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The reactivity of transition metal dioxide anions of cobalt, nickel, copper and zinc with sulfur dioxide has been investigated in the gas phase by mass spectrometric techniques and theoretical calculations. Dioxide anions of the late transition metals Cu and Zn promote an unprecedented fast and efficient oxidation to the sulfate radical anion through a double oxygen transfer, whereas the earlier transition metals Ni and Co only undergo the addition of two SO2 molecules. The ligation of a water molecule to ZnO2 − switches the reaction path to a single oxygen anion transfer, forming SO3 −.

The new nitrogen-rich oxide N 3 O has been detected in the gas phase by mass spectrometric experiments. The radical has been assigned a minimum lifetime of 0.7 μs and an open-chain NNNO structure in the quartet state. Structures and energies of the N 3 O + precursor ion and the N 3 O radical have been investigated by ab initio calculations.

H 2 O 2 + ions are shown to be formed in ionized O2/CH4 mixtures from the secondary reaction of CH2O+ with O2. By high-resolution mass spectrometric techniques, the peroxide HOOH structure and the H2CO structure of ionized formaldehyde are assigned to the [H2O2]+ ions and the [CH2O]+ reactants, respectively. The [CH4O2]+ intermediate of the reaction between O 2 + and CH4 is characterized as a mixture of the loosely bound [ CH 4 O 2 ] + complex and the theoretically predicted insertion product CH3OOH+. The CH3OOH+ intermediate, precursor of the most abundant ionic product CH2OOH+, has been characterized for the first time as a long-lived species.

The HOOO− anion, the conjugate base of dihydrogen trioxide, HOOOH, is suggested as a key intermediate in ozonation reactions relevant to atmospheric chemistry, water purification, preparative organic chemistry, etc. Whereas recent theoretical calculations characterize HOOO− as a long-lived metastable species, so far the anion proved experimentally elusive, defying attempts at its direct detection even in solution, where it should be stabilized by solvation. We report the preparation and the proof-positive detection of HOOO− as an isolated species with a lifetime >0.8 μs. The preparation proved particularly difficult, since the only viable approach is the indirect route involving the preliminary formation of HOOO upon neutralization of HOOO+, followed by electron attachment to the radical, utilizing neutralization–reionization ( + NR − ) mass spectrometry. Direct two-electron reduction of the HOOO+ cation to the HOOO− anion by the vertical process typical of charge reversal (CR) experiments proved inefficient. This observation, and application of neutral and ion decomposition difference (NIDD) mass spectrometry, pioneered by Schwarz and coworkers [Int. J. Mass Spectrom. Ion Processes 172 (1998) 181] suggest that the geometries of HOOO+ and HOOO− are significantly different, whereas that of the HOOO radical is structurally intermediate between them, which accounts for the diversity of the Franck–Condon factors characterizing the mutual vertical transitions of the three species. This inference is consistent with the available optimized geometries of HOOO and HOOO+, computed at various levels of theory, and with that of the anion recently reported by Plesnicar et al. [J. Am. Chem. Soc. 124 (2002) 8462].

Recent mass spectrometric studies of simple inorganic species (both charged and neutral) of main-group elements are reviewed, focusing attention on radicals and ions of interest to the chemistry of the atmosphere and its pollution. The examples illustrated concern the detection of HO 3 , hydrogen trioxide, the O 2 /O 3 isotope exchange and its charged intermediate O 5 + , the reactions promoted by ionization of ozone/halocarbon mixtures in atmospheric gases, the ion chemistry of NO x oxides, and that of elemental chlorine and chlorine fluoride. Among the results of specific interest to gas-phase ion chemistry, the examples illustrated concern the intracluster ligand-switching reactions in ternary NO + complexes, the NO 2 + reactivity towards ethylene and acetylene, the gas-phase basicity of Cl 2 , the formation and characterization of Cl 2 X + ions (X = Cl, F) and of [H 3 C-Cl-Cl] + , a new isomer of protonated dichloromethane.

[CH3SO2]+, [CH3SO3H2]+ and [CH3SO2OCH3H]+, positive even-electron ions from hexavalent sulfur compounds, have been studied by Fourier transform ion cyclotron resonance and collision-induced dissociation (CID). These ions showed no evidence of undergoing S → O methyl group shift, at variance with easy skeletal rearrangements frequently postulated for ionized sulfones and sulfonyl derivatives. A proton affinity value of 184 ± 2 kcal mol−1 has been obtained for methanesulfonic acid by the bracketing procedure, but the site of proton attachment in [CH3SO3H2]+ ions could not be established, either by CID or kinetic evidence. The methylation of SO2 by (CH3)2F+ ions has been ascertained to yield [CH3SO2]+ ions methylated on oxygen, in analogy to the same reaction observed in a superacid medium.

The reactivity of the SiF+3 cation towards oxygen bases (H2O, CH3OH, and CH3CH2OH) and nitrogen bases (NH3 and CH3NH2) has been studied using Fourier-transform ion cyclotron resonance mass spectrometry. The SiF+3 ion exclusively attacks the n-centre of the selected bases, yielding excited onium intermediates that undergo fragmentation by elimination of either an alkyl cation (CH3OH, CH3CH2OH, and CH3NH2) or an HF molecule (H2O, NH3, and CH3NH2). In H2O, various protonated fluorosilicic and silicic acids are formed which can be readily converted into their esters and amides by reaction with alcohols and ammonia, respectively. The structure and the reactivity of several such species have been investigated by mass-analyzed ion kinetic energy-collision induced dissociation spectroscopy and ab initio calculations. The extreme affinity of SiF+3 toward n-type electrons ranks it as a powerful gaseous “Lewis superacid”, suitable for generating long-lived, highly reactive ions, e.g. CH+3, in “non nucleophilic” gaseous media, such as, for instance, SiF4.

Collisional activation mass spectrometry and radiolytic techniques have been used to study the structure, the stability and the interconversion of gaseous C9H+11 ions from different reactions, i.e. (i) protonation of 3-phenyl-1-propene, 2-phenylpropene, cis- and trans-1-phenyl-1-propene, cyclopropylbenzene and indane; (ii) protodehydration and protodehydrohalogenation of Ph-C3H6X (X = OH, Br) substrates; (iii) addition of C3H+5 ions to PhH and of Ph+ ions to propene and cylcopropane. Several model structures have been identified, namely, Ph C + (Me)2 (a), Ph C + HEt (b), PhCH2 C + HMe (c) and C9H+11 arenium ions from indane. Protonation of cyclopropylbenzene in the presence of MeOH yields exclusively Ph-CH(OMe)Et, which is likely to arise from the cleavage of the protonated, intact C3 ring induced by the attack of the nucleophile. No rearrangement of the linear-chain ions b and c nor of protonated phenylcyclopropane into the branched-chain ion a has been detected under chemical ionization conditions, nor in the radiolytic systems, while significant c → b isomerization occurs in the period of time, ca. 10−8 s, required for the capture of the ionic intermediates by the nucleophile.