1H and 17O NMR study of H-bond dynamics in picolinic acid N-oxide solutions in acetonitrile-h3 and acetonitrile?d3: Novel aspects of old casus

Article abstract of DOI:10.1016/j.chemphys.2018.06.020

Despite apparent similarity, two isotopomers of acetonitrile (ACN-h₃ and ACN-d₃) show unexpected differences in their structural and dynamic properties. These differences cannot be fully understood within the usual isotope effect and therefore called as unresolved casus. Sensing the solvent properties, the H-bond dynamics of picolinic acid N-oxide (PANO) in ACN was studied using ¹H and ¹⁷O NMR. Several overlapping factors, like the presence of micro-traces of water, liquid-liquid equilibrium, effect of ionic ingredient, etc., which may cause the observed anomalies in the dependences of PANO chemical shift and signal width on temperature, were analyzed. The deuterium isotope effect on the phase separation in organic solute/water mixtures was deduced to be the crucial factor inducing those surprising differences between ACN-h₃ and ACN-d₃. Thus the casus could be resolved within the second-order phase transition model, in which the vibrational contributions to the intermolecular interaction energies would be properly taken into account.