Magnitude and origin of the attraction and directionality of the halogen bonds of the complexes of C _6F _5X and C _6H _5X (X=I, Br, Cl and F) with pyridine

摘要

The geometries and interaction energies of complexes of pyridine with C _6F _5X, C _6H _5X (X=I, Br, Cl, F and H) and R FI (R _F=CF _3, C _2F _5 and C _3F _7) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (E _(int)) for the C _6F _5X-pyridine (X=I, Br, Cl, F and H) complexes at the basis set limit were estimated to be -5.59, -4.06, -2.78, -0.19 and -4.37 kcalmol ~(-1), respectively, whereas the E _(int) values for the C _6H _5X-pyridine (X=I, Br, Cl and H) complexes were estimated to be -3.27, -2.17, -1.23 and -1.78 kcalmol ~(-1), respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C _6F _5X. The values of E int estimated for the R _FI-pyridine (R _F=CF _3, C _2F _5 and C _3F _7) complexes (-5.14, -5.38 and -5.44 kcalmol ~(-1), respectively) are close to that for the C _6F _5I-pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short-range (charge-transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C _6F _5I- and C _2F _5I-pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water-formaldehyde complex. The calculations suggest that the C-I and C-Br halogen bonds play an important role in controlling the structures of molecular assemblies, that the C-Cl bonds play a less important role and that C-F bonds have a negligible impact. Bond directionality: The directionality of halogen bonds is higher than that of conventional hydrogen bonds. The electrostatic interactions of the atomic quadrupole moments of the halogen atoms are the origin of the attraction and directionality of the halogen bond (see figure).