Empty level structure and dissociative electron attachment cross section in (bromoalkyl)benzenes.

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for the series of bromoalkyl benzenes C6H5(CH2)nBr (n=0-3), where the bromine atom is directly bonded to a benzene ring or separated from it by 1 to 3 CH2 groups, and the dihalo derivative 1-Br-4-Cl-benzene. The relative DEA cross sections (essentially due to the Br – fragment) are reported and the absolute cross sections are also evaluated. HF/6-31G and B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral state molecules. The p* VOEs, scaled with empirical equations, satisfactorily reproduce the corresponding experimental vertical electron attachment energies (VAEs). According to the calculated localization properties, the LUMO (as well as the singly occupied MO of the lowest-lying anion state) of C6H5(CH2)3Br is largely localized on both the benzene ring and the C-Br bond, in spite of only a small p*/s*C-Br interaction and in contrast to the chlorine analogue where the LUMO is predicted to possess essentially ring p* character. This would imply a less important role of intramolecular electron transfer in the bromo derivative for production of the halogen negative fragment through dissociation of the first resonant state. The VAEs calculated as the anion/neutral energy difference with the 6-31+G* basis set which includes diffuse functions are relatively close to the experimental values, but do not parallel their sequence. In addition the SOMO of some compounds is not described as a valence MO with large p* character, but as a diffuse s* MO.