On the microscopic fluctuations driving the NMR relaxation of quadrupolar ions in water

Journal Article · Sat Nov 21 00:00:00 EST 2015 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4935496· OSTI ID:22493255

Carof, Antoine; Salanne, Mathieu; Rotenberg, Benjamin ^[1]; Charpentier, Thibault ^[2]

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1. Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire PHENIX, Case 51, 4 Place Jussieu, F-75005 Paris (France)
2. CEA, IRAMIS, NIMBE, LSDRM, UMR CEA-CNRS 3685, F-91191 Gif-sur-Yvette Cedex (France)

Nuclear Magnetic Resonance (NMR) relaxation is sensitive to the local structure and dynamics around the probed nuclei. The Electric Field Gradient (EFG) is the key microscopic quantity to understand the NMR relaxation of quadrupolar ions, such as {sup 7}Li{sup +}, {sup 23}Na{sup +}, {sup 25}Mg{sup 2+}, {sup 35}Cl{sup −}, {sup 39}K{sup +}, or {sup 133}Cs{sup +}. Using molecular dynamics simulations, we investigate the statistical and dynamical properties of the EFG experienced by alkaline, alkaline Earth, and chloride ions at infinite dilution in water. Specifically, we analyze the effect of the ionic charge and size on the distribution of the EFG tensor and on the multi-step decay of its auto-correlation function. The main contribution to the NMR relaxation time arises from the slowest mode, with a characteristic time on the picosecond time scale. The first solvation shell of the ion plays a dominant role in the fluctuations of the EFG, all the more that the ion radius is small and its charge is large. We propose an analysis based on a simplified charge distribution around the ion, which demonstrates that the auto-correlation of the EFG, hence the NMR relaxation time, reflects primarily the collective translational motion of water molecules in the first solvation shell of the cations. Our findings provide a microscopic route to the quantitative interpretation of NMR relaxation measurements and open the way to the design of improved analytical theories for NMR relaxation for small ionic solutes, which should focus on water density fluctuations around the ion.

• OSTI ID: 22493255
• Journal Information: Journal of Chemical Physics, Vol. 143, Issue 19; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CATIONS
CHARGE DISTRIBUTION
CHLORINE 35
CHLORINE IONS
CORRELATION FUNCTIONS
CORRELATIONS
DILUTION
ELECTRIC FIELDS
FLUCTUATIONS
LITHIUM 7
MAGNESIUM 25
MOLECULAR DYNAMICS METHOD
MOLECULES
NUCLEAR MAGNETIC RESONANCE
POTASSIUM 39
RELAXATION TIME
SODIUM 23
SOLUTES
SOLVATION
WATER