Reaction of aqueous 2-(N-methylamino)ethanol solutions with carbon dioxide.: Chemical species and their conformations studied by vibrational spectroscopy and ab initio theories

The reaction systems of aqueous 2-(N-methylamino)ethanol (MAE) solutions with dissolved CO2 (CO2-MAE-H2O system) and of liquid MAE with dissolved CO2 (CO2-MAE system) have been studied by vibrational spectroscopy and ab initio theories. The chemical species, molecular conformations, and intramolecular and intermolecular hydrogen bonding in these systems have been elucidated. The species produced in the CO2-MAE-H2O system are the carbamate anion (MAECO(2)(-)), the protonated cation (MAEH(+)), hydrogencarbonate ion (HCO3-), and carbonate ion (CO32-), while the species produced in the CO2-MAE system are MAECO(2)(-) and MAEH(+). In the CO2-MAE-H2O system, MAE (secondary amine) reacts with CO2 to form MAECO(2)(-) and MAEH(+), and MAECO(2)(-) subsequently reacts with H2O and CO2 to form MAEH(+) and HCO3-. This reaction mechanism is contrasted with the mechanism for 2-(N,N-dimethylamino)ethanol (DMAE, tertiary amine), which reacts with CO2 and H2O to form directly DMAEH(+) and HCO3-. In the liquid state and in aqueous solution, MAECO(2)(-) molecules assume predominantly the S(-)G(-)g(+) conformation with the formation of strong intramolecular 1,6-OH ... O- hydrogen bonding, and MAEH(+) molecules assume predominantly the TG(+/-)t conformation with the formation of strong intramolecular 1,4-NH+... O hydrogen bonding. MAE molecules in the condensed phases, on the other hand, assume several conformations such as TG(-)x (x = t, g(-), and g(+)), TG(+)g(-), and G(-)G(-)g(+) as the result of the competition of intermolecular interactions such as OH ... O or OH ... N hydrogen bonding and intramolecular interactions such as the 1,4-OH ... N hydrogen bonding. The interconversion, depending on pH or addition/removal of CO2, of strong ionic hydrogen bonding such as NH+... O, NH+... O-, and OH ... O- and nonionic hydrogen bonding such as OH ... N and OH ... O is clearly one of the important structural features in the CO2-amine-H2O system and probably in biomolecular systems, where intramolecular and/or intermolecular interactions between nitrogen and oxygen atoms are involved.