Mass spectrometry as a tool to probe the gas-phase reactivity of neutral molecules

Neutralization-reionization (NR) and charge-reversal (CR) mass spectrometric experiments can be combined to investigate the reactivity of neutrals generated in high energy collisions. Provided that the species under study exists as anion, neutral, and cation, the reactions of neutral molecules can be distinguished from those of projectile and recovery ions by taking neutral and ion decomposition difference (NIDD) mass spectra. The scope and limitation of this approach are discussed in detail for several diatomic species and selected polyatomic molecules. The NIDD spectra of tightly bound diatomics exhibit only minor signals which can be interpreted within the framework of vertical electron transfer processes and their Franck-Condon factors. More significant features arise for systems with weaker bonds in either the neutral or one of the charged states, for example, the different oxidation states of chlorine and hypochlorite, i.e. Cl-2(+.)/Cl-2/Cl-2(-.) and ClO+/(ClOClO-)-Cl-., respectively. The NIDD spectra of some polyatomic compounds demonstrate the performance of the method for the elucidation of the neutrals' structures and reactivities. The [C,H,O-2](-/./+) system is studied in detail and may serve as a model system for prototype carboxylate and acylium ions, HCOO- and HOCOO+, respectively. The NIDD spectra of peroxide molecular cations reveal the intrinsic features of peroxidic systems in their different oxidation states, i.e. preferential O-O bond rupture in the neutral species as compared to a favorable O-O bond cleavage in the ionic systems. The gas-phase reactivity of small alkoxy radicals is dominated by alpha-cleavages, and Barton-type 1,5-hydrogen migrations are observed for larger alkoxy radicals. Application of NIDD to probe the neutral species generated by electron detachment from the (CH2COO-)-C-. distonic ion reveals the potential of the method to study diradicals. Finally, two C2N3 isomers are discussed as extreme examples in which the neutrals do not exhibit any distinct reactivity under the experimental conditions chosen. (C) 1998 Elsevier Science B.V.