Intramolecular H Atom Transfer Reactions in Alkyl Radicals and the Ring Strain Energy in the Transition Structure

Abstract

Ab initio calculations were performed on prototypical intramolecular H atom transfer reactions in alkyl radicals, namely, the identity reactions of 1,2-, 1,3-, 1,4-, and 1,5 intramolecular isomerizations in the ethyl, 1-propyl, 1-butyl, and 1-pentyl radicals, respectively. The equilibrium geometries and the transition structures have been optimized at the UHF/6-31G* and/or UMP2/6-311G** levels. The activation energies at 0 K were calculated with the MP-SAC2 and BAC-MP4 methods. The computed barrier heights (41.1, 41.6, 24.6, 17.2 kcal mol-1 at the MP-SAC2 and 43.1, 41.0, 25.1, and 18.8 kcal mol-1 at the BAC-MP4 level of theory) follow the trend expected and are in reasonable agreement with experimental data on related reactions. The differences between the ab initio barrier heights of alkyl isomerizations and that of the corresponding bimolecular reaction are in good agreement with the strain energies of the corresponding cycloalkanes.

Capturing pressure‐dependence in automated mechanism generation: Reactions through cycloalkyl intermediates

“…We are aware of the work by Viskolcz et al [45,46] to develop such rules, but that work ignored hindered rotations, which can be important for detemining Arrhenius A-factors; the "rotor loss" effects are included in the rules of Table II. Parameters for H-shift types not listed (e.g., 1,3 secondary to allylic radical) were estimated by Evans-Polanyi rules derived from the results of the above ab initio calculations, combined with an estimate of the ring-strain energy for the particular H-shift transition state to predict the activation energy:…”

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

“…However, nonArrhenius behavior may occur at intermediate temperatures when equilibration of the addition path competes with abstraction. Further complications appear when larger alkyl radicals are involved since the barriers for internal 1,4 and 1,5 H-atom transfers are low [9,10].…”

“…Earlier, we studied a large family of unimolecular reactions of alkyl radicals, the internal H-atom transfers via cyclic transition states extensively [9,10]. We found kinetic parameters for these reactions which are generally useful for the modeling of complex systems [11].…”

Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase

“…They can be formed by a first H • transfer from the chain to an ionized heteroatom [5] or by ring opening as studied by Tabet and coworkers [9]. From experiment [5,9,10] and from calculation [11], it has been also shown that the energy barrier for the transfers to a carbon radical site decreases from the 1,2-H • migration up to the 1,5 one. More generally, the energy barrier for intramolecular 1,n-H • transfers is the lowest when the hydrogen to be transferred, the acceptor atom and donor atom are in line in the transition state.…”