The thermal decomposition of syn-ethanal-oxide (syn-CHCHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CHCHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients. This work suggests that, under boundary layer conditions in the atmosphere, thermal unimolecular decomposition is the most important sink of syn-CHCHOO. Thus, the title reaction should be included into atmospheric modeling. The fate of cold VHP, the intermediate stabilized by collisions with a third body, has also been investigated.
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http://dx.doi.org/10.1063/1.4964393 | DOI Listing |
J Phys Chem A
December 2024
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.
Radical-radical reaction channels are important in the pyrolysis and oxidation chemistry of perfluoroalkyl substances (PFAS). In particular, unimolecular dissociation reactions within unbranched -perfluoroalkyl chains, and their corresponding reverse barrierless association reactions, are expected to be significant contributors to the gas-phase thermal decomposition of families of species such as perfluorinated carboxylic acids and perfluorinated sulfonic acids. Unfortunately, experimental data for these reactions are scarce and uncertain.
View Article and Find Full Text PDFProc Natl Acad Sci U S A
December 2024
Department of Chemistry, University of California, Berkeley, CA 94720.
Polycyclic aromatic hydrocarbons (PAHs) play a major role in the chemistry of combustion, pyrolysis, and the interstellar medium. Production (or activation) of radical PAHs and propagation of their resulting reactions require efficient dehydrogenation, but the preferred method of hydrogen loss is not well understood. Unimolecular hydrogen ejection (i.
View Article and Find Full Text PDFJ Phys Chem A
November 2024
Quantum Theory Project, Departments of Chemistry and Physics, University of Florida, Gainesville, Florida 32611, United States.
The thermal unimolecular decay of ethoxy is important in high-temperature combustion environments where the ethoxy radical is a key reactive intermediate. Two dissociation pathways of ethoxy, including the β-C-C scission to yield CH + CHO and the H-elimination to make H + CHCHO, were characterized using a high-level coupled-cluster-based composite quantum chemical method (mHEAT-345(Q)). The former route is found to be dominant while the latter is insignificant, in agreement with previous experimental and theoretical studies.
View Article and Find Full Text PDFPhys Chem Chem Phys
November 2024
Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), 00014 Helsinki, Finland.
We have performed direct kinetic measurements to determine the thermal unimolecular-decay rate coefficient of (CH)COO as a function of temperature (223-296 K) and pressure (4-100 torr) using time-resolved UV-absorption spectroscopy. The stabilised (CH)COO Criegee intermediate was produced by photolysing 3-bromo-3-iodopentane ((CH)CIBr) with 213 nm radiation in the presence of O. We performed quantum-chemistry calculations and master-equation simulations to complement the experimental work.
View Article and Find Full Text PDFJ Phys Chem A
October 2024
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States.
A transient carbon-centered hydroperoxyalkyl intermediate (•QOOH) in the oxidation of cyclopentane is identified by IR action spectroscopy with time-resolved unimolecular decay to hydroxyl (OH) radical products that are detected by UV laser-induced fluorescence. Two nearly degenerate •QOOH isomers, β- and γ-QOOH, are generated by H atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental and first overtone OH stretch transitions and combination bands of •QOOH are observed and compared with anharmonic frequencies computed by second-order vibrational perturbation theory.
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