arXiv subjects

Xuefei Xu

Publications and source records attributed to Xuefei Xu.

4 recordsLinked to original sources

arXiv2024

Propensity of water self-ions at air(oil)-water interfaces revealed by deep potential molecular dynamics with enhanced sampling

The preference of water self-ions (hydronium and hydroxide) towards air/oil-water interfaces is one of the hottest topics in water research due to its importance for understanding properties, phenomena, and reactions of interfaces. In this work, we performed enhanced-sampling molecular dynamics simulations based on state-of-the-art neural network potentials with approximate M06-2X accuracy to investigate the propensity of hydronium and hydroxide ions at air/oil(decane)-water interfaces, which can simultaneously describe well the water autoionization process forming these ions, recombination of ions, and ionic distribution along the normal distance to the interface by employing a set of appropriate Voronoi collective variables. A stable ionic double-layer distribution is observed near the air-water interface, while the distribution is different at oil-water interfaces, where hydronium tends to be repelled from the interface into the bulk water, whereas hydroxide, with an interfacial stabilization free energy of -0.6 kcal/mol, is enriched in the interfacial layer. Through simulations of oil droplets in water, we further reveal that the interfacial propensity of hydroxide ions is caused by the positive charge distribution of the oil-water interface contributed by hydrogens of the dangling OH bonds of interfacial water layer and the outmost layer decane molecules laying flat on the droplet. The present results may aid in understanding the acid-base nature of water interfaces with wide applications.

physics.chem-ph↗

arXiv2024

Unimolecular Chemical Kinetics in the Interstellar: Competition of Infrared Radiation and Collision Activation Mechanisms

Unimolecular gas phase chemical reactions could be activated by both infrared (IR) radiation and inter-molecular collision in the interstellar environment. Understanding the interplay and competition between the radiation and collision activation mechanisms is crucial for assessing accurate reaction rate constants with an appropriate model. In this work, guided by an extended version of Lindemann theory that considers the contribution of both the radiation-activation and collision-activation to the rate constant of unimolecular reactions, we show that the relative importance of the two mechanisms can be measured by a dimensionless number $PR$ that is the ratio of the collision frequency to the radiation absorption rate of the molecule. The reaction kinetic is dominated by collision-activation or by radiation activation depending on whether $PR$ is larger or smaller than a reference value $PR^*$, which is determined to be $PR^* \approx 10$ based on magnitudes of molecular properties and is verified by detailed calculations of a number of typical interstellar unimolecular reactions. This method of evaluating the relative importance of the two mechanisms is checked against master equation calculations of two interstellar reactions: the dissociation reaction of silicilic acid around the asymptotic giant branch (AGB) star and the methyl association in Titan's atmosphere, and the validity is verified. The method can be used in the future to help determine the appropriate and effective modeling approach for chemical reactions in astrophysical environments.

astro-ph.GA↗

arXiv2024

High-Temperature Non-Equilibrium Atom-Diatom Collisional Energy Transfer

The change of the vibrational energy within a molecule after collisions with another molecule plays an essential role in the evolution of molecular internal energy distributions, which is also the limiting process in the relaxation of the gas towards equilibrium. Here we investigate the energy transfer between the translational motion and the vibrational motion of the diatom during the atom-diatom collision, the simplest case involving the transfer between inter-molecular and intra-molecular energies. We are interested in the situation when the translational temperature of the gas is high, in which case there are significant probabilities for the vibrational energy to change over widely separated energy levels after a collision. Data from quasi-classical trajectory simulations of the N+N$_2$ system with \textit{ab initio} potential energies suggest that the transition probability dependence on the collisional energy possesses an ``activation-saturation'' behavior and can be described by a simple model. The model allows for explicit evaluation of the vibrational state-to-state transition rate coefficients, from which the evolution of the vibrational energy distribution from any initial conditions can be solved by the master equation approach. An example of the vibrational energy relaxation in the N+N$_2$ system mimicking the gas behind strong shocks in a hypersonic flow is shown and the results are in good agreement with available data.

physics.chem-ph↗

arXiv2023

Intramolecular and water mediated tautomerism of solvated glycine

The understanding of prototropic tautomerism in water and the characterization of solvent effects on protomeric equilibrium pose significant challenges. Using molecular dynamics simulations based on state-of-the-art deep learning potential and enhanced sampling methods, we provide a comprehensive description of all configurational transformations in glycine solvated in water and determine accurate free energy profiles of these processes. We observe that the tautomerism between the neutral and zwitterionic forms of solvated glycine can occur by both intramolecular proton transfer in glycine and intermolecular proton transfer in the contact ion pair (anionic glycine and hydronium ion) or the separated ion pair (cationic glycine and hydroxide ion).

cond-mat.soft↗