How approximate is the experimental evaluation of quadrupole coupling constants in liquids? A novel computational study

Hardy, Edme H.; Müller, Markus; Vogt, Patrick; Bratschi, Christoph; Kirchner, Barbara; Huber, Hanspeter; Searles, Debra J.

doi:10.1063/1.1602071

Cited by 11 publications

(30 citation statements)

References 27 publications

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“…Recently, a multi-exponential ansatz has been proposed, based on classical MD simulations. [42] Therein, a similar initial decay of the EFG autocorrelation was found, followed by a slower exponential evolution. The linear decrease in our simulations, however, makes the physical interpretation of a set of exponential fits difficult.…”

supporting

confidence: 58%

Beyond Isotropic Tumbling Models: Nuclear Spin Relaxation in Liquids from First Principles

et al. 2008

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supporting

confidence: 58%

Beyond Isotropic Tumbling Models: Nuclear Spin Relaxation in Liquids from First Principles

et al. 2008

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“…In fact, we know of only one study that has computed the correct biaxial 17 O TCF. 97 In previous simulation studies of supercooled water, the structural part of uniaxial TCFs has been variously fitted to two exponentials 98 or to a stretched exponential function. 48 However, in the former case, the TCF was only fitted up to C(τ max ) ≈ 0.5.…”

Section: Time Correlation Functionsmentioning

confidence: 99%

Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations

Qvist

Mattea

Sunde

et al. 2012

The Journal of Chemical Physics

141

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Structural dynamics in liquid water slow down dramatically in the supercooled regime. To shed further light on the origin of this super-Arrhenius temperature dependence, we report high-precision (17)O and (2)H NMR relaxation data for H(2)O and D(2)O, respectively, down to 37 K below the equilibrium freezing point. With the aid of molecular dynamics (MD) simulations, we provide a detailed analysis of the rotational motions probed by the NMR experiments. The NMR-derived rotational correlation time τ(R) is the integral of a time correlation function (TCF) that, after a subpicosecond librational decay, can be described as a sum of two exponentials. Using a coarse-graining algorithm to map the MD trajectory on a continuous-time random walk (CTRW) in angular space, we show that the slowest TCF component can be attributed to large-angle molecular jumps. The mean jump angle is ∼48° at all temperatures and the waiting time distribution is non-exponential, implying dynamical heterogeneity. We have previously used an analogous CTRW model to analyze quasielastic neutron scattering data from supercooled water. Although the translational and rotational waiting times are of similar magnitude, most translational jumps are not synchronized with a rotational jump of the same molecule. The rotational waiting time has a stronger temperature dependence than the translation one, consistent with the strong increase of the experimentally derived product τ(R) D(T) at low temperatures. The present CTRW jump model is related to, but differs in essential ways from the extended jump model proposed by Laage and co-workers. Our analysis traces the super-Arrhenius temperature dependence of τ(R) to the rotational waiting time. We present arguments against interpreting this temperature dependence in terms of mode-coupling theory or in terms of mixture models of water structure.

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“…Several approaches have been proposed to estimate the EFG in classical MD simulations, such as its computation on selected configurations from calculations at the Hartree-Fock 28 and QM/MM 25 levels, or the use of a function of positions fitted to prior ab initio computations on a simpler system. 29 However, most classical MD studies rely on the so-called Sternheimer approximation, which postulates the linear response of the electronic cloud to the EFG arising from the "external" charge distribution of the classical molecular model describing the surrounding solvent. This leads to the following expression of the relaxation rate:…”

Section: A Theorymentioning

confidence: 99%

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

Carof

Salanne

Charpentier

et al. 2015

The Journal of Chemical Physics

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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 (7)Li(+), (23)Na(+), (25)Mg(2+), (35)Cl(-), (39)K(+), or (133)Cs(+). 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.

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How approximate is the experimental evaluation of quadrupole coupling constants in liquids? A novel computational study

Cited by 11 publications

References 27 publications

Beyond Isotropic Tumbling Models: Nuclear Spin Relaxation in Liquids from First Principles

Beyond Isotropic Tumbling Models: Nuclear Spin Relaxation in Liquids from First Principles

Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations

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

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