Secondary vinylic radicals in 1,3-butadiene systems and their reactions with O2: A theoretical study of thermochemical properties, reaction paths, and kinetics

Document Type

Conference Proceeding

Publication Date

1-1-2007

Abstract

1,3-dienes are formed by β-scission (elimination) reactions of hydrocarbons in thermal and oxidation reaction systems of linear and branched hydrocarbons and olefins, and in the oxidation of aromatics. They are almost completely unstudied in terms of their formation reactions via abstraction, or further oxidation reaction paths or thermochemistry. The thermochemistry and reaction path kinetics of the secondary olefinic carbons in 1,3-butadiene (CH2=CH-CH=CH2) are analyzed with density functional and ab initio calculations. Enthalpies of formation are determined using isodesmic reaction analysis at the G3MP2, and CBS-QB3 composite levels, and the B3LYP and TPSS density functional methods. Entropies (S°298) and heat capacities (CP(T)) are determined using geometric parameters and vibration frequencies obtained at the B3LYP/6-311+G(2d,d,p) level of theory. Internal rotor barriers are analyzed and their contributions are included in S°298 and CP(T) values. The allylic-type radical with the allene structure (CH2=C=CH-C•H2) is shown to be the favored form of the resonant structure, requiring an intramolecular rearrangement (both internal rotor operation and bond length changes) upon radical formation. Resonant structures are shown to be significantly lower in energy than the primary or vinylic type radicals. The allylic like radicals add to O2 to form an energized peroxy adduct [CH2=C=CH-CH2-OO•]∗ with a reaction exothermicity which is lower than for vinyl or phenyl radical + O2 reactions. The reaction channels of the peroxy adducts include reverse reaction, isomerization via hydrogen shift with subsequent β-scission or RO-OH bond cleavage, and cyclization to 4, 5, and 6 member rings. The cyclic peroxide radicals can undergo unimolecular reaction via radical attack on either peroxy oxygen or via conventional β-scission to different isomers.

Identifier

84943571737 (Scopus)

ISBN

[9781604238112]

Publication Title

5th US Combustion Meeting 2007

First Page

1110

Last Page

1121

Volume

2

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