3 research outputs found
Ring-Polymer Molecular Dynamics Rate Coefficient Calculations for Insertion Reactions: X + H<sub>2</sub> → HX + H (X = N, O)
The thermal rate constants of two prototypical insertion-type reactions, namely, N/O + H<sub>2</sub> → NH/OH + H, are investigated with ring polymer molecular dynamics (RPMD) on full-dimensional potential energy surfaces using recently developed RPMDrate code. It is shown that the unique ability of the RPMD approach among the existing theoretical methods to capture the quantum effects, e.g., tunneling and zero-point energy, as well as recrossing dynamics quantum mechanically with ring-polymer trajectories leads to excellent agreement with rigorous quantum dynamics calculations. The present result is encouraging for future applications of the RPMD method and the RPMDrate code to complex-forming chemical reactions involving polyatomic reactants
Ring-Polymer Molecular Dynamics for the Prediction of Low-Temperature Rates: An Investigation of the C(<sup>1</sup>D) + H<sub>2</sub> Reaction
Quantum mechanical calculations are
important tools for predicting
the rates of elementary reactions, particularly for those involving
hydrogen and at low temperatures where quantum effects become increasingly
important. These approaches are computationally expensive, however,
particularly when applied to complex polyatomic systems or processes
characterized by deep potential wells. While several approximate techniques
exist, many of these have issues with reliability. The ring-polymer
molecular dynamics method was recently proposed as an accurate and
efficient alternative. Here, we test this technique at low temperatures
(300–50 K) by analyzing the behavior of the barrierless C(<sup>1</sup>D) + H<sub>2</sub> reaction over the two lowest singlet potential
energy surfaces. To validate the theory, rate coefficients were measured
using a supersonic flow reactor down to 50 K. The experimental and
theoretical rates are in excellent agreement, supporting the future
application of this method for determining the kinetics and dynamics
of a wide range of low-temperature reactions
Unimolecular Reaction Pathways of a γ‑Ketohydroperoxide from Combined Application of Automated Reaction Discovery Methods
Ketohydroperoxides
are important in liquid-phase autoxidation and
in gas-phase partial oxidation and pre-ignition chemistry, but because
of their low concentration, instability, and various analytical chemistry
limitations, it has been challenging to experimentally determine their
reactivity, and only a few pathways are known. In the present work,
75 elementary-step unimolecular reactions of the simplest γ-ketohydroperoxide,
3-hydroperoxypropanal, were discovered by a combination of density
functional theory with several automated transition-state search algorithms:
the Berny algorithm coupled with the freezing string method, single-
and double-ended growing string methods, the heuristic KinBot algorithm,
and the single-component artificial force induced reaction method
(SC-AFIR). The present joint approach significantly outperforms previous
manual and automated transition-state searches – 68 of the
reactions of γ-ketohydroperoxide discovered here were previously
unknown and completely unexpected. All of the methods found the lowest-energy
transition state, which corresponds to the first step of the Korcek
mechanism, but each algorithm except for SC-AFIR detected several
reactions not found by any of the other methods. We show that the
low-barrier chemical reactions involve promising new chemistry that
may be relevant in atmospheric and combustion systems. Our study highlights
the complexity of chemical space exploration and the advantage of
combined application of several approaches. Overall, the present work
demonstrates both the power and the weaknesses of existing fully automated
approaches for reaction discovery which suggest possible directions
for further method development and assessment in order to enable reliable
discovery of all important reactions of any specified reactant(s)