Substituent effects on the cooperativity of halogen bonding.

DFT calculations (B97-1) with the 6-31+G(d,p)-LanL2DZdp basis set were used to analyze the intermolecular interactions in 4-Z-Py···XCN···XCN triads (Z = H, F, OH, OCH3, CH3, NH2, NO2, and CN; Py = pyridine; and X = Cl and Br) that are connected by halogen-bond interactions. To understand the properties of the systems better, the corresponding dyads are also studied. Particular attention is given to parameters such as cooperative energy. All complexes show cooperative energy ranging from -1.39 to -3.46 kJ mol(-1) and -2.61 to -5.84 kJ mol(-1) for X = Cl and Br, respectively. We show that the effect of the substituents on the title interactions strongly depends on the nature of the substituents (Z). Thus, the electron-donor and electron-acceptor substituents increase and decrease the stability of complexes, respectively. The electronic properties of the complexes have been analyzed using molecular electrostatic potential (MEP) and minimum average local ionization energy, and the parameters were derived from the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies.