Energy Redistribution following CO2 Formation on Cold Amorphous Solid Water

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO2 formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(1Σ+) + O(1D) recombination the average translational and internal energy of the water molecules increases on the ∼10 ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO2 and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid ∼10 ps and a considerably slower ∼1 ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the reaction products generated

Energy Redistribution following CO2 Formation on Cold Amorphous Solid Water

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO2 formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(1Σ+) + O(1D) recombination the average translational and internal energy of the water molecules increases on the ∼10 ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO2 and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid ∼10 ps and a considerably slower ∼1 ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the reaction products generated

Thermal and Vibrationally Activated Decomposition of the syn-CH3_3CHOO Criegee Intermediate

The full reaction pathway between the syn-CH$_3$CHOO Criegee Intermediate via vinyl hydroxyperoxide to OH+CH$_2$COH is followed for vibrationally excited and thermally prepared reactants. The rates from vibrational excitation are consistent with those found from experiments and tunneling is not required for reactivity at all initial conditions probed. For vibrationally excited reactant, VHP accumulates and becomes a bottleneck for the reaction. The two preparations - relevant for laboratory studies and conditions in the atmosphere - lead to a difference of close to one order of magnitude in OH production (~ 5 % vs. 35 %) on the 1 ns time scale which is an important determinant for the chemical evolution of the atmosphere.Comment: 40 pages , 16 figure

Molecular Simulation for Atmospheric Reaction Exploration and Discovery: Non-Equilibrium Dynamics, Roaming and Glycolaldehyde Formation Following Photo-Induced Decomposition of syn-Acetaldehyde Oxide

The decomposition and chemical dynamics for vibrationally excited syn-CH$_3$CHOO is followed based on statistically significant numbers of molecular dynamics simulations. Using a neural network-based reactive potential energy surface, transfer learned to the CASPT2 level of theory, the final total kinetic energy release and rotational state distributions of the OH fragment are in quantitative agreement with experiment. In particular the widths of these distributions are sensitive to the experimentally unknown strength of the O--O bond strength, for which values $D_e \in [22,25]$ kcal/mol are found. Due to the non-equilibrium nature of the process considered, the energy-dependent rates do not depend appreciably on the O--O scission energy. Roaming dynamics of the OH-photoproduct leads to formation of glycolaldehyde on the picosecond time scale with subsequent decomposition into CH$_2$OH+HCO. Atomistic simulations with global reactive machine-learned energy functions provide a viable route to quantitatively explore the chemistry and reaction dynamics for atmospheric reactions.Comment: 28+11pages, 7+14figure

Genesis of Polyatomic Molecules in Dark Clouds: CO2_2 Formation on Cold Amorphous Solid Water

Understanding the formation of molecules under conditions relevant to interstellar chemistry is fundamental to characterize the chemical evolution of the universe. Using reactive molecular dynamics simulations with model-based or high-quality potential energy surfaces provides a means to specifically and quantitatively probe individual reaction channels at a molecular level. The formation of CO$_2$ from collision of CO($^1 \Sigma$) and O($^1$D) is characterized on amorphous solid water (ASW) under conditions typical in cold molecular clouds. Recombination takes place on the sub-nanosecond time scale and internal energy redistribution leads to stabilization of the product with CO$_2$ remaining adsorbed on the ASW on extended time scales. Using a high-level, reproducing kernel-based potential energy surface for CO$_2$, formation into and stabilization of CO$_2$ and COO is observed.Comment: 33 pages, 13 figure

Quantum and quasi-classical dynamics of the C(3P) + O2(3Σ −g) → CO(1Σ+) + O(1D) reaction on its electronic ground state

The dynamics of the C((3)P) + O(2)((3)Σ(−)(g)) → CO((1)Σ(+)) + O((1)D) reaction on its electronic ground state is investigated by using time-dependent wave packet propagation (TDWP) and quasi-classical trajectory (QCT) simulations. For the moderate collision energies considered (E(c) = 0.001 to 0.4 eV, corresponding to a range from 10 K to 4600 K) the total reaction probabilities from the two different treatments of the nuclear dynamics agree very favourably. The undulations present in P(E) from the quantum mechanical treatment can be related to stabilization of the intermediate CO(2) complex with lifetimes on the 0.05 ps time scale. This is also confirmed from direct analysis of the TDWP simulations and QCT trajectories. Product diatom vibrational and rotational level resolved state-to-state reaction probabilities from TDWP and QCT simulations agree well except for the highest product vibrational states (v′ ≥ 15) and for the lowest product rotational states (j′ ≤ 10). Opening of the product vibrational level CO(v′ = 17) requires ∼0.2 eV from QCT and TDWP simulations with O(2)(j = 0) and decreases to 0.04 eV if all initial rotational states are included in the QCT analysis, compared with E(c) > 0.04 eV obtained from experiments. It is thus concluded that QCT simulations are suitable for investigating and realistically describe the C((3)P) + O(2)((3)Σ(−)(g)) → CO((1)Σ(+)) + O((1)D) reaction down to low collision energies when compared with results from a quantum mechanical treatment using TDWPs

Thermal Activation of Methane by MgO+: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling

The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simula- tions. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5 . 9 ± 1 . 5 × 10 − 10 ( T/ 300 K) − 0 . 5 ± 0 . 2 cm 3 s − 1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0 . 08 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate- limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels

Thermal and Vibrationally Activated Decomposition of the syn-CHCHOO Criegee Intermediate

The full reaction pathway between the syn-CH3CHOO Criegee Intermediate via vinyl hydroxyperoxide (VHP) to CH2COH+OH is followed for vibrationally excited and thermally prepared reactants. Reactivity along the entire pathway was characterized from an aggregate of more than 10 μs of reactive MD simulations using energy functions with accuracies at the Møller–Plesset second order level of theory. Reaction times for OH elimination are on the nanosecond time scale, and the energy dependence of the rates is consistent with experiments in the jet. The actual rates depend on the O–O dissociation energy (DeOO = 31.5 kcal/mol at the MP2/aug-cc-pVTZ level or DeOO = 23.5 kcal/mol closer to earlier CASPT2 calculations). Also, the initial preparation of the reactants influences both the VHP formation/OH elimination rates and the OH yields. For most conditions with initial vibrational excitation 80% or more of syn-CH3CHOO progress to OH elimination on the 5 ns time scale. However, for internally cold conformational ensembles generated at low temperature (50 K) only 20% to 30% of VHP eliminate OH on the 5 ns time scale which increases to 55% to 67% depending on excitation energy from simulations on the 15 ns time scale. For thermal preparation of syn-CH3CHOO, which is relevant in the atmosphere, 35% of the trajectories lead to OH-elimination within 1 ns. This compares with experimentally reported yields of 24% to 64% in a collisional environment. The estimated thermal rate at 300 K is 103 s–1, extrapolated from results at 500 to 900 K, is consistent with the experimentally measured rate of 182 ± 66 s–1

Quantitative molecular simulations

All-atom simulations can provide molecular-level insights into the dynamics of gas-phase, condensed-phase and surface processes. One important requirement is a sufficiently realistic and detailed description of the underlying intermolecular interactions. The present perspective provides an overview of the present status of quantitative atomistic simulations from colleagues' and our own efforts for gas- and solution-phase processes and for the dynamics on surfaces. Particular attention is paid to direct comparison with experiment. An outlook discusses present challenges and future extensions to bring such dynamics simulations even closer to reality