Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

ab-Initio Study of Hydrogen Bond Networks in 1,2,3-Triazole Phases

The research in storage and conversion of energy is an everlasting process. The use of fuel cells is very tempting but up to now there are still several conceptual challenges to overcome. Especially, the requirement of liquid water causes difficulties due to the temperature limit. Therefore, imidazoles and triazoles are increasingly investigated in a manifold of experimental and theoretical publications as they are both very promising in overcoming this problem. Recently, triazoles were found to be superior to imidazoles in proton conduction. An ab-initio molecular dynamics simulation of pure triazole phases for investigating the behavior of both tautomer species of the triazole molecule has never been done. In this work, we investigate the structural and dynamical properties of two different solid phases and the liquid phase at two different temperatures. We are able to show how the distinct tautomers contribute to the mechanism of proton conduction, to compute dynamical properties of the four systems and to suggest a mechanism of reorientation in solid phase.

Quantum fluctuations of a fullerene cage modulate its internal magnetic environment

To investigate the effect of quantum fluctuations on the magnetic environment inside a C₆₀ fullerene cage, we have calculated the nuclear magnetic shielding constant of protons in H₂@C₆₀ and HD@C₆₀ systems by on-the-fly ab initio path integral simulation, including both thermal and nuclear quantum effects. The most dominant upfield from an isolated hydrogen molecule occurs due to the diamagnetic current of the C₆₀ cage, which is partly cancelled by the paramagnetic current, where the paramagnetic contribution is enlarged by the zero-point vibrational fluctuation of the C₆₀ carbon backbone structure via a widely distributed HOMO-LUMO gap. This quantum modulation mechanism of the nuclear magnetic shielding constant is newly proposed. Because this quantum effect is independent of the difference between H₂ and HD, the H₂/HD isotope shift occurs in spite of the C₆₀ cage. The nuclear magnetic constants computed for H₂@C₆₀ and HD@C₆₀ are 32.047 and 32.081 ppm, respectively, which are in reasonable agreement with the corresponding values of 32.19 and 32.23 ppm estimated from the experimental values of the chemical shifts.

Influence of Intramolecular Charge Transfer and Nuclear Quantum Effects on Intramolecular Hydrogen Bonds in Azopyrimidines

Intramolecular hydrogen bonds (IMHBs) in 5-azopyrimidines are investigated by NMR spectroscopy and DFT computations that involve nuclear quantum effects. A series of substituted 5-phenylazopyrimidines with one or two hydrogen bond donors able to form IMHBs with the azo group is prepared by azo coupling. The barrier of interconversion between two rotamers of the compounds with two possible IMHBs is determined by variable temperature NMR spectroscopy and it is demonstrated that the barrier is significantly affected by intramolecular charge transfer. Through-hydrogen-bond scalar coupling is investigated in ¹⁵N labeled compounds and the stability of the IMHBs is correlated with experimental NMR parameters and rationalized by path integral molecular dynamics simulations that involve nuclear quantum effects. Detailed information on the hydrogen bond geometry upon hydrogen-to-deuterium isotope exchange is obtained from a comparison of experimental and calculated NMR data.

“On-the-fly” coupled cluster path-integral molecular dynamics: impact of nuclear quantum effects on the protonated water dimer

“On-the-fly” coupled cluster-based path-integral molecular dynamics simulations predict that the effective potential of the protonated water–dimer has a single-well only.

Effects of quantum nuclear delocalisation on NMR parameters from path integral molecular dynamics

The influence of nuclear delocalisation on NMR chemical shifts in molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory calculations of shielding tensors. Nuclear quantum effects are shown to explain previously observed systematic deviations in correlations between calculated and experimental chemical shifts, with particularly large PIMD-induced changes (up to 23 ppm) observed for carbon atoms in methyl groups. The PIMD approach also enables isotope substitution effects on chemical shifts and J couplings to be predicted in excellent agreement with experiment for both isolated molecules and molecular crystals. An approach based on convoluting calculated shielding or coupling surfaces with probability distributions of selected bond distances and valence angles obtained from PIMD simulations is used to calculate isotope effects.

Spectroscopic fingerprints of toroidal nuclear quantum delocalization via ab initio path integral simulations

We investigate the quantum-mechanical delocalization of hydrogen in rotational symmetric molecular systems. To this purpose, we perform ab initio path integral molecular dynamics simulations of a methanol molecule to characterize the quantum properties of hydrogen atoms in a representative system by means of their real-space and momentum-space densities. In particular, we compute the spherically averaged momentum distribution n(k) and the pseudoangular momentum distribution n(kθ). We interpret our results by comparing them to path integral samplings of a bare proton in an ideal torus potential. We find that the hydroxyl hydrogen exhibits a toroidal delocalization, which leads to characteristic fingerprints in the line shapes of the momentum distributions. We can describe these specific spectroscopic patterns quantitatively and compute their onset as a function of temperature and potential energy landscape. The delocalization patterns in the projected momentum distribution provide a promising computational tool to address the intriguing phenomenon of quantum delocalization in condensed matter and its spectroscopic characterization. As the momentum distribution n(k) is also accessible through Nuclear Compton Scattering experiments, our results will help to interpret and understand future measurements more thoroughly.