CCSD(T) level interaction energy for halogen bond between pyridine and substituted iodobenzenes: origin and additivity of substituent effects.

The CCSD(T) level interaction energies at the basis set limit (E(int)) were calculated for 33 halogen bonded pyridine complexes with substituted iodobenzenes. The CCSD(T) level electron correlation correction substantially decreases the magnitude of attraction in comparison with the MP2. The E(int) for the pyridine complexes with mono substituted iodobenzenes varies from -3.14 to -4.42 kcal mol(-1), depending on the substituent. The electron-withdrawing substituents such as NO2 enhance the attraction, while the effects of electron-donating substituents reduce. The additivity of the substituent effects is observed for the E(int) of the pyridine complexes with multiple substituted iodobenzenes. The electrostatic interactions are mainly responsible for the substituent effects on the magnitude of the attraction in the halogen-bonded complexes. The electrostatic energy depends significantly on the substituent. They have a strong correlation with the E(int). On the other hand the effects of the substituent on the dispersion energy are small, however the dispersion does contribute greatly to the attraction.