Temperature Dependence of NMR Parameters Calculated from Path Integral Molecular Dynamics Simulations

The influence of temperature on NMR chemical shifts and quadrupolar couplings in model molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory (DFT) calculations of shielding and electric field gradient (EFG) tensors. An approach based on convoluting calculated shielding or EFG tensor components with probability distributions of selected bond distances and valence angles obtained from DFT-PIMD simulations at several temperatures is used to calculate the temperature effects. The probability distributions obtained from the quantum PIMD simulations, which includes nuclear quantum effects, are significantly broader and less temperature dependent than those obtained with conventional DFT molecular dynamics or with 1D scans through the potential energy surface. Predicted NMR observables for the model systems were in excellent agreement with experimental data.

Anomalous temperature dependence of nuclear quadrupole interactions in strongly hydrogen-bonded systems from first principles

We present an analysis of the effect of finite temperatures on the deuteron nuclear quadrupole coupling constants in a strongly hydrogen-bonded molecular crystal by means of first-principles Car-Parrinello molecular-dynamics simulations. Our findings agree well with experiments and provide a microscopic explanation of the anomalous increase of the quadrupole coupling in this class of systems. We show that a simple model based on the anharmonicity of the hydrogen bond potential fails to describe the temperature dependence of the couplings even qualitatively. Instead, the inclusion of fluctuations and disorder in terms of atomic motion of the surrounding molecules turns out to be important to obtain the correct magnitude of the temperature effect.