Multiple hydrogen-bonding interactions between macrocyclic triurea and F-, Cl-, Br-, I- and NO3(-): a theoretical investigation.

The binding behaviors of the 27-membered macrocyclic triurea 1 towards the five anions, F(-), Cl(-), Br(-), I(-) and NO(3)(-), through multiple hydrogen-bonding interactions, were investigated at the B3LYP/6-311++G(d,p)//B3LYP/6-31(1)++G(d,p) (6-31(1)++G(d,p) is a hybrid basis set; for more details see computational methods) level. Three binding modes (I, II, and III) were found for all the five anions in the gas phase, and seven structural parameters have been used to describe these binding modes. Binding mode I and II have similar binding geometries and their coordination number of anions is six. Binding mode III exhibits completely different binding characteristics and the coordination number is three except for NO(3)(-). Our calculation revealed that the binding strength of binding modes follows the trend, mode II > I > III, with the exception of F(-) complex. The binding affinity of anions in the gas phase goes in this order: F(-) > Cl(-) > NO(3)(-) > Br(-) > I(-). The changes in the binding affinity for all 15 urea-anion complexes under the influence of solvent environment were examined using the IEF-PCM continuum solvation model. Although the binding affinities are weakened substantially because of solvent effect, these drastic changes do not affect the affinity order in the gas phase. The experimentally observed affinity strength in chloroform, Cl(-) > NO(3)(-) > Br(-), was confirmed by this work. Moreover, we found a high correlation between ΔE(bind) (1) and ΔE(In) (1,3-dimethylurea) for all three binding modes, implying that the affinity strength of 1 to these five anions is determined mainly by the proton-accepting ability of anions, not by steric effect.