Hydrogen Delocalization Research Articles

Ultralow-Friction at Cryogenic Temperature Induced by Hydrogen Correlated Quantum Effect.

Temperature is one of the governing factors affecting friction of solids. Undesired high friction state has been generally reported at cryogenic temperatures due to the prohibition of thermally activated processes, following conventional Arrhenius equation. This has brought huge difficulties to lubrication at extremely low temperatures in industry. Here, the study uncovers a hydrogen-correlated sub-Arrhenius friction behavior in hydrogenated amorphous carbon (a-C:H) film at cryogenic temperatures, and a stable ultralow-friction over a wide temperature range (103-348 K) is achieved. This is attributed to hydrogen-transfer-induced mild structural ordering transformation, confirmed by machine-learning-based molecular dynamics simulations. The anomalous sub-Arrhenius temperature dependence of structural ordering transformation rate is well-described by a quantum mechanical tunneling (QMT) modified Arrhenius model, which is correlated with quantum delocalization of hydrogen in tribochemical reactions. This work reveals a hydrogen-correlated friction mechanism overcoming the Arrhenius temperature dependence and provides a new pathway for achieving ultralow friction under cryogenic conditions.

On the tunneling splitting in a cyclic water trimer

We propose an alternative explanation of the “bifurcation” splittings observed for the water trimer in the VRT experiments of Saykally’s group [Chem. Rev. 103 (2003) 2533]. In our interpretation, the splittings originate from the quantum delocalization of hydrogen bonded protons in the mean field potential between two oxygen neighbors. The pattern and the order of our calculated splittings is in the range of experimentally observed values. Consequently, quantum delocalization of protons should be considered seriously as the origin of experimentally observed fine splittings. The presented model can be extended to a water pentamer and, hopefully, advance our understanding of liquid water.

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.

Real-Time Observation of Formation and Relaxation Dynamics of NH4 in (CH3OH)m(NH3)n Clusters

The formation and relaxation dynamics of NH4(CH3OH)m(NH3)n clusters produced by photolysis of ammonia-methanol mixed clusters has been observed by a time-resolved pump-probe method with femtosecond pulse lasers. From the detailed analysis of the time evolutions of the protonated cluster ions, NH4(+)(CH3OH)m(NH3)n, the kinetic model has been constructed, which consists of sequential three-step reaction: ultrafast hydrogen-atom transfer producing the radical pair (NH4-NH2)*, the relaxation process of radical-pair clusters, and dissociation of the solvated NH4 clusters. The initial hydrogen transfer hardly occurs between ammonia and methanol, implying the unfavorable formation of radical pair, (CH3OH2-NH2)*. The remarkable dependence of the time constants in each step on the number and composition of solvents has been explained by the following factors: hydrogen delocalization within the clusters, the internal conversion of the excited-state radical pair, and the stabilization of NH4 by solvation. The dependence of the time profiles on the probe wavelength is attributed to the different ionization efficiency of the NH4(CH3OH)m(NH3)n clusters.

Binary hydrogen‐tetrahydrofuran clathrate hydrate formation kinetics and models

AbstractBinary H2‐THF clathrate hydrate formation kinetics were investigated with a pressure decay method at temperatures from 266.7 to 275.1 K, at initial pressures from 3.6 to 8.4 MPa, and at stoichiometric THF hydrate concentrations for particle sizes between 212 and 1,400 μm. Formation rate increased for smaller particle sizes, higher pressures and lower‐temperatures. A hydrogen delocalization model and a proposed hydrogen hydrate phase diffusion (HHPD) model were used to analyze the formation mechanisms. The HHPD model assumes that the H2‐THF hydrate phase is formed due to hydrogen adsorption onto the particle surface that is followed by subsequent diffusion of hydrogen into the clathrate hydrate. The HHPD model could express the kinetics quantitatively at the experimental conditions studied. Values of the hydrogen diffusion coefficient in the clathrate hydrate estimated from the bulk data and the phase thickness in the HHPD model agreed well with the literature. © 2008 American Institute of Chemical Engineers AIChE J, 2008

A theoretical study of hydrogen adsorption and diffusion on a W(1 1 0) surface

We have used density functional theory to investigate hydrogen adsorption and diffusion on a W(1 1 0) surface. Hydrogen adsorption structures were examined from low coverage to one monolayer, and a threefold hollow site was found to be the most stable site at all coverages. In contrast to previous assertions, the work function decrease is not due to electron transfer from the hydrogen atoms to the W surface, but due to electron depletion at the vacuum region above the hydrogen atoms. Hydrogen atoms can diffuse via short-bridge sites and long-bridge sites at a coverage of θ = 1.0. Although the calculated activation energy for hydrogen diffusion via a short-bridge site is as small as 0.05 eV, field emission microscope experiments have shown that the activation energy for hydrogen diffusion is about 0.20 eV, which agrees fairly well with our calculated value of the activation energy via a long-bridge site. This discrepancy can be related to hydrogen delocalization on the W(1 1 0) surface, which has been suggested by electron energy loss spectroscopy experiments.

Impedance study near the ferroelastic transition in TlH2PO4

The high temperature phase in TlH2PO4, a hydrogen-bonded antiferroelectric/ferroelastic material, was investigated by ac impedance measurements. A Warburg diffusion was manifested in the low-frequency region in the Cole-Cole plots near the ferroelastic transition temperature Tc′=357K, indicating a surface instability. The temperature dependent impedance, analyzed by a simple equivalent circuit model, was discussed in view of hydrogen delocalization on the surface.

Quantum Delocalization of Hydrogen in the Li2NH Crystal

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Quantum Delocalization of Hydrogen in the Li2NH Crystal

By mapping out potential energy surfaces from density-functional theory (DFT) and solving a protonic Schrödinger equation, we find that the H atom in a unit cell of the Li2NH crystal shows remarkably strong quantum behavior, leading to the delocalization of H over six octahedral sites around each N. This can be rationalized in terms of rapid coherent tunneling among these equivalent octahedral sites. Structural and dynamical consequences of our finding are discussed. Since the Li-N-H compounds are considered promising candidates for H-storage, understanding of these fundamental properties will be useful toward improving the performance of the material.

Hydrogen Delocalization in Cyclic Water Clusters

An alternative model of the dynamics in the low energy states of the water trimer is presented. In this model the dynamically averaged pseudorotational levels reflect the D3h molecular symmetry group. In contrast to the currently accepted C3h model, in the present model transitions through much higher energy barriers, such as clockwise to counterclockwise rearrangements, are allowed. This leads to a significantly different picture of the trimer dynamics. This paper identifies a difference between the two models which can be tested experimentally and will thereby facilitate the selection of the model that is closer to nature.

Ground state and excited state hydrogen tunnelling in Nb 1− xTi xH y

Ti represents a substitutional dopant in Nb and at low temperatures acts as hydrogen trap. For Nb 0.999Ti 0.001H 0.001 at 1.5 K we observe a pronounced (ground state) tunnelling peak at 0.2 meV and a second weak peak at 0.4 meV in high resolution neutron spectra. The vibrationally excited state has been studied by neutron energy loss spectroscopy on Nb 0.99Ti 0.01H 0.0094; the (fundamental) vibrational peak around 110 meV exhibits a splitting into three tunnelling lines seperated by about 4 meV. Both tunnelling phenomena indicate hydrogen delocalization over four (or perhaps six) sites. This conclusion is supported by muon spin relaxation measurements between 10 and 200 K and a QENS study at 150 K, where in both cases a localized motion (jump rotation) of the light interstitials over four sites was found.

Low-temperature delocalization of hydrogen in crystalline silicon

The hydrogen delocalization in crystalline silicon, a property related to the quantum nature of the proton, is studied by path-integral Monte Carlo simulations in the temperature range from 50 to 300 K. The SiH interaction potential was fitted to results of earlier pseudopotential-density-functional calculations. The lowest-energy position for hydrogen corresponds to the bond-center (BC) site, with an average distance of the proton to the SiSi axis higher than 0.15 Å. At T = 50 K, the proton delocalization around the BC site results to be (Δ r p) 2 = 4.4 × 10 −2 A ̊ 2 . Anharmonicities of the interatomic potential contribute to increase this delocalization with respect to a one-particle harmonic approximation.

Hydrogen bonding: the role of hydrogen delocalization in the AHB bond

The behavior of the A-H...B multiple minimum hydrogen bond can be understood in terms of an analysis similar to the molecular orbital method in molecular electronic quantum theory. Delocalization of hydrogen about two or more wells is described in terms of a linear combination of Gaussian orbitals, each referred to a well minimum. The analysis yields an effective A-B potential mediated by hydrogen. The role of a model three-center interaction in the potential is found to be important and can account for much of the strength of the hydrogen bond

Existence of superdiffusion of hydrogen in vanadium.

Using an electromigration-relaxation method we determined the diffusion coefficient D of interstitial hydrogen in homogeneously strained monocrystalline and polycrystalline vanadium samples. In sharp contrast to Suzuki et al., who claimed to have observed superdiffusion due to stress-induced delocalization of hydrogen, we found that within experimental error D remains constant for uniaxial stresses in the [111] direction up to 115 MPa. The ``superdiffusion'' of Suzuki et al. is probably due to an inadequate analysis of their experimental data.