Theoretical Study of the Straight-Chain C₄H₇ Radical Isomers and Their Dissociation and Isomerization Transition States

Abstract

We use the G3//B3LYP method to calculate structures, harmonic vibrational frequencies, and energies at selected local minima and saddle points on the C 4 H 7 potential energy surface. These include the straightchain C 4 H 7 radical isomers (of which there are twelve conformers of five structural isomers: 1-buten-1-yl, 2-buten-2-yl, 1-buten-2-yl, 1-methylallyl, and 3-buten-1-yl), sets of dissociation products, and dissociation, isomerization, and conformational transition states. Analogy with smaller hydrocarbon systems reveals why, for some reaction channels, more than one transition state may be accessed from the conformers of a structural isomer en route to the corresponding products. We compare results of the G3 and G3//B3LYP methods for selected C 4 H 7 species.

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

“…The calculations used vibrational frequencies and rotational constants for the radical and transition states taken from the Supporting Information of ref 3. Table 1 reports the results of RRKM calculations when the external moments of inertia are treated as adiabatic and the torsional modes associated with rotation of the methyl groups are not treated as free internal rotors.…”

“…The 2-buten-2-yl radical is one of several straight-chain isomers of the C 4 H 7 unsaturated hydrocarbon radical species. Several groups [1][2][3] have sought to characterize the important dissociation and isomerization channels of the C 4 H 7 radical isomers. Previous electronic structure G3//B3LYP calculations in our group 3 have characterized the relative energies and isomerization barriers of the C 4 H 7 radical isomers, as well as the enthalpies and transition states for the available dissociation reactions of the radicals ( Figure 1).…”

“…Several groups [1][2][3] have sought to characterize the important dissociation and isomerization channels of the C 4 H 7 radical isomers. Previous electronic structure G3//B3LYP calculations in our group 3 have characterized the relative energies and isomerization barriers of the C 4 H 7 radical isomers, as well as the enthalpies and transition states for the available dissociation reactions of the radicals ( Figure 1). Listed below are the reactions that are energetically possible for 2-chloro-2-butene at 193 nm, with the dissociation pathways open to the 2-buten-2-yl radicals and the C-C fission pathways open to the HCl elimination cofragments indented.…”

“…This agrees, within error, with the G3//B3LYP value of 9.3 kcal/mol for the exit barrier of the CH 3 + propyne channel. 3 The experiments described herein utilize 2-chloro-2-butene, CH 3 C(Cl)CHCH 3 , as a photolytic precursor for the radical. The experiments measure the velocity distributions of the nascent 2-buten-2-yl radical and its subsequent dissociation products.…”

Unimolecular Dissociation of the CH3OCO Radical: An Intermediate in the CH3O + CO Reaction

“…This is the case when the vinyl radical dissociates to H + acetylene 42 and also when the 2-buten-2-yl radical accesses its C-C and C-H bond fission channels. 43 The overall photolysis of methyl chloroformate is complex, with overlapping signal from several different channels making data analysis difficult. Performing these experiments with a longer photolysis wavelength would be advantageous in that the nascent CH 3 OCO radicals would have lower internal energy, possibly avoiding formation of excited-state radicals.…”