Substituent effects on the hydrogen-bonded complex of aniline–H2O: a computational study

Abstract

Substituent effects (X = –CH₃, –NH₂, –OH, –F, –SiH₃, –PH₂, –H, –Cl, –CN, –NO₂, –CHO) on the hydrogen-bonded complex of para-substituted aniline with one water molecule are studied at the B3LYP/6-311++G(d,p) level of theory. The nature of H-bond interactions and the origin of substituent effects are explored by means of natural bond orbital (NBO) and atoms in molecules (AIM) analysis as well as a series of good correlation equations obtained. The result suggests that the substitution induces changes in the electron density transfer from the water molecule to the aniline derivative by means of influencing the interaction of n_O → σ*_N–H while the change in the electron density transfer would give rise to the variation on the electron densities in the proton donating N–H bond and the N–H⋯O hydrogen bond, and ultimately, influence the length and the frequency of the N–H bond, the H⋯O distance, the binding energies of complexes, the pK_a of the substituted aniline and the ¹H chemical shift. In addition, the correlations obtained reveals that the H-bond parameters calculated (such as ∇²ρ_H⋯O, ρ_H⋯O, R_H⋯O), reflecting both the intramolecular (substituent effects) and intermolecular (water effects) interactions, are found to perform better than substituent constants in rationalizing the substitution induced variations on the structure, the binding energy, ¹H chemical shift and experimental pK_a.