Vibrational Spectroscopic and ab Initio Studies on Conformations of the Chemical Species in a Reaction of Aqueous 2-(N,N-Dimethylamino)ethanol Solutions with Carbon Dioxide. Importance of Strong NH~+…O Hydrogen Bonding

摘要

The process of a reaction of aqueous 2-(N,N-dimethylamino)ethanol (DMAE) solutions with carbon dioxide and the conformations of the chemical species present in the reaction system have been studied by infrared and Raman spectroscopy and ab initio molecular orbital and density functional theories. The products of the reaction are protonated ion of DMAE (DMAEH~+), hydrogencarbonate ion, and carbonate ion. The DMAEH~+ species in the aqueous solution assumes predominantly the G~(-+)G~±t conformation around the HN~+-C-C-OH bonds. DMAEH~+ in solid DMAEH~+Cl~- and in acidic aqueous solution also assumes the same conformation. This conformational property of DMAEH~+ is contrasted with the results for the DMAE molecules in the liquid state and in aqueous solution that they assume the G~(-+)Tx (x = t, g~(-+), or g~±) and G~(-+)G~±g~(-+) conformations around the LN-C-C-OH bonds (., lone pair). The G~(-+)G~±t conformation of DMAEH~+ is stabilized by strong intramolecular 1,4-NH~+…O hydrogen bonding, while the G~(-+)Tx and G~(-+)G~±g~(-+) conformations of DMAE are made stable through the competition of intermolecular hydrogen bonding and intramolecular 1,4-OH…N hydrogen bonding. The interconversion, depending on the acidity/basicity of the environment, of strong NH~+…O hydrogen bonding and OH…N hydrogen bonding or N…O repulsion is certainly one of the important structural factors in chemical and biological reaction processes.