Structure-reactivity relationships for the self- reactions of linear secondary alkylperoxy radicals: An experimental investigation

Abstract

The self-reactions of the linear pentylperoxy (C 5 H 11 O 2 ) and decylperoxy (C 10 H 21 O 2 ) radicals have been studied at room temperature. The technique of excimer laser flash photolysis was used to generate pentylperoxy radicals, while conventional flash photolysis was used for decylperoxy radicals. For the former, the recombination rate coefficients were estimated for the primary 1-pentylperoxy isomer (n-C 5 H 11 O 2 ) and for the secondary 2-and 3-pentylperoxy isomers combined (Љsec-C 5 H 11 O 2 Љ) by creating primary and secondary radicals in different ratios of initial concentrations and simulating experimental decay traces using a simplified chemical mechanism. The values obtained at 298 K were:Ϫ14 cm 3 molecule Ϫ1 s Ϫ1 . Quoted errors are 1, whereas the total relative combined uncertainties correspond to an estimated uncertainty factor around 1.65. For decylperoxy radicals, the kinetics of all the types of secondary peroxy isomers reacting with each other were considered equivalent and grouped as sec-C 10 H 21 O 2 (as for sec-C 5 H 11 O 2 ). The UV absorption spectrum of these secondary radicals was measured, and the combined self-reaction rate coefficients then derived as: k(sec-C 10 H 21 O 2 ϩ sec-C 10 H 21 O 2 ) ϭ (9.4 Ϯ 1.3) ϫ 10 Ϫ14 cm 3 molecule Ϫ1 s Ϫ1 at 298 K. Again, quoted errors are 1 and the total uncertainty factor corresponds to a value around 1.75. The sec-dodecylperoxy radical was also investigated using the same procedure, but only an estimate of the rate coefficient could be obtained, due to aerosol formation in the reaction cell: k(sec-C 12 H 25 O 2 ϩ sec-C 12 H 25 O 2 ) Ϸ 1.4 ϫ 10 Ϫ13 cm 3 molecule Ϫ1 s Ϫ1 , with an uncertainty factor of about 2. Despite the fairly high uncertainty factors, a relationship has been identified between the room-temperature rate coefficient for the selfreaction and the number of carbon atoms, n, in the linear secondary radical, suggesting: log(k(sec-RO 2 ϩ sec-RO 2 )/cm 3 molecule Ϫ1 s Ϫ1) ϭ Ϫ13.0-3.2 ϫ exp(Ϫ0.64 ϫ (n-2.3)). Concerning

Estimation of rate coefficients and branching ratios for reactions of organic peroxy radicals for use in automated mechanism construction

“…The data show that the self-reaction reactivity, relative to that of alkyl peroxy radicals, is activated by the presence of numerous functional groups (including allyl-, benzyl-, hydroxy-, alkoxy-, oxo-and acyl-groups), and that the rate coefficients follow the general trend of decreasing reactivity, primary > secondary > tertiary, for peroxy radicals containing otherwise similar functionalities. It also appears that reactivity tends to increase with the size of the organic group towards a "plateau" value, as most clearly demonstrated by the systematic study of secondary alkyl peroxy radicals reported by Boyd et al (1999). Based on optimization to the complete secondary alkyl peroxy radical dataset, an expression almost identical to that recommended by Boyd et al (1999) is thus derived as a reference rate coefficient for secondary peroxy radicals at 298 K, as illustrated in Fig.…”

Modelling the evolution of organic carbon during its gas-phase tropospheric oxidation: development of an explicit model based on a self generating approach

“…Alkoxy radicals undergo three reaction types under tropospheric conditions, namely reaction with O 2 , beta scission decomposition and 1,5-H-shift isomerization: Lesclaux (1997); b kinetic and mechanistic parameters from Tyndall et al (2001); c rate constant based on Boyd et al (1999) for the reaction of C 5 H 11 O 2 and C 10 H 21 O 2 and using the activation energy and the branching ratio for ipropyl peroxy radical. ; d based on kinetic and mechanistic parameters for CH 3 CH(OH)CH(OO·)CH 3 ; e based on kinetic and mechanistic parameters for (CH 3 ) 3 C(OO·); f based on kinetic and mechanistic parameters for CH 3 C(CH 3 )(OH)C(OO·)(CH 3 )CH 3 .…”

Alpha-pinene oxidation by OH: simulations of laboratory experiments

“…Lightfoot et al (1992) and Boyd et al (1999) measured the self-reaction rate of the linear primary radicals up to n-C 5 H 11 O 2 . However, the presence of a double bond or a substitution in β in a primary radical appears to increase its k self as e.g.…”

“…Since the k self for non-linear alkyl radicals increases slightly with carbon number, we set the self-reaction rate to 4.0×10 −12 cm 3 molec −1 s −1 for the R1R class. Boyd et al (1999) parameterized the self-reaction rates of linear secondary alkyl peroxy radicals (R2R) as a function of the carbon number n. Based on this relationship, and assuming that this rate is somewhat higher due to the numerous allyl and substituted secondary radicals present in the mechanism, a k self of 4.0×10 −13 cm 3 molec −1 s −1 is chosen for the R2R class.…”