Frozen shell approximation violation in negative ion formation from halogenated benzenes via dissociative attachment

A series of benzene derivatives (R1C6H4R2) has been studied by means of ele ctron capture negative ion mass spectrometry (ECNI-MS), and PM3 quantum che mical calculations. The dissociation channel M-. --> Hal(-) + (M - Hal)(.) is analysed from the point of view of symmetry conservation. Generally, a s ymmetry ban on dissociation may be avoided in at least two ways: (i) out-of -plane vibrations of the halogen atom in the molecular negative ion (MNI), mixing pi- and sigma-states of the anion; (ii) symmetrical in-plane vibrati on of the C-Hal bond, changing the order of the empty levels in the MNI wit h subsequent radiationless conversion into a sigma-state. Our analysis show s that neither of them provides a satisfactory explanation of the ECNI mass spectra for chlorobenzene, if one retains the usual assumption that an add itional electron goes into the LUMO of the neutral molecule. Thus, it may b e concluded that in this case electron capture causes a significant perturb ation of the energy ordering of vacant orbitals, thus making the frozen she ll approximation inapplicable. Copyright :(C) 2000 John Wiley & Sons, Ltd.