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Reactive Molecular Dynamics Simulations and Quantum Chemistry Calculations To Investigate Soot-Relevant Reaction Pathways for Hexylamine Isomers

Summary

Recent yield sooting index (YSI) experiments for nitrogen-containing hydrocarbons provide a quantitative ordering of particulate tendency in doped methane/air diffusion flames, but detailed elementary pathways for larger amines are often incomplete in kinetic models. Building on those YSI measurements, this work studies three C6H15N isomeric amines—dipropylamine (DPA), diisopropylamine (DIPA), and 3,3-dimethylbutylamine (DMBA)—with ReaxFF MD at 1400, 1600, and 1800 K (temperatures where soot formation is identified in the YSI experiment) plus quantum-mechanical refinement of reaction networks. The reactive trajectories use a methane-rich radical-pool framework in which test fuel molecules interact with radicals and intermediates generated from rich methane combustion, as described in the article, so the chemistry is explicitly embedded in a co-combustion context rather than isolated gas-phase pyrolysis of a single amine. ReaxFF ranks reactivity DIPA > DPA > DMBA across those temperatures. Major nonaromatic soot precursors differ by isomer (C2H4-, C3H6-, and C4H8-type product families in the authors’ classification); combined ReaxFF and QM sooting tendency order matches experimental YSI trends, and the authors highlight agreement between the two theoretical methods on reactivity, major intermediates, and major nonaromatic precursors.

Methods

1 — MD application (ReaxFF). ReaxFF NVT molecular dynamics in LAMMPS on C\(_6\)H\(_{15}\)N isomers (dipropylamine DPA, diisopropylamine DIPA, 3,3-dimethylbutylamine DMBA) at 1400, 1600, and 1800 K within a methane-rich radical-pool setup (species definitions and supercell construction in the article). NVT uses a thermostat to hold each target temperature; N/A — NPT / barostat in the main production MD; N/A — electric field; N/A — umbrella or metadynamics (standard ReaxFF MD). PBC; femtosecond timestep; ns-scale or long ps production per article. Hydrostatic GPa pressure: N/A in typical NVT gas-like boxes.

3 — Static QM (DFT). DFT refines reaction pathways and barriers/energetics for key intermediates after ReaxFF screening (functional, basis, and NEB/path details per paper).

FF training — N/A — application paper using an existing ReaxFF training for hydrocarbon/amine–type chemistry, not a new public parameter fit documented as the main result here.

Findings

ReaxFF ranks relative reactivity as DIPA > DPA > DMBA across the temperatures sampled. Major nonaromatic soot precursors differ by isomer (families involving C\(_2\)H\(_4\), C\(_3\)H\(_6\), C\(_4\)H\(_8\)-type products in the authors’ classification). Combined ReaxFF and QM trends for sooting propensity align with the YSI ordering reported experimentally for these amines.

Limitations

Isomer subset only; nitrogen chemistry under real flame conditions is richer than the modeled gas-phase subset. Exact barriers and reaction graphs are in the JCP text and SI; this page paraphrases the publication only.

Relevance to group

Penn State combustion collaboration tying ReaxFF to biomimetic/bio-derived fuel nitrogen chemistry and YSI metrics.

Citations and evidence anchors

papers/Kwon_JPC_2020_Soot_growth.pdf — abstract (fuels, temperatures, reactivity order, precursor species). https://doi.org/10.1021/acs.jpca.0c03355

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