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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 p-systems. The calculated p* MO energies correlate linearly with the energies of electron attachment to the p* 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 s* MO rather than a valence p* MO. The relative Cl À anion currents measured in the DEA spectra of the present molecular systems are compared to those previously found in benzene analogues. The Cl À yield reflects the efficiency of intramolecular electron transfer from the p-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 À current, ascribed to the lower energy of the empty s* Si-C MOs and consequent greater ability to promote through-bond coupling between the p* and s* C-Cl MOs. Comparison between the corresponding benzene, ethene and ethyne derivatives reveals that the Cl À current is also significantly influenced by the nature of the p-functional group, in agreement with the inverse dependence on energy of the lifetime of the temporary p* anion state.

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The temporary anion states of gas-phase furan, isoxazole, oxazole, pyrrole, pyrazole, imidazole, thiophene, isothiazole, and thiazole are characterized by means of electron transmission spectroscopy. The measured energies of vertical electron attachment are compared with the virtual orbital energies of the neutral state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental attachment energies. Replacement of a ring CH group with a nitrogen atom increases the electron-acceptor properties, although the stabilization of the π* anion states is not as large as that of the π cation states, in line with the bond length variations caused by aza-substitution. In the spectra of thiophene and isothiazole the first π* resonances display sharp vibrational structure with energy spacing of about 80 meV. The spectrum of isothiazole presents clear evidence for a low-energy (1.61 eV) resonance ascribed to the lowest σ* anion state.