Intramolecular hydrogen bonding and conformational behaviour of N-methyldiethanolamine and triethanolamine

Abstract

In N-methyldiethanolamine and triethanolamine, bicyclic five- and eight-membered ring intramolecular hydrogen bonds are formed such that the lone pairs of an oxygen whose hydroxy proton is already involved in an intramolecular hydrogen bond with the amino nitrogen participates in further hydrogen bonding with the second and third hydroxy groups, respectively. In triethanolamine, the participation of all three hydroxy groups in intramolecular hydrogen bonding was brought about by the formation of a cis intramolecularly hydrogen-bonded O—H⋯N five-membered ring and two intramolecularly hydrogen-bonded O—H⋯O eight-membered rings located symmetrically on either side of the five-membered ring. In both ethanolamines, nearly all the molecules had one intramolecularly hydrogen-bonded hydroxy group forming a five-membered ring. In half of these species there were one or two additional hydroxy groups involved in eight-membered ring intramolecular hydrogen bonds.