Andreina Ricci

Ionic Fluorination of Carbon Monoxide as a Route to Gasphase Carbonylation of Inert CH and NH Bonds

Gaseous FCO+ ions from the ionization of mixtures of nitrogen trifluoride and carbon monoxide execute selective and efficient CO-functionalization of the C-H bonds of benzene and toluene and of the N-H bond of ammonia. The occurrence of these carbonylation reactions has been unambiguously ascertained by Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry, and the details of the structure and the mechanism of formation of the precursor FCO+ ions have been investigated. FT-ICR experiments show that these ions, structurally assigned as F-C-O+ by collisionally activated dissociation (CAD) spectrometry, arise from the reaction of CO.+ with NF3 and of NF+2 with CO. Combining the latter F+ transfer with the independently observed fluoride-ion abstraction by FCO+ from NF3 results in a catalytic cycle in which gaseous NF+2 ions promote the conversion of carbon monoxide into carbonic difluoride, F2 CO, with nitrogen trifluoride as the source of F.

PROTON INDUCED METHYL GROUP SHIFTS IN GASEOUS XYLENIUM IONS. DISTINGUISHING ISOMERS BY GAS-PHASE TITRATION

Abstract The differences in the gas-phase basicities (GBs) of isomers can be used for a semi-quantitative mixture analysis. For this “gas-phase titration” method, the mixture of isomers is completely protonated in the external ion source of a FT-ion cyclotron resonance spectrometry (ICR) spectrometer and undergoes selective, stepwise deprotonation by reactions with appropriate bases within the FT-ICR cell. This method is demonstrated by the analysis of a mixture of three isomeric alkylpyridines differing in GB by 4 kJ mol −1 . “Gas-phase titration” is employed to detect the isomerization of gaseous para -xylenium ions X p H + into the more stable ortho and meta isomers X o H + and X m H + by 1,2-methyl shifts around the aromatic ring. Strongly exoenergetic protonation of para -xylene X p in the external ion source by chemical ionization (CI) (methane) and gas-phase titration of the resulting XH + after transfer into the FT-ICR cell reveals isomerization of X p H + into a mixture of ≤ 15 mol% X p H + , 25 ± 3 mol% X o H + , and 60 ± 4 mol% X m H + . The degree of isomerization depends clearly on the exoenergicity of the initial protonation and is significantly reduced for XH + ions generated by CI (dimethyl ether). This effect is confirmed by an investigation of the controlled protonation of X p by selected proton donors AH + (A = C 2 H 5 CN, CH 3 OH, C 6 H 6 , C 3 H 6 , H 2 O, C 2 H 4 ) in the FTICR cell. This study shows that the chemical nature of A is also important for the degree of isomerization. The results are explained convincingly by assuming isomerization of the X p H + ions through multiple 1,2-methyl shifts within a long lived ion/molecule complex [A⋯H + X p ], formed by proton transfer from AH + to X p . The rearrangement is driven by the excess energy of the complex as a result of exothermic proton transfer and electrostatic activation of the complex, and competes with the dissociation of the excited complex.

Kaempferol glycosides from Lobularia maritima and their potential role in plant interactions.

Six kaempferol glycosides, four of them characterized for the first time, were isolated from the leaf extract of Lobularia maritima. The structural elucidation was performed by a combined approach using Electrospray-Ionization Triple-Quadrupole Mass-Spectrometric (ESI/TQ/MS) techniques, and 1D- and 2D-NMR experiments (1H, 13C, DEPT, DQ-COSY, TOCSY, ROESY, NOESY, HSQC, HMBC, and HSQC-TOCSY). The isolated kaempferol derivatives have different disaccharide substituents at C(3) and four of them have a rhamnose unit at C(7). To evaluate their potential allelopathic role within the herbaceous plant community, the compounds, as well as the aglycone obtained from enzymatic hydrolysis, have been tested in vitro on three coexisting plant species, Dactylis hispanica, Petrorhagia velutina, and Phleum subulatum. The results obtained allow us to hypothesize that the type of the sugar modulates the biological response. The bioassay data, analyzed by a multivariate approach, and grouping the compounds on the basis of the number of sugar units and the nature of carbohydrates present in the disaccharide moiety, indicate a structure-activity relationship.

Protonated thiohypofluorous acid, FSH2+. Theoretically predicted to be stable and experimentally observed in the gas phase

Protonated FSH has been theoretically investigated at the Gaussian-2 level of theory. The S-protonated isomer 1 was found to be more stable than the fluorine-protonated isomer 3 by 28.6 kcal mol−1, and the proton affinity of FSH at the sulphur atom is computed as 162 kcal mol−1 at 298 K. The interconversion of the two ions occurs through the three-centre transition structure 6, less stable than 1 by 53 kcal mol−1. The experimental observation of gaseous FSH2+ ions, obtained from the ionization of a mixture of H2S and NF3, is fully consistent with the theoretical prediction of at least one deep energy minimum on the potential energy surface and corroborates and extends previous experimental evidence for stable H2SF+SbF6− salt in anhydrous HF at 195 K.

Acid-catalysed glucose dehydration in the gas phase: a mass spectrometric approach.

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.

Positive ion chemistry of gaseous boric and polyboric acids

Abstract The positive ion chemistry of H 3 BO 3 has been studied by chemical ionization (CI) and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. From the results of equilibrium and “bracketing” experiments, the gas-phase basicity (GPB) and the proton affinity (PA) of H 3 BO 3 are estimated to be 167.8 ± 0.5 and 176 ± 2 kcal mol −1 respectively. The latter value is in fair agreement with the PA of 181.2k cal mol −1 from MO SCF calculations at the MP3/6-31G**//6-31G* + ZPVE (6-31G*) level of theory. The PA of HBO 2 calculated at the MP4(SDTQ)/6-31G**//6-31G* + ZPVE (6-31G*) level is 181.2 kcal mol −1 , which falls within the interval set by ICR “bracketing” experiments. The H 4 BO + 3 ion undergoes condensation with H 3 BO 3 yielding protonated polyboric acids containing up to seven B atoms belonging to three different classes, (H n +2 B n O 2 n +1 )H + , (H n B n O 2 n H + and (H n −2 B n O 2 n −1 )H + , formed via sequences established by FT-ICR experiments involving isolation of the ionic reactants by multiple resonance. The structures of the polyions observed are discussed and compared to those of the corresponding anions, also observed in the gas phase, and of the few anions whose structures have been established in crystalline borates. The fast isotope exchange with H 2 18 O undergone by H 4 BO + 3 suggests that the H 6 BO + 4 ion observed in the CI spectra of H 3 BO 3 may contain a tetrahedrally coordinated B atom, as present in many borate and polyborate anions. The reactions of H 3 BO 3 and/or of H 4 BO + 3 with MeOH and HCOOH, yielding protonated esters, respectively anhydrides, of boric and polyboric acids are surveyed.

Structure determination of chamaedryosides A-C, three novel nor-neo-clerodane glucosides from Teucrium chamaedrys, by NMR spectroscopy.

Three new nor‐neo‐clerodane diterpenes, named chamaedryoside A (1), B (2) and C (3), have been isolated from the organic extracts of Teucrium chamaedrys (L.) and their structural characterization has been accomplished by 1H and 13C‐NMR spectra, and DEPT, by COSY, TOCSY, HSQC, HSQC‐TOSCY and HMBC experiments, as well as by ESI‐MS/MS techniques. The stereostructures have been elucidated by NOESY and computational calculations. Copyright

The Mechanism of 2-Furaldehyde Formation from d -Xylose Dehydration in the Gas Phase. A Tandem Mass Spectrometric Study

AbstractThe mechanism of reactions occurring in solution can be investigated also in the gas phase by suited mass spectrometric techniques, which allow to highlight fundamental mechanistic features independent of the influence of the medium and to clarifying controversial hypotheses proposed in solution studies. In this work, we report a gas-phase study performed by electrospray triple stage quadrupole mass spectrometry (ESI-TSQ/MS) on the dehydration of d-xylose, leading mainly to the formation of 2-furaldehyde (2-FA). It is generally known in carbohydrate chemistry that the thermal acid catalyzed dehydration of pentoses leads to the formation of 2-FA, but several aspects on the solution-phase mechanism are controversial. Here, gaseous reactant ions corresponding to protonated xylose molecules obtained from ESI of a solution containing d-xylose and ammonium acetate as protonating reagent were allowed to undergo collisionally activated decomposition (CAD) into the triple stage quadrupole analyzer. The product ion mass spectra of protonated xylose are characterized by the presence of ionic intermediates arising from xylose dehydration, which were structurally characterized by their fragmentation patterns. As expected, the xylose triple dehydration leads to the formation of the ion at m/z 97, corresponding to protonated 2-FA. On the basis of mass spectrometric evidences, we demonstrated that in the gas phase, the formation of 2-FA involves protonation at the OH group bound to the C1 atom of the sugar, the first ionic intermediate being characterized by a cyclic structure. Finally, energy resolved product ion mass spectra allowed to obtain information on the energetic features of the d-xylose→2-FA conversion. Figureᅟ

Reaction of CH3NO3 with resonance stabilized anions the gas-phase counterpart of alkyl nitrate nitration in solution

Abstract The gas-phase reactions of methyl nitrate with anions from active methylene compounds (ketones, nitriles, ring-substituted toluenes, methylpyridines) have been studied by Fourier transform ion cyclotron resonance (FT–ICR) mass spectrometry. An interesting variety of mechanistic pathways is observed. SN 2 processes dominate for anions of aliphatic ketones and nitriles. E CO 2 reactions yielding NO − 2 ions from methylnitrate are also observed. Nucleophilic attack of the negatively charged carbon of methylpyridines and substituted toluenes primarily leads to formation of nitronate ions. It seems likely that gas-phase alkyl nitrate nitration proceeds according to the mechanism postulated in solution. The remarkable specifity of mechanisms is discussed in terms of potential energy profiles and the reaction efficiency trend observed is explained considering the role played by charge delocalization of the anion.

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.