Nuclear magnetic resonance spin-lattice relaxation of lithium ions in aqueous solution by NMR and molecular dynamics

Abstract

We study the aqueous solvation dynamics of lithium ions using nuclear magnetic resonance spectroscopy, molecular dynamics, and viscosity measurements. Several relaxation mechanisms are examined to explain the strong increases of spin-lattice relaxation toward high concentrations. The use of both 6Li and 7Li isotopes is helpful to identify the quadrupolar contribution to the relaxation rate. In particular, it is found that the quadrupolar interaction constitutes the strongest contribution above a concentration of ∼10 molal. The next-strongest contribution arises from interactions that scale with the square of the gyromagnetic ratio (mostly the dipolar interaction), and the experimental relaxation rates appear to be fully accounted for when these mechanisms are combined over the concentration range up to the saturation concentration. The study of solvation dynamics, particularly at high concentrations, could be of relevance for electrolyte dynamics in aqueous Li-ion rechargeable batteries.

NMR Relaxation Rates of Quadrupolar Aqueous Ions from Classical Molecular Dynamics Using Force-Field Specific Sternheimer Factors

“…The value of 0.0247 (0.0007) for 7 Li + in the Amber14 FF is in very good agreement with the experimental value. Recently, the latter model has been used to determine the concentration dependence of the quadrupolar contribution to the rate of 7 Li + in simulations, yet a value of γ ∞ = 0.17 was used to account for the electronic cloud polarization effects and ultimately resulted in some differences between the simulation and experimental results at lower concentrations . Here, we show that this discrepancy can be overcome by considering the model specific Sternheimer factor γ eff .…”

“…In principle, ab initio molecular dynamics (MD) can provide both features, − yet the entailed computational cost often limits the ability to converge the corresponding ACF with sufficient accuracy for a reliable estimate of its integral . This motivated the use of classical MD to sample configurations over the relevant time scales, yet the accurate estimation of the EFG at the nucleus position has remained a formidable challenge in classical MD. ,− Such a challenge comes from the fact that the dominant contribution to the total EFG V αβ is due to the intra-atomic electronic charge distribution around a nucleus that is not available in atomistic classical models and is even difficult to obtain accurately with electronic structure calculations. Yet, quantum density functional theory (DFT) allows quite accurate computation of EFGs for main group elements and some transition metals, although a complete understanding of the solvation effects on the EFGs has remained challenging.…”

NMR relaxation rates of quadrupolar aqueous ions from classical molecular dynamics using force-field specific Sternheimer factors

“…Recently, the latter model has been used to determine the concentration dependence of the quadrupolar contribution to the rate of 7 Li + in simulations, yet a value of γ ∞ = 0.17 was used to account for the electronic cloud polarization effects and ultimately resulted in some differences between the simulation and experimental results at lower concentrations. 11 Here we show that this discrepancy can be overcome by considering the model specific Sternheimer factor γ eff .…”

“…6 This motivated the use of classical MD to sample configurations over the relevant time scales, yet the accurate estimation of the EFG at the nucleus position has remained a formidable challenge in classical MD. 2,[7][8][9][10][11] Such challenge comes from the fact that the dominant contribution to the total EFG V αβ is due to the intraatomic electronic charge distribution around a nucleus that is not available in atomistic classical models and is even difficult to obtain accurately with electronic structure calculations. In the case of monoatomic ions, the widespread electrostatic models of the EFG relaxation [12][13][14][15] consider that the EFG at the nucleus is created by the inhomogeneous distribution of external charges that polarize the electronic cloud of the given ion.…”

Weak nuclear spin singlet relaxation mechanisms revealed by experiment and computation

“…7,8 The mechanisms that ultimately lead to the decay of SO are often difficult to identify and quantify. Molecular dynamics (MD) and ab initio calculations (including ab initio MD) have been used for the calculation, prediction, and analysis of NMR processes, [9][10][11][12][13][14][15] but this approach has not been extended to nuclear spin singlet states, with the exception of the work of Håkansson. 16 That work presented a unique approach to use a fitting algorithm to interpolate between different MD snapshots for which ab initio quantities were calculated.…”