Thermochemical properties, Δ fH o(298), S o(298), and C p o(298), (T), for n-butyl and n-pentyl hydroperoxides and the alkyl and peroxy radicals, transition states, and kinetics for intramolecular hydrogen shift reactions of the peroxy radicals

Document Type

Article

Publication Date

7-19-2007

Abstract

Alkyl radicals in atmospheric and combustion environments undergo a rapid association with molecular oxygen ( 3O 2) to form an alkyl peroxy radical (ROO). One important reaction of these peroxy radicals is the intramolecular H-shift (intramolecular abstraction) to form a hydroperoxide alkyl radical (R′ ̇COOH), where the hydroperoxide alkyl radical may undergo chemical activation reaction with O 2 and result in chain branching at moderate to low temperatures. The thermochemistry and trends in kinetic parameters for the hydrogen shift reactions from each carbon (4-8-member-ring TST's) in n-butyl and n-pentyl peroxy radicals (CCCCOO ̇ and CCCCCOO ̇) are analyzed using density functional and ab initio calculation methods. Thermochemical properties, Δ fH o(298 K), C-H bond energies, S o(29S K), and C p oT) of saturated linear C4 and C5 aliphatic peroxides (ROOH), as well as the corresponding hydroperoxide alkyl radicals (R′ ̇COOH), are determined. Δ fH o(298 K) are obtained from isodesmic reactions and the total energies of the CBS-QB3 and B3LYP computational methods. Contributions to the entropy and the heat capacity from translation, vibration, and external rotation are calculated using the rigid-rotor-harmonic-oscillator approximation based on the CBS-QB3 frequencies and structures. The results indicate that pre-exponential factors, A(T), decrease with the increase of the ring size (4-8-member-ring TS, H-atom included). The ΔH‡ for 4-, 5-, 6-, and 7-member rings in n-butyl (and n-pentyl) peroxy are 40.8 (40.8), 31.4 (31.5), 20.5 (20.0), 22.6-p (19.4) kcal mol -1, respectively. The ΔH‡ for the 8-member ring in n-pentylperoxy is 23.8-p kcal mol -1, All abstractions are from secondary (-CH 2-) groups except those marked (-p), which are from primary sites. Enthalpy and barrier values from the B3LYP/6-311++G(2d,p) and BHandHLYP/6-311G(d,p) methods are compared with CBS-QB3 results. The B3LYP results show good agreement with the higher level CBS-QB3 calculation method; the BHandH barriers for the intramolecular peroxy H-shifts are not acceptable. © 2007 American Chemical Society.

Identifier

34547526933 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp070342s

ISSN

10895639

First Page

6361

Last Page

6377

Issue

28

Volume

111

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