NMR and quantum chemistry study of mesoscopic structuring in 3-methylpyridine/water/NaBr mixture in the vicinity of the phase separation boundary

Abstract

²³Na and ⁸¹Br NMR spin-lattice relaxation times and signal half widths (Δ_1/2) have been measured in 3-methylpyridine (3MP)/H₂O/NaBr mixture along T = 294 and 301 K isotherms gradually increasing the mass fractions of salt (X) up to the phase separation boundary. The extreme narrowing condition and thus 1/T ₁ = 1/T ₂ = πΔ _1/2 was found to be valid in all cases. Discontinuous changes in slope of 1/T _1,2 = f(X) were detected, and then corresponding points on the phase diagram (X, T) were attributed to the borderline between the molecular-ionic solution and the area of enhanced mesoscopic structuring. A very strong relaxation effect was observed for ⁸¹Br nuclei reaching relaxation rates of 14,000 s^–1. ²³Na and ⁸¹Br NMR relaxation data together with calculations of quantum chemistry model of electrical field gradient tensor evidence the migration of 3-methylpyridinium at increasing X from the anions hydration shells towards cations. An interchange from migration to steady distribution regimes is observed for anions and vice versa for cations at the borderline of the structured phase.

Keywords:

• NMR spectroscopy
• phase transition
• structural change

Acknowledgements

The Alexander von Humboldt Foundation is kindly acknowledged for the support of joint research project “Structure and phase transitions in ionic solutions”. We thank the Danish Center for Scientific Computing (Copenhagen, Denmark) for the possibility to use computational facilities and Gaussian 03 program package and the Supercomputer Centre of Vilnius Gediminas Technical University for the possibility to use their cluster and GAMESS package. We are grateful to Prof. W. Schröer, Prof. H. Weingärtner and Prof. M.A. Anisimov for stimulating discussions and comments.