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Thin films derived by the deposition of charged microdroplets generated in the ESI source of a mass spectrometer act as highly concentrated reaction vessels in which the final products of an ion-molecule reaction can be isolated by their precipitation onto a solid surface under ambient conditions. In this study, the ESI Z-spray source supplied to a Q-TOF Ultima mass spectrometer was used to investigate the d-fructose acid-catalyzed reactions by microdroplets deposition onto a stainless-steel target surface. High conversion ratios of d-fructose into 5-hydroxymethylfuraldehyde (5-HMF), 5-methoxymethylfuraldehyde (5-MMF), and difructrose anhydrides (DFAs) were obtained with HCl and KHSO

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 reactivity of carbonate cluster ions with sulfur dioxide has been investigated in the gas phase by mass spectrometric techniques. SO2 promotes the displacement of carbon dioxide from carbonate clusters through a stepwise mechanism, leading to the quantitative conversion of the carbonate aggregates into the corresponding sulfite cluster ions. The kinetic study of the reactions of positive, negative, singly, and doubly charged ions reveals very fast and efficient processes for all the carbonate ions.

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.

N-heterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESI-MS) to the detection of charge-tagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the charge-tagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry.

The gas-phase ion chemistry of protonated 5-hydroxymethyl-2-furaldehyde (5-HMF), the main product of hexose sugar dehydration reaction, was investigated by means of mass spectrometric and theoretical calculations. Calculations at the B3LYP-D3/6-311++G** and the G3 (MP2, CCSD(T)) levels of theory indicated the carbonyl oxygen as the 5-HMF preferred protonation site. The Cooks’ “extended” kinetic method was used to measure the unknown proton affinity (PA) and gas-phase basicity (GB) of 5-HMF. The experimentally measured values, PA= 207.3 kcal mol−1 and GB = 200.0 kcal mol−1, were found to be in good agreement with those calculated by means of G3 (MP2, CCSD(T)) composite methods. Among the seven different 5-HMF optimized protomers, the theoretically predicted potential energy surface of [5-HMF]H+ ions indicated that the carbonyl oxygen protonated most stable species, as well as each one of the four ring-protonated less stable isomers, were trapped in deep potential wells and, at least in principle, could be formed from the gas phase acid-catalyzed hexose sugar decomposition.

In this work pentose sugar (D-xylose, D-ribose and D-arabinose) gas phase dehydration reaction was investigated by means of mass spectrometric techniques and theoretical calculations. The ionic species derived from the dehydration reaction of protonated D-ribose and D-arabinose were structurally characterized by their fragmentation patterns and the relative dehydration energies measured by energy resolved CAD mass spectra. The results were compared with those recently obtained for D-xylose in the same mass spectrometric experimental conditions. Dehydration of C1-OH protonated sugars was theoretically investigated at the CCSD(T)/cc-pVTZ//M11/6-311++G(2d,2p) level of theory. Protonated pentoses are not stable and promptly lose a water molecule giving rise to the dehydrated ions at m/z 133. D-xylose, D-ribose and D-arabinose dehydration follows a common reaction pathway with ionic intermediates and transition states characterized by similar structures. Slightly different dehydration energies were experimentally measured and the relative trend was theoretically confirmed. The overall dehydration activation energy follows the order arabinose < ribose < xylose. Gas-phase pentose sugar dehydration leads to the formation of protonated 2-furaldehyde as final product. Based on the experimental and theoretical evidence a new mechanistic hypothesis starting from C1-OH protonation was proposed.

The synthesis of cationic mono oxo MoIV PNP pincer complexes of the type [Mo(PNPMe-iPr)(O)X]+ (X = I, Br) from [Mo(PNPMe-iPr)(CO)X2] is described. These compounds are coordinatively unsaturated and feature a strong Mo≡O triple bond. The formation of these complexes proceeds via cationic 14e intermediates [Mo(PNPMe-iPr)(CO)X]+ and requires both molecular oxygen and water. ESI MS measurements with 18O labeled water (H2 18O) and molecular oxygen (18O2) indicates that water plays a crucial role in the formation of the Mo≡O bond. A plausible mechanism based on DFT calculations is provided. The X-ray structure of [Mo(PNPMe-iPr)(O)I]SbF6 is presented.

Unprecedented hydrolysis reactions are observed in the gas phase in clusters containing doubly-charged vanadium oxides and hydroxide anions, SO2 and H2O. The experimental and computational study shows the cooperative effects of the vanadium species, V2O62− and HV3O92−, and sulfur dioxide on the enhancement of the hydrogen bonds of water that allows the acid–base reaction and charge separation in the doubly-charged ion.

In this study, the ability of an unmodified ESI Z-spray source to carry out ambient ion soft landing (AISL) and microdroplet reactions (MR) was tested. To this aim, heme proteins were ionized and landed onto the working area of multi-walled carbon nanotubes electrodes. The voltammetric and spectroscopic analyses of the modified electrodes evidenced a strong interaction of unfolded structures of the heme proteins with the carbon nanotubes surface. Moreover, the possibility to use the ESI Z-spray source to study microdroplet reactions was attempted by exploring the accelerated acid-catalyzed decomposition of carbohydrates. The study demonstrated that a commercial ESI Z-spray source can provide successful modifications of surfaces by AISL, and represents a suitable platform for the study of accelerated microdroplet reactions.

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 gas-phase reactivity of doubly-charged vanadium hydroxides anions with SO2 has been studied by experimental and computational methods. The obtained results highlight the role of sulfur dioxide in promoting unprecedented bond-forming reactions, which produce singly-charged products by breaking the Vx Oy skeleton or a terminal V-O bond.

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.

In order to investigate the gas-phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan, we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6-31 + G(d,p) level of theory. The gaseous reactant species, the protonated AA, [C6 H8 O6 ]H+ , were generated by electrospray ionization of a 10-3 M H2 O/CH3 OH (1 : 1) AA solution. In order to structurally characterize the gaseous [C6 H8 O6 ]H+ ionic reactants, we estimated the proton affinity and the gas-phase basicity of AA by the extended Cooks's kinetic method and by computational methods at the B3LYP/6-31 + G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity of 875.0 ± 12 kJ mol-1 and protonation entropy ΔSp 108.9 ± 2 J mol-1 K-1 , a gas-phase basicity value of AA of 842.5 ± 12 kJ mol-1 at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations. Copyright © 2016 John Wiley & Sons, Ltd.

Furan derivatives, potentially carcinogenic to humans, can be formed, in addition to carbohydrates and other sources, from the degradation of ascorbic acid (AA). At present, the mechanisms involved in the ascorbic acid degradation are not yet fully understood. In this study, we reported a gas-phase investigation, performed using Triple Quadrupole (TQ/MS) and Ion Trap Mass Spectrometry (QIT/MS) together with quantum mechanical calculations at the B3LYP/6-31+G(d,p) level of theory, on the non-oxidative degradation mechanism of l-ascorbic acid (AA) to furan derivatives. Gaseous protonated ascorbic acid ions, the AAH+ ionic reagents, were generated by Electrospray Ionization (ESI) of an aqueous AA solution added with a mild protonation reagent. The Collisionally Induced Dissociation (CID) mass spectra of the AAH+ ions displayed gaseous fragment ions corresponding to the degradation intermediates and reaction products, which were structurally characterized and identified. Precise mechanistic insights were achieved by using l-[1-13C]-AA. On the basis of experimental and computational results, the gas phase non-oxidative acid catalyzed degradation mechanism of ascorbic acid is proposed, a benchmark point for the comprehension of the mechanism in the condensed phase.

Understanding on a molecular level the acid-catalysed decomposition of the sugar monomers from hemicellulose and cellulose (e.g. glucose, xylose), the main constituent of lignocellulosic biomass is very important to increase selectivity and reaction yields in solution, key steps for the development of a sustainable renewable industry. In this work we reported a gas-phase study performed by electrospray triple quadrupole mass spectrometry on the dehydration mechanism of d-glucose. In the gas phase, reactant ions corresponding to protonated d-glucose were obtained in the ESI source and were allowed to undergo collisionally activated decomposition (CAD) into the quadrupole collision cell. The CAD mass spectrum of protonated d-glucose is characterized by the presence of ionic dehydrated daughter ion (ionic intermediates and products), which were structurally characterized by their fragmentation patterns. In the gas phase d-glucose dehydration does not lead to the formation of protonated 5-hydroxymethyl-2-furaldehyde, but to a mixed population of m/z 127 isomeric ions. To elucidate the d-glucose dehydration mechanism, 3-O-methyl-d-glucose was also submitted to the mass spectrometric study; the results suggest that the C3 hydroxyl group plays a key role in the reaction mechanism. Furthermore, protonated levulinic acid was found to be formed from the monodehydrated d-glucose ionic intermediate, an alternative pathway other than the known route consisting of 5-hydroxymethyl-2-furaldehyde double hydration. Copyright © 2015 John Wiley & Sons, Ltd.

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.

Understanding on a molecular level the acid-catalysed decomposition of the sugar monomers from hemicellulose and cellulose (e.g. glucose, xylose), the main constituent of lignocellulosic biomass is very important to increase selectivity and reaction yields in solution, key steps for the development of a sustainable renewable industry. In this work we reported a gas-phase study performed by electrospray triple quadrupole mass spectrometry on the dehydration mechanism of D-glucose. In the gas phase, reactant ions corresponding to protonated D-glucose were obtained in the ESI source and were allowed to undergo collisionally activated decomposition (CAD) into the quadrupole collision cell. The CAD mass spectrum of protonated D-glucose is characterized by the presence of ionic dehydrated daughter ion (ionic intermediates and products), which were structurally characterized by their fragmentation patterns. In the gas phase D-glucose dehydration does not lead to the formation of protonated 5-hydroxymethyl-2-furaldehyde, but to a mixed population of m/z 127 isomeric ions. To elucidate the D-glucose dehydration mechanism, 3-O-methyl-D-glucose was also submitted to the mass spectrometric study; the results suggest that the C3 hydroxyl group plays a key role in the reaction mechanism. Furthermore, protonated levulinic acid was found to be formed from the monodehydrated D-glucose ionic intermediate, an alternative pathway other than the known route consisting of 5-hydroxymethyl-2-furaldehyde double hydration.

The new radical HONF has been detected in the gas phase by neutralization-reionization mass spectrometry (NRMS). The radical has been identified and directly characterized as a gaseous isolated species, having a lifetime of at least 1 microsecond and a linear cis-trans structure of H-O-N-F connectivity. Detection of this molecule, which is highly unstable towards the dissociation into HF and NO and kinetically sufficiently stable to be observed, represents an advance in the search for high-energy 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].

The oxidation of SO2 by nonmetal oxoanions in the gas phase is investigated in an experimental and theoretical study of the structure of the species involved and the reaction kinetics and mechanism. SO3 , SO3(.-) and SO4(.-) are efficiently produced by reaction of On XO(-) anions (X=Cl, Br, and I; n=0 and 1) with SO2 ; XO(-) ions mainly react to give SO3 by oxygen-atom transfer, whereas OXO(-) ions mainly give SO3(.-) by oxygen-anion transfer. On descending the halogen group from chlorine to iodine, the SO3 /SO3(.-) ratio decreases and increases for reactions involving XO(-) and OXO(-) anions, respectively, whereas the formation of SO4(.-) is particularly significant with OIO(-). Kinetic factors play a major role in the reactions of On XO(-), depending on the halogen atom and its oxidation state.

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.

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 .

Charged species structurally related to several isomers of Cl(2)O(2), of considerable importance in atmospheric chemistry, were obtained by chemical ionization (CI) and characterized by collisionally activated dissociation (CAD) mass spectrometry. The ClOClO(+) and [Cl(2)-O(2)](+) species were prepared by positive ion Cl(2)/CI of ClO(2) and O(2), respectively, whereas the ClClO(2)(+) isomer proved an elusive species of considerably lower stability. The ClClO(2)(-) anion was obtained from the negative ion Cl(2)/CI of ClO(2). The formation process, structure and stability of the ions are discussed in connection with available theoretical results and related to the recent preparation of (Cl(2)O(2))(+)SbF(6)(-) and (Cl(2)O(2))(+)Sb(2)F(11)(-) salts in the solid phase. Copyright 1999 John Wiley & Sons, Ltd.

The regio- and stereochemistry of the nucleophilic attack of (S)-trans-3-hexen-2-ol (MS) and (S)-trans-4-hexen-3-ol (ES) on the corresponding O-protonated (L = H) and -methylated (L = CH3) derivati...

It has long been assumed that ionizing neutral acetonitrile produces ions with the same atomic connectivity, CH(3)CN(+*). Recent calculations on the C(2)H(3)N(+*) potential energy surface have suggested that it may be difficult to generate pure CH(3)CN(+*) when ionizing acetonitrile. We have probed the interconversion of CH(3)CN(+*) and its lower energy isomer CH(2)CNH(+*) by calculation, collision-induced dissociation mass spectrometry and ion-molecule reaction. The latter ion, ionized ketenimine, is co-generated upon electron or chemical ionization of neutral acetonitrile in the ion source of a mass spectrometer. An estimate of the ratio of the two isomers can be obtained from their respective ion-molecule reactions with CO(2) or COS. CH(3)CN(+*) reacts by proton-transfer with CO(2) and charge transfer with COS, whereas CH(2)CNH(+*) is unreactive.

The activation of C-halogen bonds by the SO

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 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.

The Proton Affinity and Gas-Phase Basicity of Sulfur Dioxide