Empty level structure and dissociative electron attachment cross sections in saturated and unsaturated bromohydrocarbons

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for nine (normal, secondary, and tertiary) bromoalkanes and ten bromoalkenes where the bromine atom is directly bonded to an ethene carbon atom or separated froth the double bond by 1-4 CH2 groups. The relative DEA cross sections (essentially due to the Br- fragment) are reported and compared with those of the corresponding chlorides. B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral states of the bromohydrocarbons. The calculated VOEs satisfactorily reproduce the trends of the vertical electron attachment energies (VAEs) measured in the ET spectra. Electron attachment to the sigma*(C-Br) MO of the saturated bromides occurs at about 1.2 eV, the energy of the resonance being slightly stabilized with increasing branching. The corresponding peak in the DEA cross section occurs at about 0.6 eV in the normal bromoalkanes and 0.9 eV in the secondary and tertiary bromoalkanes. In vinyl bromide, the lowest resonance is associated with the ethene pi* LUMO, whereas in the CH2=CH(CH2)(n)Br alkenes with n > 2, the LUMO is the sigma*(C-Br) MO. Consistently, in the latter compounds the energy at the peak of the DEA cross section and the magnitude of the latter are comparable to those of the normal bromoalkanes.