Normal Hydrogen Atoms Research Articles

Thermal transport across TiO2-H2O interface involving water dissociation: Ab initio-assisted deep potential molecular dynamics.

Water dissociation on TiO2 surfaces has been known for decades and holds great potential in various applications, many of which require a proper understanding of thermal transport across the TiO2-H2O interface. Molecular dynamics (MD) simulations play an important role in characterizing complex systems' interfacial thermal transport properties. Nevertheless, due to the imprecision of empirical force field potentials, the interfacial thermal transport mechanism involving water dissociation remains to be determined. To cope with this, a deep potential (DP) model is formulated through the utilization of abinitio datasets. This model successfully simulates interfacial thermal transport accompanied by water dissociation on the TiO2 surfaces. The trained DP achieves a total energy accuracy of ∼238.8meV and a force accuracy of ∼197.05 meV/Å. The DPMD simulations show that water dissociation induces the formation of hydrogen bonding networks and molecular bridges. Structural modifications further affect interfacial thermal transport. The interfacial thermal conductance estimated by DP is ∼8.54 × 109 W/m2K, smaller than ∼13.17 × 109 W/m2K by empirical potentials. The vibrational density of states (VDOS) quantifies the differences between the DP model and empirical potentials. Notably, the VDOS disparity between the adsorbed hydrogen atoms and normal hydrogen atoms demonstrates the influence of water dissociation on heat transfer processes. This work aims to understand the effect of water dissociation on thermal transport at the TiO2-H2O interface. The findings will provide valuable guidance for the thermal management of photocatalytic devices.

CATALYTIC MECHANISM OF ALL-TRANS-RETINOIC ACID 4-HYDROXYLATION MEDIATED BY CYTOCHROME P450 2C8: HOW DOES ARGININE 241 AFFECT THEC–HBOND ACTIVATION?

Experiments revealed that cytochrome P450 2C8 enzyme (CYP2C8) has two distinct substrate binding sites to the physiologically important molecules, retinoic acids, and the main difference between these two binding sites is whether there is a salt bridge interaction between the anionic carboxylate tail of retinoic acids and the surrounding protein environment. However, the influence of such salt bridge interaction toward catalysis is still elusive. In the present paper, density functional theory (DFT) calculations were employed to research the reaction mechanism of all-trans-retinoic acid (atRA) 4-hydroxylation mediated by CYP2C8. Our DFT calculations revealed that such salt bridge interaction has obvious effects on the reaction mechanism of atRA 4-hydroxylation. In the binding site containing a salt bridge interaction between the anionic carboxylate tail of atRA and the cationic guanidine group of Arg241, C – H bond activation proceeds via a normal hydrogen atom transfer (HAT) mechanism; in the other site without this salt bridge interaction, however, C – H bond activation is achieved via a stepwise electron transfer and hydrogen atom transfer, thus, a novel ET/HAT mechanism. These findings enrich the mechanism patterns of C – H bond activation catalyzed by metalloenzymes and their biomimetics. Meanwhile, the self-interaction error (SIE) problem encountered during our calculations in vacuum was affected and removed by the inclusion of an external electric field in the calculations.

Blended hydrogen atom abstraction and proton-coupled electron transfer mechanisms of closed-shell molecules

The paper addresses the surging topic of H-abstractions by closed-shell molecules, such as MnO4−, α-methylstyrene, ketones, metal-oxo reagents, etc. It is found that in the normal hydrogen atom transfer (HAT) regime, closed-shell abstractors require high barriers for H-abstraction. Under certain conditions a closed-shell abstractor can bypass this penalty via a proton-coupled electron transfer (PCET) mechanism. This occurs mainly in the identity reactions, e.g. MnO4− abstracting a hydrogen atom from MnO4H−·, but not in the corresponding non-identity reactions with alkanes. The usage of the valence bond (VB) diagram model allows us to characterize the HAT/PCET mechanistic relationship and bridge their reactivity patterns. It is thus shown that in the normal HAT regime, high barriers for closed-shell abstractors occur due to the additional promotion energy that is required in order to create a radical center and “prepare” the abstractor for H-abstraction. Mixing of the PCET states into the HAT states mitigates however these high barriers. The variable HAT/PCET mixing in a reaction series is discussed and its consequences for reactivity are outlined. It is shown that non-identity reactions sample PCET characters that depend, among other factors, on the C–H bond strength of the alkane, and hence may cause the Marcus analysis to produce different identity barriers for the same identity reaction.

Radiative collisional quenching of metastable muonic hydrogenpμ−(2s)and the metastable muonic helium ionαμ−(2s)

Fully quantum-mechanical calculations have been performed to determine the radiative collisional cross sections for quenching of metastable muonic hydrogen $p{\ensuremath{\mu}}^{\ensuremath{-}}(2s)$ by normal hydrogen atoms and for quenching of the metastable muonic helium ion $\ensuremath{\alpha}{\ensuremath{\mu}}^{\ensuremath{-}}(2s)$ by normal helium atoms. The interatomic potential curves and the radiative transition rates for the mixed electronic-muonic systems are calculated using molecular-structure methods. The nuclear motion is treated as adiabatic and the interaction is written in terms of a complex potential. The cross sections are evaluated by direct numerical integration of the Schr\"odinger equation. The two-body thermal (300 K) quenching rate constants obtained are 3.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ ${\mathrm{cm}}^{3}$/sec for $p{\ensuremath{\mu}}^{\ensuremath{-}}$ and 6.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{3}$/sec for $\ensuremath{\alpha}{\ensuremath{\mu}}^{\ensuremath{-}}$.

The long-range interaction between hydrogen atoms

The long-range interaction energy between two normal hydrogen atoms has been calculated directly, without recourse to laborious variational methods. The result E2(R) = -12 998R-6-248.80R-8-6571R-10-... confirms that of Chan and Dalgarno as opposed to that of Hirschfelder and Löwdin.

Impact parameter treatments of certain hydrogen-proton and hydrogen-hydrogen excitation collisions

Consideration is given to the impact parameter form of the Born approximation and of the pRA approximation (i.e. the first-order approximation to which the perturbed stationary state, or PSS, approximation tends). Calculations are carried out on the excitation of normal hydrogen atoms to the 2 s or 2 p states in encounters with protons and other normal hydrogen atoms. The results obtained provide some information on the range of validity of the Born approximation. The impact-parameter treatment corresponding to a simplified version of the second Born approximation is given and discussed. Attention is drawn to the fact that for certain transitions the pRA approximation fails seriously when the encounter is very close.

The Excitation of the Discrete Levels of Atomic Hydrogen by Fast Electrons

The cross sections for the excitation of normal hydrogen atoms to the fifth and higher levels have been obtained by interpolation from the Born cross sections for the excitation to the second, third and fourth levels, and for ejection into the continuum. The cross section describing the excitation of all levels and the energy loss sustained by the incident electron are hence derived.

The coefficients of absorption and opacity of a partially degenerate gas

1. The absorption of radiation in a highly degenerate gas has been investigated by Chandrasekhar, Majumdar, and the present writer. Chandrasekhar has also considered very fully the absorption in a classical gas. It appears that the absorption in a highly degenerate gas is much less than that which would be calculated for a classical gas at the same density and temperature. The present paper is concerned with the absorption in a gas for which the degeneracy, as measured by Sommerfeld’s parameter, is small. As in a stellar configuration with a degenerate-gas core there must be a transition zone, where the gas is only slightly degenerate, it may be of value for astrophysical purposes to know the behaviour of the absorption coefficient for incipient degeneracy. In 2 the coefficients of absorption and opacity due to “free-free” transitions are evaluated; in 3 and 4 the contributions to the atomic absorption coefficient due to “bound-free” transitions from the lower energy levels are calculated. In 5 the absorption coefficient per normal hydrogen atom is found and the mass absorption coefficient deduced, and in 6 the extrapolation for other atoms is discussed.

The Continuous Electron Affinity Spectrum of Hydrogen

A wave-mechanical calculation has been made of the intensity distribution in the continuous emission spectrum due to the capture of electrons by normal hydrogen atoms and the coefficient of absorption in the corresponding absorption spectrum due to the neutralization of negative hydrogen ions. Approximate wave functions for the continuous range of upper energy levels have been combined with the discrete state wave function for ${\mathrm{H}}^{\ensuremath{-}}$ given by Hylleraas to give the matrix components for the desired transitions. The computed results are graphically illustrated and their significance discussed. It is shown that an experimental attempt to find the electron affinity emission spectrum of normal hydrogen atoms may encounter at least two difficulties: (1) insufficient intensity compared to that in experiments on the recombination between ${\mathrm{H}}^{+}$ ions and electrons, and (2) spectral distribution such that the spectral limit is too diffuse for positive identification. The relation of these results to the interpretation of experiments on the electron affinity spectrum of halogen atoms is discussed.

Absorption Bands of Hydrogen Cyanide Gas in the Near Infrared

The absorption spectrum of gaseous hydrogen cyanide has been investigated by photographic methods in the region $\ensuremath{\lambda}7000\ensuremath{-}9200$. Two weak bands of very simple structure were found, having $P$ and $R$ branches but no $Q$ branches. The band at $\ensuremath{\lambda}7912$ is apparently a harmonic of a fundamental band at 3.04\ensuremath{\mu}, and the very weak band at $\ensuremath{\lambda}8563$ is a combination band. The hydrogen cyanide molecule is linear in the normal state, and has a moment of inertia $I=18.79\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}40}$ g\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{cm}}^{2}$. The distance of separation of the carbon and nitrogen atoms is estimated to be 1.15\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ cm. Hydrogen cyanide is discussed in regard to its three fundamental oscillations which have frequencies 3290, 2090, and 710, respectively, and in regard to its dissociation energy and dissociation products. The evidence requires a molecular structure represented by the formula HCN, and shows that the normal molecule is built from a normal hydrogen atom and a normal CN radical. The absorption of cyanogen gas has also been investigated in the photographic infrared, but no absorption bands could be detected.