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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.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.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.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.

The reactivity of [NaL·ClO2]− cluster anions (L = ClOx−; x = 0–3) with sulphur dioxide has been investigated in the gas phase by ion–molecule reaction experiments (IMR) performed in an in-house modified Ion Trap mass spectrometer (IT-MS). The kinetic analysis revealed that SO2 is efficiently oxidised by oxygen-atom (OAT), oxygen-ion (OIT) and double oxygen transfer (DOT) reactions. The main difference from the previously investigated free reactive ClO2− is the occurrence of intracluster OIT and DOT processes, which are mediated by the different ligands of the chlorite anion. This gas-phase study highlights the importance of studying the intrinsic properties of simple reacting species, with the aim of elucidating the elementary steps of complex processes occurring in solution, such as the oxidation of sulphur dioxide.

A gas‐phase investigation of the D‐fructose dehydration reaction in the presence of base has been performed by the joint application of mass spectrometric techniques and theoretical calculations. Protonated addition products of D‐fructose and base were generated in the gas phase by electrospray ionization using several bases of different proton affinity. The intermediates, products and decomposition channels were investigated by ion trap mass spectrometry. Electronic structure calculations allowed the identification of the ionic intermediates and products of a selected system containing NH3, helping to rationalize the observed reaction pathways. The obtained results show that the final product, the protonated 5‐hydroxymethyl‐2‐furaldheyde [5‐HMF]H+, is better formed using selected bases and only if these remain clustered until the end of the dehydration process.

The creation of carbon–carbon bonds is still a highly required topic in organic chemistry for the capacity to give a wide variety of new products of industrial value. Accordingly, the Knoevenagel condensation between activated methylene compounds and aldehydes is one of the most known carbon coupling reactions. In this work, the catalytic activity of imidazolium-based ionic liquids (1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium chloride, 1-methyl-3-carboxymethylimidazolium chloride) was studied under solvent-free conditions and compared with that of sodium salts (sodium chloride, sodium acetate). Mass spectrometric techniques were used to monitor the formation of the reaction products and to detect the key intermediates of the process. Based on the catalyst employed different reaction mechanisms were highlighted, thus laying the foundation for the design of more specific and efficient catalysts.

Unprecedented redox reactions of chromium oxide anions have been observed in the gas phase and investigated by mass spectrometric techniques and electronic structure calculations. The effective oxidizing and reducing power of chromium oxide anions is unravelled by sulfur dioxide: CrO4 − and CrO2 − generate CrO3 − as the reduction and oxidation product, respectively, together with SO3 and SO as the oxidation and reduction products of SO2, respectively. Computations predict that the reactions of CrO4 − and CrO2 − proceed through rather stable ion-dipole complexes and that CrO4 - produces also a strongly bound [CrSO6] - species.

Fulminates containing the CNO(-) ion have been widely utilized as high-energy density materials (HEDMs) for more than 120 years. Yet no purely covalently bound CNO molecule, i.e., nitrile oxide, is known to behave as an HEDM. In this study, we performed a thorough investigation of the potential energy surface of nitrile oxide ONCNO and related isomers, applying various sophisticated methods including G4, CBS-QB3, W1BD, CCSD(T)/CBS, and CASPT2/CBS. The Gibbs free energy calculations showed that the decomposition of ONCNO to the considerably endothermic products CNO + NO is favored compared to that into the highly exothermic products CO2 + N2. Thus, ONCNO fails to be the long expected nitrile oxide HEDM. However, with the rate-determining barrier of 23.3 kcal mol(-1) at the W1BD level, ONCNO should be experimentally accessible.

The new radical FBNCO and the new cation FBNCO+, containing all the p-block elements, have been identified in the gas phase, as species having a dissociation energy of 50 and 161 kcal mol−1, respectively.

The activation of C-halogen bonds by the SO.+ radical cation is investigated in the gas phase. The CCl bond of CH2 Cl2 is activated by fast and effective chloride-ion transfer, whereas the CF bond of CH2 F2 is activated by a very slow fluoride-ion transfer. In both cases, thermodynamic and kinetic factors account for the type of process and the observed selectivity, that is, carbon-halogen versus carbon-hydrogen activation, mainly owing to the presence of the sulfur atom. The cleavage of the CCl bond of CH2 Cl2 by SO.+ is a rare example of a very effective and selective chloride-ion abstraction by a metal-free, sulfur-centred radical cation.

The H(.)-atom transfer (HAT) reaction is investigated in the gas phase, starting from two different entrance channels, O(2)(.+)/CH(2)X(2) and CH(2)X(2)(.+)/O(2) (X=F, Cl), that correspond to a step of hydride transfer and to HAT, respectively. Analysis of the spin and charge along the reaction pathway shows that HAT occurs through the same reacting configuration, irrespective of whether the reactants are formed within the complex or are free isolated species.

The room-temperature C-H activation of ethane by metal-free SO2 center dot+ 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 H2SO2 center dot+ the radical cation of sulfoxylic acid. The mechanism of the double C-H activation, in the absence of the single activation product HSO2+, 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 SO2 center dot+ 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.

The new radical HS(4) containing three sulfur-sulfur bonds has been detected in the gas phase by mass spectrometric experiments. Structures and energies of HS(4) in the three oxidation states +/0/- have been investigated by ab initio calculations. HS(4) is characterized by the HSSSS open-chain structure, and it is stable toward dissociation into S(2) and HS(2) by 23 kcal mol(-1). The first detection of HS(4)(-), the conjugate base of the strong acid H(2)S(4) is also reported.

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.

A novel O-H bond activation reaction is reported: thermal SO2 center dot+ radical cations activate water in the gas phase forming OH center dot radicals with 100% efficiency.

The O-exchange reaction between CO2+ and CO is triggered by two different entrance channels in 13C-labeled systems. The unimolecular dissociation of the C13CO3(+) reactive intermediate is investigated at varying energies by mass spectrometric techniques. An almost "one-way" O-exchange at energies close to the dissociation threshold discloses a remarkably large intramolecular 12C/13C kinetic isotope effect ranging from 6.0 to 7.3 +/- 0.5.

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.

The HS3 radical and the HS3(-) anion, the sulfur analogues of HO3 and HO3(-), have been detected for the first time by mass spectrometric experiments performed in the gas phase. The structural and energetic features of HS3 and HS3(-) have been investigated by ab initio calculations. Both HS3 and HS3(-) are characterized by HSSS open-chain structures, stable toward the dissociation into S2 and HS(0/-). HS3 adds to HS and HS2 as the known HS n species, and HS3(-) is the conjugate base of a strong Brønsted acid, the trisulfane HSSSH.

A hitherto unknown, isotope-exchange reaction is studied in ionized gaseous mixtures containing disulfur monoxide and water. The kinetics, mechanism, and intermediate of the reaction are investigated by experimental and theoretical methods. The reactivity of the S(2)O(*+) cation with water is investigated under a wide range of pressures ranging from 10(-7) to 10(-4) Torr, by FT-ICR, TQ, and high-resolution CAD mass spectrometry. In the high-pressure limit the reaction proves to be a route to strongly bound sulfur-containing species.

A theoretical study performed at B3LYP and CCSD(T) level of theory of the reaction of formation of S3O+ in ionized carbonyl sulphide/disulphur monoxide mixtures is reported here. The interaction of carbonyl sulphide with disulphur monoxide cation leads to three different [CS3O2]+ isomers, essentially formed by a S3O+ cation solvated by a CO molecule. The S3O+ cation is present in the three structures cis, trans and ring, showed also by the isolated S3O+ species. The complex with the ring S3O+ unit is the most stable, but a direct path to its formation starting from OCS and S2O+ has not been found in the potential energy surface of [CS3O2]+. This species can be obtained from the isomerization of the complexes containing the cis or trans S3O+ unit, overcoming relatively low barrier heights.

The oxywater cation H2OO+ was generated in the gas phase by ionization of mixtures containing O3, O2 and H2O. Different ionic mixtures, rich in either the H2OO+ or the HOOH+ isomer, were obtained by O3/O2/H2O–O2/H2O CI, and by CH3OH/O2–CH2O/O2 CI, respectively. The H2O2+ ions were characterized by high-resolution CAD mass spectrometry, and structural and energetic effects were examined.

The first experimental detection of CS(2)OH is reported. CS(2)OH was observed for about one microsecond after its formation, as an intact isolated species in the gas phase. It was generated by electron transfer to the CS(2)OH(+) ion, prepared in the source of a multisector mass spectrometer by suitable ion-molecule reactions. The vertical formation process allowed characterization of CS(2)OH by structural analysis of CS(2)OH(+). Theoretical calculations were performed at the B3LYP/6-311+G(2d,p) and CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,p) levels of theory. The computed structure and stability of CS(2)OH and CS(2)OH(+) as well as the energetics of the involved processes satisfactorily fit with the experimental results.

The N3+ reactivity with SO2, N2O, CO2 and CO is studied by mass spectrometric techniques under a wide range of pressures ranging from 10-7 to 10-4 Torr. The N+ transfer features the N3+ reactivity with these simple inorganic molecules. The kinetics, reaction mechanism and the role of vibrationally excited ions are investigated by experimental and theoretical methods. Key distinguishing features of the N3+ reactivity are evidenced by comparison to N+ and N2+ ions, that mainly undergo charge exchange reactions. The N+ transfer to SO2 prompts formation of NO+ ions and neutral oxides NO and SO. The N+ transfer to N2O also leads to NO+ by a process not allowed by spin conservation rules. In both cases no reaction intermediate is detected, whereas CO2 and CO are captured to form the very stable NCO2+ and NCO+ ions. NCO2+ ions are characterized for the first time as strongly bound triplet ions of NOCO and ONCO connectivity. DFT and CCSD(T) computations have been carried out to investigate the structural and energetic features of the NCO2+ species and their formation process.

An ionic route to the CS2 degradation was studied by mass spectrometric and theoretical methods. Ionized mixtures containing carbon disulfide and ozone prove an effective source of S2+andSO+ ions, formed by remarkable molecular reorganization and open to further reactions with ozone and surrounding species. Both the O-transfer and dissociative oxidation processes, typical of the O3 reactivity in ionized mixtures, were observed.

A hitherto unknown, atmospherically relevant, isotope-exchange reaction was studied in ionised gaseous mixtures containing carbon dioxide and monoxide. The mechanism of the O exchange, proceeding over a double-minimum potential-energy surface, was positively established by mass spectrometric and theoretical methods that also allowed the identification and characterisation of the C2O3+ intermediate. The increase of internal energy displaces the observed reactivity towards an endothermic reaction path that involves only CO2 and represents an indirect route to the dissociation of carbon dioxide.

Two N2O2 isomers containing N2/O2 and NO/NO subunits, respectively, were detected by neutralization-reionization mass spectrometry (NRMS) as metastable species with lifetimes exceeding 1 µs.

I. Introduction 251 II. Positive-Ion Chemistry of Ozone 253 A. The Reactivity of O3+ and O3 254 A. Ozone and Oxygen 256 C. Ozone and Nitrogen 257 D. Ozone and Carbonyl Sulfide 258 E. Ozone and HFCFs–HFCs 260 F. Ozone and NOx 262 III. Links Between Neutral and Ionic Atmospheric Chemistry 265 A. An Ionic Route to N2O 265 B. An Ionic Route to O3 267 C. A New Sulfur Oxide 269 References 269 The importance of ionic processes that occur in terrestrial, planetary, and stellar atmospheres is receiving increasing recognition. Actually, ions play important, often crucial, roles in a variety of atmospheric processes throughout the universe, and a strong link with the neutral chemistry is also apparent. In the terrestrial atmosphere, the ionic reactions are most relevant in those transient and fleeting events, e.g., lightning, coronas (in thunderstorm clouds and along power lines), where the local ion density is much higher than in unperturbed air, and the chemical systems are typically far from equilibrium. In such cases, ozone, a key molecule for the terrestrial atmosphere, is also present in high local concentrations; it is formed from O2 by the same transient event. Accordingly, this review provides a survey of the positive ion chemistry of ozone with several of the most important “atmospheric” species: the reactions, the products, and the importance of the examined processes are discussed also in the light of the local thermodynamic disequilibrium (LTD) approach to the chemistry of transient atmospheric events. In all such studies, mass spectrometry is traditionally, and remains today, the experimental technique of choice. The novel application of mass spectrometry to the study of neutral species (NRMS), highly successful for the preparation and positive detection of long-sought, otherwise inaccessible, short-lived neutrals, makes mass spectrometry the most powerful tool now available for the study of the species and processes that are relevant to atmospheric chemistry. Selected examples of the interlink between the neutral and the ionic chemistry are also illustrated. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 22:251–271, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mas.10053

A new instrumental arrangement dedicated to the study of ion-molecule reactions and consists of by an electron impact/chemical ionization source mounted 420 mm away from an ion trap is described. It has been designed and developed to exclude diffusion into the ion trap of the neutral reactants from the EI/CI source and so to avoid undesired side-reactions. Its instrumental parameterization is described and some preliminary results are illustrated that show promise for the successful application of the system to the study of ion-molecule reactions.

The course of the reaction of electronically and vibronically excited metastable O(2) (+)((4)Pi(u), nu') ions with O(2), known to produce O(3) (+), was examined by the joint application of computational and mass spectrometric methods. The results show that the reaction does not proceed by a direct mechanism and that it involves instead the intermediacy of the [O(2) (+)((4)Pi(u)) x O(2)] and [O(3) (+)((4)A(2)) x O] complexes, both theoretically characterized, and the latter one positively identified by structurally diagnostic mass spectrometric techniques. The reaction is a potential source of stratospheric ozone, in that O(3) (+) ions are known to undergo efficient charge exchange with oxygen to yield neutral O(3).

Recent mass spectrometric studies of species and processes relevant to the chemistry of atmospheres are reported. New species have been detected, including HO(3); a stratospheric reservoir of OH radicals; the [H(2)O(+)O(2)(-)] charge transfer complex, central to the atmospheric photonucleation theory; and the OSOSO oxide and its cation, likely present in the Io's atmosphere. As to ionic processes, a new route to tropospheric N(2)O in air ionized by lightning and coronas is reported, as well as the complex chemistry promoted by ionization of ozone/freon and ozone/carbonyl sulfide mixtures and the formation of O(5)(+), relevant to the problem of (18)O excess in stratospheric ozone.

Gaseous ionized carbonyl sulfide/ozone mixtures have been studied by mass spectrometric techniques and theoretical calculations. The evidence obtained points to this system as an effective route to the SO+ ion, which in turn promotes a rich ion chemistry leading to SnO+ (n = 2−5) and Sn+ (n = 2−9) ions via oligomerization processes.

The gas-phase reaction of SO+ with SO2, relevant to the atmospheric chemistry of Io, was investigated with mass spectrometric and computational methods. Consistent with previous reports, no net chemical change was observed at 10-8−10-7 Torr by FT-ICR spectrometry. However, formation of a transient S2O3+ adduct of OSOSO connectivity, a, was suggested by the fast (k = 6.0 ± 1.5 × 10-10 cm-3 s-1 molec-1) 34SO+/32SO2 isotope exchange. The adduct was directly observed in SO2/CI experiments, and structurally probed by MIKE and CAD spectrometry, the results of which are also consistent with connectivity a. Computational results at the B3LYP/6-311+G(2d) level of theory, complemented by single-point CCSD(T) calculations, confirmed that a planar S2O3+ ion of connectivity a is more stable at 298 K than the isomer b of connectivity OSSO2 by 23.4 kcal mol-1 at the CCSD(T) level of theory. NR spectrometry of S2O3+ allowed detection of a hitherto unknown sulfur oxide, S2O3, also of OSOSO connectivity, characterized as a...

No Abstract

Complexes that undergo extensive molecular reorganization upon unimolecular metastable dissociation are obtained after ionization of atmospheric gases containing ozone and chlorofluorocarbons [Eq. (1)]. These unusual processes, which require fission of all bonds initially present in the CHX(2) unit (X=Cl, F) and combination of C with one of the O atoms of ozone, were demonstrated and their mechanism rationalized by the joint application of mass spectrometric and theoretical methods.

FT-ICR (Fourier Transform Ion Cyclotron Resonance) experiments showed that the reaction of Cl2H+ with H2 (rate coefficient 6.3±1.0 cm3 s−1 molecule−1 at 298 K) yields ClH2+ ions, identified as protonated hydrochloric acid by CAD (Collisionally Activated Decomposition) mass spectrometry. Calculations performed at the B3LYP and CCSD(T) levels of theory show that the reaction involves insertion of the terminal Cl atom of Cl2H+ into the H–H bond, which accounts for the experimental observation that ClD2+ but not ClHD+ is formed from the reaction of Cl2H+ with D2. The same Cl+ insertion mechanism characterizes the reaction of Cl2H+ with CH4, whose product was identified as protonated methyl chloride, CH3ClH+, by CAD mass spectrometry.

An intracluster proton-promoted reaction leading to a ligand switching in mixed NO 2 + complexes of nitric acid and its derivatives is described. The proton affinity and the NO 2 + binding energy are demonstrated to be the factors affecting the process, the trend of which is shown to reflect the peculiar properties of nitric acid and its derivatives.

The chemistry of the title cations was investigated in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and by theoretical methods. Cl2H+ (1) was obtained from the protonation of Cl2 by gaseous Bronsted acids and the chlorination of HCl by Cl2F+ and XeCl+. The proton affinity (PA) of Cl2 measured by the FT-ICR equilibrium method is 131.4 ± 1 kcal mol-1 at 298 K, which compares with the 132.0 kcal mol-1 value computed at the CCSD(T) level of theory. 1 has the Cl−Cl−H connectivity, as indicated by its 2-fold reactivity as a Bronsted acid and a chlorinating agent, and confirmed by the theoretical results. The FClH+ cation was obtained from the fluorination of HCl by XeF+ and its most stable protomer (2) has the F−Cl−H connectivity, as shown by its behavior as a pure Bronsted acid and by the theoretical results that identify 2 as the global minimum, whereas protomer 3, having the H−F−Cl connectivity, is computed to be less stable by 5.0 kcal mol-1 at the CCSD(T) level of t...

H 4 PO + 3 ions have been generated in a mass spectrometer by proton-transfer to H 3 PO 3 from different Bronsted acids. The proton affinity of H 3 PO 3 has been estimated by bracketing and kinetic methods to be 198.6 ± 2 kcal mol −1 . Gaseous H 4 PO + 3 ions have been structurally assessed by metastable ion kinetic energy (MIKE) and collisionally induced dissociation (CID) mass spectrometry leading to the detection of a single isomeric species. The chemistry of H 2 PO − 3 is characterized by facile addition-elimination reactions leading to formation of polyanions. Species containing up to six P atoms have been detected.

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.

The proton affinity (PA) of hydrazoic acid has been reevaluated with experimental and ab initio methods in order to resolve the disturbing discrepancy between experiment and theory introduced by a recent computational study performed at the CISD (fc)/6-311G(df,p)//MP2(full)/6-31G * + ΔZPVE (MP2(full)/6-31G * ) level of theory, giving a value, 186.4 kcal mol -1 at 0 K, corresponding to ca 187.9 kcal mol -1 at 298 K, much higher than our earlier experimenal value of 176.6 kcal mol -1 at 298 K