NMR probing of structural peculiarities in ionic solutions close to critical point

Abstract

(1)H, (23)Na, (35)Cl, (79)Br, and (81)Br NMR chemical shifts (delta) and signal half widths (Delta(12)) have been measured in aqueous electrolyte mixtures [tetrahydrofuran/H(2)ONaCl and 3-methylpyridine (3MP)H(2)ONaBr] at different mass fractions of salt (X) in the one-phase region, close to their lower critical solution points (T(CL)). Discontinuous changes in slope of delta=f(X) and Delta(12)=f(X) have been found in (23)Na and (81)Br NMR spectra of 3MP/water/NaBr solution at X approximately 0.1 and T=301 K. The dependency of (1)H NMR signals of 3MP is continuous over the whole investigated range of X=0.002-0.2, whereas changes in the slope of H(2)O chemical shifts are hardly noticeable. In the two-phase region, i.e., at T>T(CL), a doubling of all NMR signals has been observed. The sensitivity of NMR parameters depends more on composition of solution for anions (Cl(-) and Br(-)) than for cations (Na(+)). A very strong relaxation effect for (81)Br nuclei with relaxation rates reaching 14 000 s(-1) was observed. The results are interpreted in terms of ion-molecular clustering and changes in coherency of dipole configurations of water molecules during supramolecular restructuring of solutions.

The isotropic nuclear magnetic shielding constants of acetone in supercritical water: A sequential Monte Carlo/quantum mechanics study including solute polarization

“…Among the molecular properties, the nuclear shielding constants are very sensitive to changes in the environment and the solvent shift of the shielding constants of a solute molecule constitutes a very useful parameter for probing local modifications of intermolecular interactions in solution. [19][20][21][22][23][24][25] The local magnetic field on the atomic nucleus generated by the response of the electronic structure to an external magnetic field is affected by the solvent. This effect is of great interest in normal fluids but only limited knowledge is available for SC fluids and SCW.…”

Magnetic shielding properties of water in various molecular and molecular-ionic structures

“…Several water structures, closely related to peculiar restructuring effects close in the vicinity of the phase transition [5,6], have been chosen, and their magnetic screening properties have been studied. All calculated molecular aggregates, values of isotropic part of magnetic screening tensors of 1 H and 17 O nuclei, as well as their chemical shifts δ , where .…”

“…They exhibit many strange, sometimes even paradoxical features of fundamental importance. Several extremely challenging problems are met there: hydrophobic and structure effects [4,5], non-equilibrium restructuring, criticality and crossover between the solvophobic and ionic regimes [6][7][8], supercritical state of water [9]. This made the water systems as a certain 'firing ground' for many experimental works, NMR among them, and for many developments in theory of condensed matter statistical physics and quantum chemical calculations.…”

A13C NMR and density functional theory study of critical behaviour of binary water/2,6-lutidine solution

“…Multinuclear ( 1 H, 2 H, 23 Na, 81 Br) NMR was successfully applied studying water / derivatives of pyridine systems and phenomena therein [7][8][9]. However analogous studies using the 13 C NMR are absent, and thus the capabilities of this method are still not well-recognized.…”

“…On the other hand, phase separations in completely miscible water / pyridine solutions can be induced by the introduction of an ionic subsystem [3,4]. All these phenomena are studied using a wide range of modern experimental techniqueslight scattering [3][4][5], small angle X-ray scattering [6], NMR spectroscopy [7][8][9], shear viscosity and refraction index measurements [10,11], etc. Several extremely challenging problems have been revealed during these studies, namely appearance of microheterogeneous phases or mesoscopic (of about hundred nanometres) aggregates with very large equilibration times, non-equilibrium restructuring, criticality and crossover between the solvophobic and ionic regimes, etc.…”