Reactions of chemically activated pentenyl radicals. Kinetic parameters of 1,4 hydrogen shifts and the cis-trans isomerization of homoallylic radicals

Abstract

The reactions resulting from adding H atoms to 2-pentyne in the gas phase have been studied using several deactivating gases. Measured decomposition products are propyne, 1-butyne, and 1,2-butadiene, resulting from decompositions of initially formed radicals at rates consistent with thermal results. 1,3-Butadiene, resulting from isomerization of initially formed 2-penten-2-yl first via a 1,4 H shift and then via a cis-trans isomerization of homoallylic 2-penten-5-yl, was also formed. Information about the rates of these isomerizations was obtained. 1,4 H shift activated complex models could be tested using this work and literature results, and the use of looser activated complex models is supported. Using loose 1,4 H shift activated complex frequencies and a 0°K primary vinylic C-H bond dissociation energy of 110.2 kcal/mol, a 1,4 H shift critical energy of 18.9 kcal/mol, and a cis-trans homoallylic isomerization critical energy of 23.4 kcal/mol are obtained. In addition, there is evidence of a possible 1,3 H shift of 2-penten-3-yl occurring with a critical energy of about 30 kcal/mol.

Vibrational Energy Transfer Modeling of Nonequilibrium Polyatomic Reaction Systems

“…Non-Steady-State Chemical Activation Systems. In chemical activation, an exothermic chemical reaction produces an excited species which can react further via isomerization and unimolecular decomposition, or be stabilized by collisions. − For example, the exothermic reactions of atoms and free radicals with olefins produce vibrationally excited free radicals: where the asterisk denotes vibrational excitation. According to ab initio calculations, the vibrational excitation resulting from reaction 14a is ∼30 kcal mole -1 .…”

“…In chemical activation, an exothermic chemical reaction produces an excited species which can react further via isomerization and unimolecular decomposition, or be stabilized by collisions. [145][146][147][148] For example, the exothermic reactions of atoms and free radicals with olefins produce vibrationally excited free radicals:…”

Isomerization of chemically activated 1-buten-1-yl and 1-buten-4-yl radicals

“…Study of the isomerization of nonyl-2 to nonyl-3 radical through 1,6-H atom shift has given the value of 16.0 kcal/mol as the threshold energy.3 Recently, 1,2-and 1,3-H atom shifts have been substantiated in chemical activation systems and they have high threshold energies, 30-34.5 kcal/mol. 5,13,14 In the calculation of the RRKM specific reaction rate, two factors are required; one is the vibrational frequency assignments of the radical and H-atom transfer activated complex, and the other is the critical energy for isomerization. The former can be obtained by using the formulation developed by Rabinovitch and coworkers.3,15 For the latter we proposed in a previous paper14 that the critical energy can be approximately estimated by the expression, Eo = £ab + Es, where Eo is the critical energy, E&b the activation energy for a bimolecular H atom abstraction, and Es the ring strain energy.…”