Role of Paramagnetic Impurities and Surface Roughness on NMR Relaxation Times: Insights from Molecular Dynamics Simulations

Wang, You; Amaro-Estrada, Jorge Ivan; Torres-Verdín, Carlos

doi:10.1021/acs.jpcc.3c02580

J. Phys. Chem. C

2023

DOI: 10.1021/acs.jpcc.3c02580

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Role of Paramagnetic Impurities and Surface Roughness on NMR Relaxation Times: Insights from Molecular Dynamics Simulations

You Wang,

Jorge Ivan Amaro-Estrada,

Carlos Torres-Verdín

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2024

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Cited by 2 publications

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Molecular Modes Elucidate the Nuclear Magnetic Resonance Relaxation of Viscous Fluids

Valiya Parambathu,

Pinheiro dos Santos,

Chapman

et al. 2024

J. Phys. Chem. B

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The Bloembergen, Purcell, and Pound (BPP) theory of nuclear magnetic resonance (NMR) relaxation in fluids dating back to 1948 continues to be the linchpin in interpreting NMR relaxation data in applications ranging from characterizing fluids in porous media to medical imaging (MRI). The BPP theory is founded on assuming molecules are hard spheres with 1H–1H dipole pairs reorienting randomly; assumptions that are severe in light of modern understanding of liquids. Nevertheless, it is intriguing to this day that the BPP theory was consistent with the original experimental data for glycerol, a hydrogen-bonding molecular fluid for which the hard-sphere-rigid-dipole assumption is inapplicable. To better understand this incongruity, atomistic molecular simulations are used to compute 1H NMR T 1 relaxation dispersion (i.e., frequency dependence) in two contrasting cases: glycerol, and a (non hydrogen-bonding) viscosity standard. At high viscosities, simulations predict distinct functional forms of T 1 for glycerol compared to the viscosity standard, in agreement with modern measurements, yet both in contrast to BPP theory. The cause of these departures from BPP theory is elucidated, without assuming any relaxation models and without any free parameters, by decomposing the simulated T 1 response into dynamic molecular modes for both intramolecular and intermolecular interactions. The decomposition into dynamic molecular modes provides an alternative framework to understand the physics of NMR relaxation for viscous fluids.

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Molecular Modes Elucidate the Nuclear Magnetic Resonance Relaxation of Viscous Fluids

Valiya Parambathu,

Pinheiro dos Santos,

Chapman

et al. 2024

J. Phys. Chem. B

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Theory and modeling of molecular modes in the NMR relaxation of fluids

Pinheiro dos Santos,

Orcan-Ekmekci,

Chapman

et al. 2024

The Journal of Chemical Physics

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Traditional theories of the nuclear magnetic resonance (NMR) autocorrelation function for intra-molecular dipole pairs assume a single-exponential decay, yet the calculated autocorrelation of realistic systems displays a rich, multi-exponential behavior, resulting in anomalous NMR relaxation dispersion (i.e., frequency dependence). We develop an approach to model and interpret the multi-exponential intra-molecular autocorrelation using simple, physical models within a rigorous statistical mechanical development that encompasses both rotational diffusion and translational diffusion in the same framework. We recast the problem of evaluating the autocorrelation in terms of averaging over a diffusion propagator whose evolution is described by a Fokker–Planck equation. The time-independent part admits an eigenfunction expansion, allowing us to write the propagator as a sum over modes. Each mode has a spatial part that depends on the specified eigenfunction and a temporal part that depends on the corresponding eigenvalue (i.e., correlation time) with a simple, exponential decay. The spatial part is a probability distribution of the dipole pair, analogous to the stationary states of a quantum harmonic oscillator. Drawing inspiration from the idea of inherent structures in liquids, we interpret each of the spatial contributions as a specific molecular mode. These modes can be used to model and predict the NMR dipole–dipole relaxation dispersion of fluids by incorporating phenomena on the molecular level. We validate our statistical mechanical description of the distribution in molecular modes with molecular dynamics simulations interpreted without any relaxation models or adjustable parameters: the most important poles in the Padé–Laplace transform of the simulated autocorrelation agree with the eigenvalues predicted by the theory.

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Role of Paramagnetic Impurities and Surface Roughness on NMR Relaxation Times: Insights from Molecular Dynamics Simulations

Cited by 2 publications

References 49 publications

Molecular Modes Elucidate the Nuclear Magnetic Resonance Relaxation of Viscous Fluids

Molecular Modes Elucidate the Nuclear Magnetic Resonance Relaxation of Viscous Fluids

Theory and modeling of molecular modes in the NMR relaxation of fluids

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