The proton transfer and hydrogen bonding complexes of (2-hydroxyethyl)amines with acids: A theoretical study

The interaction of ethanolamines Me3−nN(CH2CH2OH)n (n = 1–3) with arylheteroacetic acids ArYCH2COOH (Ar = 2-CH3C6H4, 4-ClC6H4; Y = O, S, SO2) as well as acetic acid itself has been studied. For isolated state, the geometries and electronic structures of the complexes obtained were calculated using DFT method at B3LYP/6-311+G** level of theory. The energy of the compounds has been calculated by means B3LYP/6-311+G** and MP2/6-311+G**//B3LYP/6-311+G** methods. It has been shown that proton transfer (PT) and hydrogen bonding (HB) complexes are formed from triethanolamine (TEA) and the studied acids, while only НВ complexes is generated from diethanolamine and ethanolamine derivatives. The TEA complexes are stabilized by NH+⋯O− (PT) or OН⋯N (HB) hydrogen bonds and OH⋯O bonds of its hydroxyl groups and oxygen atoms of acid. The NH+⋯О− and ОH⋯N bonds in the complexes have a partial covalent character as predicted by the quantum theory of atoms in molecule (QTAIM). According to the second-perturbation energies E(2) in the NBO scheme, highest charge transfer is along the ОH⋯N hydrogen bond for HB complexes of TEA with ArYCH2COOH. The GIAO calculated 15N NMR chemical shifts are well correlated with NBO electron charges on the nitrogen atom for both type complexes.