Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

Abstract

The β bond dissociation of alkyl radicals and their reverse reactions, the addition of alkyl radicals to olefins were studied by G3MP2 level of theory to obtain a consistent kinetic data set. Both reaction families can be classified depending on the type of radical formed by β bond scission, namely the CH 3 , primary, secondary tertiary radical formed. The kinetics of the reaction classes were described by only a limited number of Arrhenius parameters. The unified A factor of 10 13.7 s -1 was found for all β bond dissociations. The Arrhenius activation energies are 125, 121, 113 and 103 kJ mol -1 , for methyl, primary, secondary, and tertiary radicals, respectively. The activation energies of 32, 25 and 18 kJ mol -1 are calculated for the terminal addition of primary (including methyl), secondary, and tertiary radicals to olefins, respectively. The biologically important nonterminal radical additions to olefins have higher barriers of 37, 31 and 35 kJ mol -1 , respectively. At room temperature both strongly exothermic additions can compete with H-atom abstraction.New groups for Benson's group additivity rules were defined to describe activation parameters for the β bond dissociation reactions. The group values were calculated by using the ab initio heats of formation of transition state structures.

Theoretical studies of atmospheric reaction mechanisms in the troposphere

“…The reverse reaction, i.e. ring closure in unsaturated alkyl radicals, was found 223,227 to be endothermic with higher barriers, as well as being entropically disfavored. Still, Greenwald et al 123 proposed cyclisation in the g-unsaturated OH-adducts on the inner carbons of isoprene as a source of unsaturated carbonyls observed experimentally and confirmed these in RRKM Master equation analysis based on a DFT potential energy surface.…”

“…220 Likewise, alkyl radicals with an a-ONO 2 decompose without barrier to {CQO + NO 2 . 221 The loss of H-atoms, alkyl radicals or other substituents to form an olefin, [222][223][224] or H-migration reactions across the alkyl chain 222,225 are typically not important at atmospheric temperatures, given the high calculated barriers. A b-OOH-substituted alkyl radical, on the other hand, can undergo an epoxidation reaction (see Fig.…”

Advances in modeling hydrocarbon cracking kinetic predictions by quantum chemical theory: A review

“…53 For alkyl decomposition reactions ( Table 1, Chain propagation 2, and β-scission), Viskolcz and Laszlo have studied a large family of unimolecular reactions, especially the decomposition of large alkyl radicals, using the high level G3(MP2, SVP) method. 54 Meanwhile, the high accuracy CCSD(T)/cc-pVTZ method has been used by Miller and Klippenstein to compute the electronic structure of a series of C 3 H 7 · dissociations 55 .…”

Ab initio study of chain branching reactions involving second generation products in hydrocarboncombustion mechanisms

“…3,10,[29][30][31][32][33] Those studies that do include secondary sites generally contain a limited number of them, or don't consider H-migration reactions. 4,5,7,[34][35][36][37][38] For example, Viskolcz and Seres focus only on b-C-C bond scissions in sec-alkyl and do not include H-migration reactions. 37 Tsang and co-workers looked at the hept-2-yl to hept-3-yl isomerization using both combustion modeling, 38 and CBS-QB3 calculations.…”

“…4,5,7,[34][35][36][37][38] For example, Viskolcz and Seres focus only on b-C-C bond scissions in sec-alkyl and do not include H-migration reactions. 37 Tsang and co-workers looked at the hept-2-yl to hept-3-yl isomerization using both combustion modeling, 38 and CBS-QB3 calculations. 34 Matheu et al and Curran et al used an estimation method developed by Benson, with refinements from experimental values, to create combustion models that included 3-through 5-member secondary-secondary transition state rings, though Curran et al limited their study to n-heptyl radicals.…”