title: An NMR investigation of the conformational equilibria of 2-(2'-pyridyl)ethylphosphonic acid in several solvents.
abstract: Vicinal proton-proton NMR couplings have been used to investigate whether the position of conformational equilibria is determined by intramolecular N-H hydrogen bonding for 2-(2'-pyridyl)ethylphosphonic acid 1 in its various possible ionization states in water, methanol, ethanol, and dimethyl sulfoxide (DMSO). With 1 in the form of its monoanion and dianion, the trans is favored, with the dianion being more trans than the monoanion for a given solvent, probably as the result of steric effects, possibly enhanced by repulsive electrostatic effects between the negatively charged phosphonic group and the lone pair on the pyridine nitrogen. For 1 and its conjugate acid, the gauche amounts, respectively, to 43% and 45% in water, 66% and 51% in methanol, 66% and 64% in ethanol, and 29% and 49% in DMSO. For these latter two species, electrostatic, steric, and hydrogen bonding-effects are all likely to play a role in determining the conformational equilibria.
authors.affiliation: Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Krag A
authors.initials: KA
authors.lastname: Petterson
authors.forename: Diana
authors.initials: D
authors.lastname: Fort
authors.forename: Peter D
authors.initials: PD
authors.lastname: Wei
authors.forename: John D
authors.initials: JD
authors.lastname: Roberts