Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1C l-AQ and 2Br-AQ display dramatically steep positive temperature dependencie s of Hal(-) ion abundance in the low electron energy region. Molecular NI i ntensity decreases rapidly with increasing temperature in the case of 1I-AQ . In the case of 2Br-AQ a metastable NI peak (m/z 22.9) corresponding to th e process BrAQ(-) --> Br- + AQ(0) was recorded. This means that the charact eristic dissociation lifetime of the molecular NI Br-AQ(-) is at least simi lar to 25 mus at the energy similar to0.67 eV in the low-temperature spectr um (T similar to 80 degreesC), and at the energy similar to0.13 eV in the h ot spectrum (T similar to 290 degreesC). Together with the observed tempera ture dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecuti ve stages in the process of molecular NI dissociation of Cl- and Br-substit uted AQ, namely, electron capture into the empty pi -orbital by means of th e shape resonance mechanism, followed by a radiationless transition into th e ground electronic pi -state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissocia tion of the molecular NI accompanied by the transition from the pi -term in to the sigma -term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of i nversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, bu t also in other experimental investigations using low-energy electron-molec ule and ion-molecule collisions. Copyright (C) 2001 John Wiley & Sons, Ltd.