structures have been characterized in gas phase by rotational spectroscopy.…”