Modulation of n * Interaction in the Complexes of p-Substituted Pyridines with Aldehydes: A Theoretical Study

n & RARR; & pi;* interaction is analogous to the hydrogenbondin terms of the delocalization of the electron density between thetwo orbitals. Studies on the intermolecular complexes stabilized bythe n & RARR; & pi;* interaction are scarce in the literature.Herein, we have studied intermolecular N & BULL;& BULL;& BULL;C UOn & RARR; & pi;* interactions in the complexes of p-substituted pyridines (p-R-Py) with formaldehyde(HCHO), formyl chloride (HCOCl), and acetaldehyde (CH3CHO)using quantum chemistry calculations. We have shown that the strengthof the n & RARR; & pi;* interaction can be modulated by varyingthe electronic substituents at the donor and acceptor sites in thecomplexes. Variation of the substituents at the para position of the pyridine ring from the electron-withdrawing groups(EWGs) to the electron-donating groups (EDGs) results in a systematicincrease in the strength of the n & RARR; & pi;* interaction. Thestrength of this interaction is also modulated by tuning the electrondensity toward the carbonyl bond by substituting the hydrogen atomof HCHO with the methyl and chloro groups. The modulation of thisinteraction due to the electronic substitutions at the n & RARR;& pi;* donor and acceptor sites in the complexes is monitored byprobing the relevant geometrical parameters, binding energies, C UOfrequency redshift, NBO energies, and electron density for this interactionderived from QTAIM and NCI index analyses. Energy decomposition analysisreveals that the electrostatic interaction dominates the binding energiesof these complexes, while the charge transfer interaction, which isrepresentative of the n & RARR; & pi;* interaction, also has asignificant contribution to these.