Intracluster Sulphur Dioxide Oxidation by Sodium Chlorite Anions: A Mass Spectrometric Study

The Peroxymonocarbonate Anion HCO4- as an Effective Oxidant in the Gas Phase: A Mass Spectrometric and Theoretical Study on the Reaction with SO2

The peroxymonocarbonate anion, HCO4-, the covalent adduct between the carbon dioxide and hydrogen peroxide anion, effectively reacts with SO2 in the gas phase following three oxidative routes. Mass spectrometric and electronic structure calculations show that sulphur dioxide is oxidised through a common intermediate to the hydrogen sulphate anion, sulphur trioxide, and sulphur trioxide anion as primary products through formal HO2-, oxygen atom, and oxygen ion transfers. The hydrogen sulphite anion is also formed as a secondary product from the oxygen atom transfer path. The uncommon nucleophilic behaviour of HCO4- is disclosed by the Lewis acidic properties of SO2, an amphiphilic molecule that forms intermediates with characteristic and diagnostic geometries with peroxymonocarbonate.

Free N-heterocyclic carbenes from Brønsted acidic ionic liquids: Direct detection by electrospray ionization mass spectrometry

RATIONALE

The occurrence of N-heterocyclic carbenes in imidazolium-based ionic liquids has long been discussed, but no spectroscopic evidence has been reported yet due to their transient nature. The insertion of an ionizable acid group into the cation scaffold of an ionic liquid which acts as a charge tag allows for the direct detection of free carbenes by mass spectrometry.

METHODS

Three different Brønsted acidic ionic liquids were synthesized: 1-methyl-3-carboxymethylimidazolium chloride (MAICl), 1-methyl-3-carboxymethylimidazolium acetate (MAIAc) and the corresponding 2-(3-methyl-1H-imidazol-3-ium-1-yl)acetate zwitterion (MAI - H). The speciation of these compounds was then analysed by electrospray ionization ion-trap mass spectrometry in the negative ion mode.

RESULTS

The C2-H deprotonation of the imidazolium cation leading to the formation of the corresponding carbene is highly affected by the basic properties of the counter-anion. In the case of MAICl and MAI - H ionic liquids, no charged species corresponding to the free N-heterocyclic carbene was detected. On the contrary, in the presence of a sufficiently basic anion, such as acetate of MAIAc ionic liquid, an intense signal related to the free carbenic species was observed without the addition of an external base.

CONCLUSIONS

In situ formation of free N-heterocyclic carbenes from Brønsted acidic ionic liquids was demonstrated, highlighting the crucial role of anion basicity in promoting the C2-H proton abstraction from imidazolium cations with a carboxylic side chain.

Regioselective Bond-Forming and Hydrolysis Reactions of Doubly Charged Vanadium Oxide Anions in the Gas Phase

The gas-phase reactivity of vanadium-containing dianions, NaV3O92− and its hydrated form H2NaV3O102−, were probed towards sulphur dioxide at room temperature by ion-molecule reaction (IMR) experiments in the collision cell of an ion trap mass spectrometer. The sequential addition of two SO2 molecules to the NaV3O92− dianion leads to the breakage of the stable V3O9 backbone, resulting in a charge separation process with the formation of new V-O and S-O bonds. On the contrary, the H2NaV3O102− hydroxide species reacts with SO2, promoting regioselective hydrolysis and bond-forming processes, the latter similar to that observed for the NaV3O92− reactant anion. Kinetic analysis shows that these reactions are fast and efficient with rate constants of the 10−9 (±30) cm3 s−1 molecule−1 order of magnitude.