Giulia de Petris

Methane Activation by Metal-Free Radical Cations : Experimental Insight into the Reaction Intermediate

A precise jab to methane: The SO(2)(*+) radical cation (see figure) effectively activates CH(4) at room temperature through a [H(3)C(*)...HOSO(+)] methyl intermediate isolated in the gas phase by mass spectrometry. Methanol and ionized methyl hydrogen sulfoxylate, CH(3)OSOH(*+), are formed by selective, direct attack of the incipient methyl radical at the O atom of the intermediate. The reaction shows radical and charge effects in the activation of methane by metal-free radical cations.

Experimental Detection of Tetraoxygen

Although suggested by Lewis in 1924 and theoretically predicted, O4 has so far proved extremely elusive, defying all attempts at the positive experimental detection of a bound, intact O4 species. The search has now been brought to an end by the conclusive proof, achieved by neutralization-re-ionization mass spectrometry, of the existence of intact O4 as a gas-phase species with a lifetime in excess of 1 μs, whose dissociation requires overcoming a barrier of the order of 10 kcal mol-1 .

Double CH activation of ethane by metal-free SO2.+ radical cations

The room-temperature C-H activation of ethane by metal-free SO(2)(*+) radical cations has been investigated under different pressure regimes by mass spectrometric techniques. The major reaction channel is the conversion of ethane to ethylene accompanied by the formation of H(2)SO(2)(*+), the radical cation of sulfoxylic acid. The mechanism of the double C-H activation, in the absence of the single activation product HSO(2)(+), is elucidated by kinetic studies and quantum chemical calculations. Under near single-collision conditions the reaction occurs with rate constant k=1.0 x 10(-9) (+/-30%) cm(3) s(-1) molecule(-1), efficiency=90%, kinetic isotope effect k(H)/k(D)=1.1, and partial H/D scrambling. The theoretical analysis shows that the interaction of SO(2)(*+) with ethane through an oxygen atom directly leads to the C-H activation intermediate. The interaction through sulfur leads to an encounter complex that rapidly converts to the same intermediate. The double C-H activation occurs by a reaction path that lies below the reactants and involves intermediates separated by very low energy barriers, which include a complex of the ethyl cation suitable to undergo H/D scrambling. Key issues in the observed reactivity are electron-transfer processes, in which a crucial role is played by geometrical constraints. The work shows how mechanistic details disclosed by the reactions of metal-free electrophiles may contribute to the current understanding of the C-H activation of ethane.

Ionic Lewis superacids in the gas phase. Part 1. Ionic intermediates from the attack of gaseous SiF+3 on n-bases

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.

Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO].+ and [SiO].+: Why Do They Differ?

The reactivity of the two diatomic congeneric systems [CO](·+) and [SiO](·+) towards methane has been investigated by means of mass spectrometry and quantum-chemical calculations. While [CO](·+) gives rise to three different reaction channels, [SiO](·+) reacts only by hydrogen-atom transfer (HAT) from methane under thermal conditions. A theoretical analysis of the respective HAT processes reveals two distinctly different mechanistic pathways for [CO](·+) and [SiO](·+), and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second-row p element.

Bile salt aggregates in the gas phase: an electrospray ionization mass spectrometric study.

Helical and ordered structures have previously been identified by X-ray diffraction analysis in crystals and fibers of bile salts, and proposed as models of the micellar aggregates formed by trimeric or dimeric units of dihydroxy and trihydroxy salts, respectively. These models were supported by the results of studies of micellar bile salt solutions performed with different experimental techniques. The study has now been extended to the gas phase by utilizing electrospray ionization mass spectrometry (ESIMS) to investigate the formation and the composition of aggregates stabilized by noncovalent interactions, including polar (ion-ion, ion-dipole, dipole-dipole, hydrogen bonding etc.) and apolar (van der Waals and repulsive) interactions. The positive and negative ESIMS spectra of sodium glycodeoxycholate (NaGDC), taurodeoxycholate (NaTDC), glycocholate (NaGC), and taurocholate (NaTC) aqueous solutions, recorded under different experimental conditions, show in the first place that aggregates analogous to those present in micellar solutions do also exist in the gas phase. Furthermore, consistently with the condensed-phase model, the positive-ion spectra show that the trimers are the most stable oligomers among the aggregates of dihydroxy salts (NaGDC and NaTDC) whilst the dimers are the most stable among the aggregates of trihydroxy salts (NaGC and NaTC). Moreover, the binding energy of the constituent glycocholate salt units in most gaseous oligomers exceeds that of the corresponding taurocholate units. The ESIMS evidence has been confirmed by vapor-pressure measurements performed on NaGC and NaTC crystals and NaGDC and NaTDC fibers, the results of which show that the evaporation enthalpy of glycocholate exceeds that of taurocholate by some 50 kJ mol(-1).

Charged and neutral NO3 isomers from the ionization of NOx and O3 mixtures.

Mass spectrometric techniques have been utilized in conjunction with theoretical methods to detect and characterize new species formed upon ionization of gaseous mixtures containing ozone and an NOx oxide. NO5+ as well as isomeric NO4+ and NO3+ ions have been identified. Moreover, utilization of neutralization reionization mass spectrometry (NRMS) has provided strong evidence for, if not a conclusive demonstration of, the existence of a new NO3 isomer, in addition to the long-known trigonal radical, as a gaseous species with a lifetime in excess of approximately 1 microsecond.

The intermolecular potential in NO–N2 and (NO–N2)+ systems: implications for the neutralization of ionic molecular aggregates

The characterization of the non covalent interaction potential, responsible for the intermolecular bond in NO-N(2) and (NO-N(2))(+) molecular aggregates, has been achieved by coupling the predictions of a semiempirical method with the results of a scattering experiment and ab initio calculations. The potential wells for the most stable configurations of the neutral and ionic state, having approximatively a T shape in both cases, fall in the same intermolecular distance range. In addition, in the ionic state, the charge is completely localized on the NO partner. Important implications on the dynamics of the neutralization process, occurring as a vertical transition from ionic to neutral state, are obtained by exploiting the analytical formulation of the interaction and calculating energy spacings and relevant Franck-Condon factors for both intramolecular and intermolecular vibration modes.

Gas-phase reactions of nitronium ions with acetylene and ethylene: an experimental and theoretical study

A comparative study of the gas-phase reactions of NO2+ with acetylene and ethylene was performed by using FT-ICR, MIKE, CAD, and NfR/ CA mass spectrometric techniques, in conjunction with ab initio calculations at the MP2/6-31+G* level of theory. Both reactions proceed according to the same mechanism, that is, 1,3-dipolar cycloaddition, but yield products of different stability. The C2H2NO2+ adduct from acetylene has an aromatic character and hence is highly stabilized with respect to the C2H4NO2+ adduct from ethylene. Both cycloadducts tend to isomerize into O-nitroso derivatives, that is, nitrosated ketene and nitrosated acetaldehyde, which represent the thermodynamically most stable products from the addition of NO2+ to acetylene and ethylene, respectively. As prototypal examples of the reactivity of free nitronium ions with most simple pi systems, the reactions investigated are useful starting points to model the mechanism of aromatic nitration.

Mass spectrometric study of simple main group molecules and ions important in atmospheric processes

Abstract 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.