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Cyclic ethers undergo H-abstraction reactions that yield carbon-centered radicals (Ṙ). The ether functional group introduces a competing set of reaction pathways: ring-opening and reaction with O2 to form peroxy radical adducts, ROȮ, which can result in stereoisomers. ROȮ derived from cyclic ethers can subsequently isomerize into hydroperoxy-substituted carbon-centered radicals, Q̇OOH, which can also undergo ring-opening reactions or pathways prototypical to alkyl oxidation. The balance of reactions that unfold from cyclic ether radicals depends intrinsically on the size of the ring and the structure of any substituents retained in the formation step. The present work examines unimolecular reactions of peroxy radicals from 2-ethyloxetane, a four-membered cyclic ether formed during n-pentane oxidation, and reveals stereoisomer-specific reaction pathways. Automated quantum chemical computations were conducted on constitutional and stereoisomers of ROȮ derived from O2-addition to 2-ethyloxetanyl radicals. Pressure-dependent rate calculations were conducted by solving the master equation from 300 – 1000 K and from 0.01 – 100 atm. Branching fractions were then calculated at 650 K and 825 K, the peak temperatures at which cyclic ethers form in alkane oxidation. Isomer-specific reaction pathways of anti-ROO and syn-ROO and resulting impact on radical production were evident. Q̇OOH ring-opening reactions were significant as were rates of bi-cyclic ether formation common in alkyl radical oxidation. Detailed prescription of rates and reaction mechanisms describing cyclic ether consumption mechanisms are important to enable accurate modeling of reactions of ephemeral Q̇OOH radicals because of the direct, isomer-specific formation pathways. In addition, detailed cyclic ether mechanisms are required to reduce mechanism truncation error. The results herein provide insight on connections between cyclic ethers and chain-reaction pathways yielding ȮH, HOȮ, and other radicals, in addition to pathways leading to performic acid (HOOC(=O)H), the decomposition of which via O–O scission results in an exothermic, chain-branching step.
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Acid Gas Capture by Nitrogen Heterocycle Ring Expansion
Acid gases including CO2, OCS, CS2, and SO2 are emitted by industrial processes such as natural gas production or power plants, leading to the formation of acid rain and contributing to global warming as greenhouse gases. An important technological challenge is to capture acid gases and transform them into useful products. The capture of CO2, CS2, SO2, and OCS by ring expansion of saturated and unsaturated substituted nitrogen-strained ring heterocycles was computationally investigated at the G3(MP2) level. The effects of fluorine, methyl, and phenyl substituents on N and/or C were explored. The reactions for the capture CO2, CS2, SO2, and OCS by 3- and 4-membered N-heterocycles are exothermic, whereas ring expansion reactions with 5-membered rings are thermodynamically unfavorable. Incorporation of an OCS into the ring leads to the amide product being thermodynamically favored over the thioamide. CS2 and OCS capture reactions are more exothermic and exergonic than the corresponding CO2 and SO2 capture reactions due to bond dissociation enthalpy differences. Selected reaction energy barriers were calculated and correlated with the reaction thermodynamics for a given acid gas. The barriers are highest for CO2 and OCS and lowest for CS2 and SO2. The ability of a ring to participate in acid gas capture via ring expansion is correlated to ring strain energy but is not wholly dependent upon it. The expanded N-heterocycles produced by acid gas capture should be polymerizable, allowing for upcycling of these materials.
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- Award ID(s):
- 2205819
- PAR ID:
- 10519160
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry A
- Volume:
- 127
- Issue:
- 48
- ISSN:
- 1089-5639
- Page Range / eLocation ID:
- 10171-10183
- Subject(s) / Keyword(s):
- Chemical reactions Free energy Nitrogen Substitution reactions Thermodynamics
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.jpca.3c06094
