Hydrogen bond interactions in hydrated acetylsalicylic acid

Post Hartree–Fock and density functional theory (DFT) methods have been employed to study the molecular properties of the conformers of acetylsalicylic acid and their hydrated forms. The structures of monomer and hydrated acetylsalicylic acid are theoretically investigated at MP2 and B3LYP level of theories, in gaseous and aqueous phase environment implementing the 6-311G (2d, 2p) atomic basis set. Their interaction energy and the strength of the hydrogen bonds have been analyzed. The complex with strong hydrogen bond is found to be the most stable among the monohydrated acetylsalicylic acid complexes. The self-consistent reaction field theory (SCRF) has been employed to optimize acetylsalicylic acid complexes in aqueous phase (ε = 78.5) environment and the solvent effect has been studied. The atoms in molecules theory (AIM) is employed to investigate H-bonding patterns in the complexes. From the above topological analysis, an excellent linear correlation is shown between electron density [ρ(r)] and its Laplacian [∇2ρ(r)] at bond critical points. The charge transfer from the proton acceptor to the anti-bonding orbital of proton donor has been analyzed through natural bond orbital (NBO) analysis. NMR chemical shift has been calculated to analyze the molecular environment as well as the delocalization of electron density.