Sulfur Dioxide Oxidation by Zinc and Zinc Oxide Nitrate/Nitrite Complexes in the Gas Phase: An Interplay between Redox-Active Ligands and Metal.

Different oxidative pathways of sulfur dioxide promoted by ZnO(NO3 )2 - , Zn(NO3 )2 - and Zn(NO2 )(NO3 )- are revealed by a joint investigation by mass spectrometry and theoretical calculations. The reactions are triggered by [Zn2+ -O- ⋅]+ or by the low-valence Zn+ through oxygen ion transfer or electron transfer to SO2 , respectively. The NOx - ligands intervene in the oxidation only when sulfur dioxide is converted to SO3 - or SO2 - , leading to the formation of zinc sulfate and zinc sulfite coordinated to nitrate or nitrite anions. Kinetic analyses show that the reactions are fast and efficient, and theory discloses the elementary steps, namely oxygen ion transfer, oxygen atom transfer and electron transfer, occurring through similar energy landscapes for the three reactive anions.

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.

Kinetic Study of the Maillard Reaction in Thin Film Generated by Microdroplets Deposition.

The Maillard reaction kinetics in the confined volume of the thin film produced by ESI microdroplet deposition was studied by mass spectrometry. The almost exclusive formation of the Amadori product from the reaction of D-xylose and D-glucose toward L-glycine and L-lysine was demonstrated. The thin film Maillard reaction occurred at a mild synthetic condition under which the same process in solution was not observed. The comparison of the thin film kinetics with that of the reaction performed in solution showed strong thin film rate acceleration factors.

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.

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

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.

From ascorbic acid to furan derivatives: the gas phase acid catalyzed degradation of vitamin C.

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.

Vanadium Hydroxide Cluster Ions in the Gas Phase: Bond-Forming Reactions of Doubly-Charged Negative Ions by SO2 -Promoted V-O Activation.

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.

Iron-Promoted C-C Bond Formation in the Gas Phase.

An unusual iron transfer and carbon-carbon coupling take place in gas-phase ionized mixtures containing ferrocene and dichloromethane. Ferrous chloride and the protonated benzenium ion are eventually formed by a thermal and efficient reaction, through stable intermediates that undergo a remarkable reorganization. The mechanism of the concerted iron extrusion, carbon-chlorine bond activation and carbon-carbon bond formation is elucidated by electronic structure calculations that show the crucial role of iron.

The oxidation of sulfur dioxide by single and double oxygen transfer paths.

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.

Stereoselectivity in electrosprayed confined volumes: asymmetric synthesis of warfarin by diamine organocatalysts in microdroplets and thin films.

The asymmetric synthesis of warfarin in microdroplets and thin films generated by an electrospray ionization (ESI) source is reported. This is one of the first examples of an enantioselective organocatalyzed reaction in electrosprayed confined volumes. The optimal conditions in terms of system setting were established for this reaction.

Photo-activated Carbon dots (CDs) as Catalysts in the Knoevenagel Condensation: A Mechanistic Study by Dual-Mode Monitoring via ESI-MS.

Carbon dots (CDs) obtained from 5-(hydroxymethyl)furfural (5-HMF) were activated by a 365 nm-UV irradiation source and employed in the Knoevenagel condensation to investigate their photocatalytic mechanism. To this end, electrospray ionization mass spectrometry (ESI-MS) was used to monitor the time progress of the condensation and follow the formation of the final product in positive and negative ion modes at once. The intervention of the superoxide radical anion in the photocatalytic mechanism of CDs was highlighted.

In-Depth Chemical Characterization of Punica granatum L. Seed Oil.

Fruit seeds belonging to the pomegranate cultivar "Granata" were subjected to extraction and oily component analysis, with the aim of obtaining information about their composition. The presence of conjugated isomers of linolenic acid (CLNA isomers) in the oily phase extracted from the seeds gives a high added value to this part of the fruit, which is too often considered and treated as waste. The separated seeds were subjected to a classic Soxhlet extraction with n-hexane or extraction with supercritical CO2, assisted by ethanol. The resulting oils were evaluated by 1H and 13C-NMR and AP-MALDI-MS techniques. Differences in the triacylglycerols composition, with particular regard to punicic acid and other CLNA content, were studied in depth. Results showed the prevalence of punicic acid in the triacylglycerol mixture up to the 75%, with clear preponderance in the extract by supercritical fluids. Consequently, other CLNA isomers are, altogether, two-fold less represented in the supercritical extract than in the Soxhlet one. The two oily residues were subjected to solid phase extraction (SPE) and to HPLC-DAD analysis for the polyphenolic isolation and characterization. In addition to HPLC analysis, which showed different content and composition, DPPH analysis to evaluate the antiradical potential showed that the extract obtained with supercritical CO2 was much more active.

Sulfuric Acid Catalyzed Esterification of Amino Acids in Thin Film.

The esterification reaction of different amino acids with methanol catalyzed by H2SO4 was first studied in the small volume of thin films generated by ESI microdroplet deposition. The reaction is promoted by the pneumatic spray of the ESI source and reaches its maximum efficiency at a thin film temperature of 70 °C. Selective esterification of the COOH moiety was demonstrated. Microdroplet size and thin film volume and lifetime are critical parameters that influenced the reaction outcome. As expected, l-tyrosine and l-phenylalanine having aromatic side chain substituents were the most reactive amino acids, reaching absolute yields of around 40-50%. The amino acid esterification catalyzed by H2SO4 in a thin film occurs under synthetic conditions in which the same reaction in the bulk is not observed.

Accelerated d-Fructose Acid-Catalyzed Reactions in Thin Films Formed by Charged Microdroplets Deposition.

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 KHSO4 as metal-free catalysts by using synthetic conditions under which the same products in bulk are not formed. Furthermore, the reaction outcome was found to be highly sensitive to the catalyst and the solvent employed as well as to the ESI source parameters influencing the thin film formation from microdroplets deposition onto the solid surface.

Charge-Tagged N-Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC-Catalysed Reactions through Electrospray Ionization Mass Spectrometry.

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.

Gas-Phase Reactivity of Carbonate Ions with Sulfur Dioxide: an Experimental Study of Clusters Reactions.

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.

Base-Assisted Conversion of Protonated d-Fructose to 5-HMF: Searching for Gas-Phase Green Models.

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.

Vitamin C: an experimental and theoretical study on the gas-phase structure and ion energetics of protonated ascorbic acid.

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.

Characterization of naproxen-polymer conjugates for drug-delivery.

The synthesis and the characterization of three new naproxen decorated polymers are described. A versatile and general approach is employed to link the drug to polymers, affording the derivatives with a very high degree of purity. The release of the drug from the conjugates proved to be exceptionally slow, even in acidic aqueous media, and the kinetic of the process seems to be triggered by their solubility in water. On the other hand, the interesting outcome of the first ex vivo drug release experiments on human blood samples makes this preliminary study valuable for future investigations on the use of these polymeric prodrugs in in vivo treatment of inflammatory states.

A mass spectrometric study of the acid-catalysed d-fructose dehydration in the gas phase.

5-hydroxymethylfuraldehyde (5-HMF) and simpler compounds, such as levulinic acid (LA) and glyceraldehyde, are platform molecules produced by the thermal acid-catalyzed dehydration of carbohydrates coming from biomass. Understanding sugar degradation pathways on a molecular level is necessary to increase selectivity, reduce degradation by-products yields and optimize catalytic strategies, fundamental knowledge for the development of a sustainable renewable industry. In this work gaseous protonated d-fructose ions, generated in the ESI source of a triple quadrupole mass spectrometer, were allowed to undergo Collisionally Activated Decomposition (CAD) into the quadrupole collision cell. The ionic intermediates and products derived from protonated d-fructose dehydration were structurally characterized by their fragmentation patterns and the relative water-loss dehydration energies measured by energy-resolved CAD mass spectra. The data were compared with those obtained from protonated d-glucose decomposition in the same experimental conditions. In the gas phase, d-fructose dehydration leads to the formation of a mixed population of isomeric [C6H6O3]H(+) ions, whose structures do not correspond exclusively to 5-hydroxymethyl-2-furaldehyde protonated at the more basic aldehydic group.