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

• Published in: Rapid communications in mass spectrometry 2001-01, Vol.15 (19), p.1869-1878
• Main Authors: Asfandiarov, N. L., Fal'ko, V. S., Lukin, V. G., Nafikova, E. P., Pshenichnyuk, S. A., Fokin, A. I., Lomakin, G. S., Chizhov, Yu. V.
• Format: Article
• Language: English
• Subjects: Anthraquinones - chemistry; Mass Spectrometry - methods; Models, Molecular; Molecular Conformation; Molecular Structure; Thermodynamics
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.446

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI‐MS). 1Cl‐AQ and 2Br‐AQ display dramatically steep positive temperature dependencies of Hal− ion abundance in the low electron energy region. Molecular NI intensity 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 the process BrAQ− → Br− + AQ0 was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br‐AQ− is at least ∼25 µs at the energy ∼0.67 eV in the low‐temperature spectrum (T ∼80 °C), and at the energy ∼0.13 eV in the hot spectrum (T ∼290 °C). Together with the observed temperature 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 consecutive stages in the process of molecular NI dissociation of Cl‐ and Br‐substituted AQ, namely, electron capture into the empty π‐orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic π‐state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the π‐term into the σ‐term, so‐called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I‐AQ, an effect of inversion 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, but also in other experimental investigations using low‐energy electron‐molecule and ion‐molecule collisions. Copyright © 2001 John Wiley & Sons, Ltd.