Dissociative electron attachment and intramolecular electron transfer in linear haloalkenes

Abstract

Intramolecular electron transfer through alkyl chains has been investigated by measuring the cross section for halide detachment following resonant π* electron capture in linear n-halo-1-alkenes, for halo = chloro, bromo and n=2–6. The magnitude of the cross section decreases with increasing chain length for all the haloalkenes, with the exception of the halopropenes, but at a considerably faster rate for the chloro than for the bromo compounds. The decrease in cross section for the chloroalkenes occurs at a rate consistent with the decrease in electron-tunneling rates in hydrocarbons with through-bond interactions. For the bromoalkenes it appears that σ*–π* coupling is quite strong and thus the results are not consistent with an electron transfer interpretation. Excluding the propenes, the energy of the cross section maximum is essentially constant for the chloralkenes while it decreases with chain length for the bromoalkenes. Hartree–Fock calculations have been used to determine the equilibrium geometries of various conformers of the n-halo-1-alkenes for n=2–5. The 4- and 5-bromo-1-alkenes show considerably smaller conformational energy differences than do the chloro compounds. Electron attachment energies have been calculated for the most stable conformers at Hartree–Fock, density functional, and Mo/ller–Plesset second order levels. Trends in calculated attachment energies parallel experimental trends in the energies of dissociative attachment maxima only for the Mo/ller–Plesett second order calculations. At the Hartree–Fock level the singly occupied molecular orbitals of the haloalkene radical anions show a somewhat greater admixture of C–halogen σ* and C=Cπ* character in the bromo compounds than in the chloroalkenes, but the distinct difference in σ*–π* coupling in the bromoalkenes compared to the chloroalkenes is represented accurately only in the calculations that include electron correlation.