Hydrogen bonding and proton transfer in cocrystals of 4,4′-bipyridyl and organic acids studied using nuclear quadrupole resonance

Abstract

Cocrystals of 4,4′-bipyridyl and several carboxylic acids were grown from the methanol solution of the cocrystal formers. Complete ¹⁴N NQR spectra of these cocrystals have been measured using ¹H–¹⁴N nuclear quadrupole double resonance. The principal values of the quadrupole coupling tensor are calculated from the ¹⁴N NQR frequencies. A large variation in the ¹⁴N quadrupole coupling constant between 1.3 MHz and 4.7 MHz is observed. A very low ¹⁴N quadrupole coupling constant, characteristic for proton transfer O–H⋯N → O⁻⋯H–N⁺, is observed in 4,4′-bipyridyl–oxalic acid (1 : 1). In 4,4′-bipyridyl–5-chlorosalycilic acid (1 : 1) the ¹⁴N NQR data show the presence of a short, strong N⋯H⋯O hydrogen bond. A correlation of the principal values of the ¹⁴N quadrupole coupling tensor is observed. The correlation is analyzed in the model, where the deformation of the lone pair electron orbital and the change of the population of the π-electron orbital produce the variation of the ¹⁴N quadrupole coupling tensor in the hydrogen bonded 4,4′-bipyridyl. The temperature variation of the ¹⁴N quadrupole coupling tensor in 4,4′-bipyridyl–5-chlorosalycilic acid (1 : 1) is analyzed. Proton displacement within the N⋯H⋯O hydrogen bond and the change of the population of the π-electron orbital at the two nitrogen positions in a 4,4′-bipyridyl molecule in the temperature interval between 157 K and 323 K are determined.