Halogen-Bond Interactions Enhanced by Charge-Assisted Hydrogen Bonds: An Ab Initio Study

The NH4+center dot center dot center dot NCX center dot center dot center dot NCY (X = F, Cl, and Br; Y = H, F, and OH) complexes are theoretically investigated to find ways to enhance the halogen-bond interaction. Cooperative effects are observed when a charge-assisted hydrogen bond (CAHB) and halogen bond coexist in the same complex. These effects are studied in terms of equilibrium geometry, interaction energy, charge-transfer properties, and N-14 nuclear quadrupole coupling constant of the complexes at the MP2/aug-cc-pVTZ level. In all triads studied, a favorable cooperativity is found with values that range between -1.44 and -4.04 kcal mol(-1). However, the increased percentage of the interaction energy depends on the interaction strength. To investigate the role of different energy terms in cooperativity between hydrogen and halogen bonds, the scheme of decomposition of the interaction energy is applied. The electrostatic interaction is found to be a dominant factor in enhancing both types of interactions.