Discovery of long-lived excited electronic states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations.

Charge exchange technique has been used to detect the presence of long-lived excited electronic states of some monosubstituted ethene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (DeltaE < or = 0), or the exoergicity rule. The A2A' states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations have been found to have long lifetimes (tens of microsecond or longer) while all the excited electronic states of vinylfluoride cation have been found to undergo rapid dissociation or nonradiative relaxation to the ground state. The long-lived states found are those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these are the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or the in-plane pi orbital of the triple bond C[triple bound]N. The present findings are very similar to those for the monosubstituted benzene cations reported previously.