Electron attachment and intramolecular electron transfer in unsaturated chloroderivatives

The electron transmission (ET) and dissociative electron attachment (DEA) spectra of chloroalkyl ethene and ethyne derivatives are reported. B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies for the optimised geometries of the neutral states of these molecules and other related pi-systems. The calculated pi* MO energies correlate linearly with the energies of electron attachment to the pi* LUMO measured in the ET spectra with a correlation coefficient of 0.993. The vertical attachment energies supplied by B3LYP/6-311+G** calculations, where the basis set includes diffuse functions, are often in significant disagreement with experiment, describing the singly occupied MO of the lowest-lying anion state as a diffuse sigma* MO rather than a valence pi* MO. The relative Cl- anion currents measured in the DEA spectra of the present molecular systems are compared to those previously found in benzene analogues. The Cl- yield reflects the efficiency of intramolecular electron transfer from the pi-system (where the extra electron is first trapped) to the remote chlorine atom. Replacement of a carbon atom with a silicon atom in the intermediate saturated alkyl chain causes a notable increase of the Cl- current, ascribed to the lower energy of the empty sigma*(Si-C) MOs and consequent greater ability to promote through-bond coupling between the pi* and sigma*(C-Cl) MOs. Comparison between the corresponding benzene, ethene and ethyne derivatives reveals that the Cl- current is also significantly influenced by the nature of the pi-functional group, in agreement with the inverse dependence on energy of the lifetime of the temporary pi* anion state.