Ab Initio Calculations on Halogen Bond Between N-Br and Electron-donating Groups

Ab initio calculations of complexes formed between N-bromosuccinimide and a series of electron-donating groups were performed at the level of MP2/Lanl2DZ^* to gain a deeper insight into the nature of the N-Br halogen bonding. For the small complexes, H₃C-Br...NH₃ and H₂N-Br...NH₃, the primary calculation has demonstrated that the N-Br in H₂N-Br...NH₃ can form a much stronger halogen-bonding complex than the C-Br. A comparison of neutral hydrogen bond complex series reveals that the electron-donating capacities of the atoms decrease in the order, N > O > S; O(sp³) >0(sp²), which is adequate for the C-Br halogen bonding. Interaction energies, in conjunction with the geometrical parameters show that the affinitive capacity of trihalide anions X^-₃ with N-bromosuccinimide are markedly lower than that of the corresponding X^- with N-bromosuccinimide, even lower than those of neutral molecules with N-bromosuccinimide. AIM analyses further confirmed the above results.