Fragmentation pathways in the dissociation of fluoro-iodo-benzene negative ions Original Research Article

In a crossed-beam configuration between a fast potassium atom beam (10–350 eV) and a thermal beam of fluoro-iodo-benzene molecules, the total yields of parent and fragment negative ions have been measured. The measurements include all three isomers: ortho-, para- and meta-fluoro-iodo-benzene. The negative ions are extracted from the interaction region by a pulsed extraction voltage. Mass selection is obtained by applying a time-of-flight (TOF) method. Apart from the parent ion C6H4IF− (m=222) the fragment ions C6H4F− (m=95), C4H3F− (m=70), C3H2F− (m=57) and C2HF− (m=44) are formed in the electron transfer collision. From the TOF spectra it is concluded that the fragmentation of energy-rich ions, for the three isomers, follows two paths. Either the I-atom is expelled, leaving the ring intact, or the ring is opened followed in a second step by further fragmentation. Calculations are performed applying the extended Hückel method to establish the orbitals in which the electron is captured. Evidence is presented that the LUMO and the second LUMO for all three isomers are only separated by a few tenths of an electron volt. Both orbitals are strongly antibonding π* ring orbitals.