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.

The Conformations of 1,4-Butanedioic Acid as a Function of Solvent Polarity in a Series of Alcohols as Determined by NMR Spectroscopy

“…That similar behavior is observed to only a minor degree in water 1 accords with its much higher dielectric constant. As pointed out elsewhere, the calculated values reported in Table for the percentage of gauche conformer, especially where those values are low, as they are for isopropyl and tert -butyl alcohols as solvents, could well be actually even lower because of uncertainties in the accuracy of the Altona−Haasnoot relations in the regime of nearly all-trans conformations.…”