Theoretical investigation of the π-π stacking interactions in substituted pyridinium ion.

The interaction between positively charged aromatic groups (π-π) is characterized by stacked pyridinium ion derivatives X-PyH||X-PyH (XH, NH2, OH, CN, F, Br and Cl) at the M06-2X/6-311++G(d,p) level of theory in the presence of three solvents. The results indicate the formation of π-π stacking in a dielectric solvent can be due to the significant stability of the charge-charge repulsion forces. The trend in the absolute value of stacking interaction energy |ΔE⋯| with respect to the solvent is formamide>water>acetone. In all complexes, the π-π stacking interactions increase with the electron-donating substituents (EDSs) and decrease with the electron-withdrawing substituents (EWSs). Excellent correlations were found between the ΔE values of the complexes and a combination of Hammett substituent constants. The results obtained from the atoms in molecules (AIM) and natural bond orbital (NBO) analysis show that the sum of electron densities calculated at BCPs and CCPs between the rings (∑r and ∑ρ) and the sum of donor-acceptor interaction energies (∑E) from one ring to other can be used as useful descriptors in the prediction of ΔE⋯ values in the complexes.