Alkoxy radicals in the gaseous phase: β-scission reactions and formation by radical addition to carbonyl compounds

Abstract

The structures and reactivities of the alkoxy radicals methoxy (CH 3 O·), ethoxy (CH 3 CH 2 O·), 1-propoxy (CH 3 CH 2 CH 2 O·), 2-propoxy ((CH 3 ) 2 CHO·), 2-butoxy (CH 3 CH 2 CH(CH 3 )O·), tert-butoxy ((CH 3 ) 3 CO·), prop-2-enoxy (CH 2 =CHCH 2 O·), and but-3-en-2-oxy (CH 2 =CHCH(CH 3 )O·) have been investigated at the B3-LYP/6-31G(d) and CBS-RAD levels of theory. Enthalpies of formation (∆ f H 298 o ) were calculated with CBS-RAD for all the alkoxy radicals, the carbonyl and radical products of β-scission reactions, and the transition structures leading to them. The mean absolute deviation between the predicted and available experimental ∆ f H 298 o values is 5.4 kJ mol -1 . Eyring (∆H 0 ‡ , ∆H 298 ‡ , ∆G 298 ‡ ) and Arrhenius (log A, E a ) activation parameters for both the forward (β-scission) and reverse (radical addition to carbonyl) pathways were calculated. Agreement with available experimental data is very good, generally within 1-5 kJ mol -1 for E a , and 0.5 for log A. The transition structures are found to be substantially polarized, with the departing radical slightly positive, the O atom negative, and the rest of the molecule positive. The barriers for the β-scission reactions decrease with decreasing endothermicity and with decreasing ionization energy of the departing radical.Résumé : Faisant appel à des calculs théoriques aux niveaux B3-LYP/6-31G(d) et CBS-RAD de la théorie, on a étudié les structures et les réactivités des radicaux alkoxy, méthoxy (CH 3 O·), éthoxy (CH 3 CH 2 O·), 1-propoxy (CH 3 CH 2 CH 2 O·), 2-propoxy [(CH 3 ) 2 CHO·], 2-butoxy [CH 3 CH 2 CH(CH 3 )O·], tert-butoxy [(CH 3 ) 3 CO·], prop-2-énoxy (CH 2 =CHCH 2 O·) et but-3-én-2-oxy [CH 2 =CHCH(CH 3 )O·]. On a calculé les enthalpies de formation, ∆ f H 298 o , au niveau CBS-RAD de la théorie pour tous les radicaux alkoxy, tous les produits carbonylés et tous les produits radicalaires provenant de réac-tions de β-scission et toutes les structures de transition qui y conduisent. La déviation absolue moyenne entre les valeurs prédites et les valeurs de ∆ f H 298 o expérimentales disponibles est de 5,4 kJ mol -1 . On a aussi calculé les paramètres d'activation d'Eyring (∆H 0 ‡ , ∆H 298 ‡ , ∆G 298 ‡ ) et d'Arrhenius (log A, E a ) pour la réaction vers la droite (β-scission) et pour la réaction inverse (addition d'un radical sur le carbonyle). L'accord entre les valeurs calculées et les valeurs expéri-mentales disponibles est bon, généralement entre 1 et 5 kJ mol -1 pour les valeurs de E a et de 0,5 pour le log A. On a trouvé que les structures de transition sont assez polarisées alors que le radical qui se détache est légèrement positif, l'atome d'oxygène est négatif et que le reste de la molécule est positif. Les barrières aux réactions de β-scission diminuent avec une augmentation du caractère endothermique et avec une diminution de l'énergie d'ionisation du radical qui se détache.

Experimental and modeling investigation of the low-temperature oxidation of n-heptane

“…Note that the kinetic parameters obtained for the reactions of β-scission (1) and (2) in Figure 16 are in good agreement with the kinetic data obtained by Rauk et al [62] Note that these new reactions have almost no effect to the global reactivity. The computed mole fraction profile of C 7 H 14 O 3 ketohydroperoxides is compared with the signal at m/z 146 obtained using SVUV photo-ionization mass spectrometry in Figure 10.…”

Methyl formate oxidation: Speciation data, laminar burning velocities, ignition delay times, and a validated chemical kinetic model

“…Our original treatment for this reaction was to adopt the values used by Dooley et al [19] to describe the analogous process in methyl butanoate oxidation, which likens the n-propylformyl radical to the vinyl radical. On the basis of quantum chemical calculations for the addition reactions of the vinyl radical to various compounds, Dooley et al [19] opted for an expression of 3.89 × 10 11 exp (−10,900 cal mol −1 /RT ) cm 3 , which comprises the preexponential factor of Dooley above and a value for the activation energy that is more akin to that recommended by Rauk et al [55] for methyl radical addition to formaldehyde than for vinyl radical addition has been used for the reaction…”

New experimental evidence and modeling study of the ethylbenzene oxidation

“…m Rate constant taken equal to those proposed by Sahetchian et al [33] in the case of alkylhydroperoxides. n Rate constant taken equal to that of the similar reaction in the case of but-3-en-1-oxy [34].…”

“…In the case of resonance stabilized C 8 H 9 radical, the resulting alcoxy radical can decompose by breaking C-C bonds, yielding methyl or phenyl radical, respectively (reactions 37 and 39), or a C-H bond yielding acetophenone (reaction 38). Rate constants were estimated as that for the decompositions of but-3-en-1-oxy radical calculated by Rauk et al [34]. Acetophenone can also be produced directly from the hydroperoxide by a roaming reaction, where the leaving OH group abstract directly the H-atom bounded to the C-atom bearing the hydroperoxide function (reaction 35).…”