Search

Benzene formation from magnesium benzoate complexes in two steps (carboxylation followed by hydrolysis) has been studied. Detailed reaction mechanisms are proposed based on evidence from tandem mass spectrometry experiments and bimolecular gas- phase reactions, supported by extensive quantum chemical calculations. Formation of intermediate Grignard compounds by decarboxylation is seen to be the rate- determining step of the overall process, but is essentially the same irrespective of charge state (+1, 0, -1). The following reaction with H2O proceeds with negative activation energy. The requirements for hydrolysis of metal benzoates to give benzene in water solution are briefly discussed.

A series of simple α-oxo acids (RCOCOOH, R = H, CH 3 , CH 3 CH 2 , CF 3 ) were deprotonated by negative electrospray ionisation and subjected to collision-induced dissociation and mass spectrometric analysis. Decarboxylation is the major process, observed for all four carboxylates. In addition, glyoxylate (R = H) and trifluoropyruvate (R = CF 3 ) undergo efficient decarbonylation. Pyruvate (R = CH 3 ) and 2-oxobutyrate (R = CH 3 CH 2 ) also lose CO, but to a minor extent. The mechanistic and kinetic details of these processes have been investigated, supported by quantum chemical and RRKM modelling. The observed reactivity trends are in good agreement with conventional structure/reactivity relationships.

Three deprotonated 2-oxocarboxylic acids, glyoxylate, pyruvate, and 2-oxobutyrate (RCOCO2−, R = H, CH3, CH3CH2) have been associated with MgCl2 and ZnCl2 to generate [RCOCO2MCl2]− (M = Mg, Zn) complexes. Upon collision-induced dissociation these complexes all undergo efficient eliminations of CO2 and CO, via an intermediate [RCOMCl2]− product, to ultimately give [RMCl2]− products. The pyruvate and 2-oxobutyrate complexes also undergo efficient elimination of HCl to produce the enolate-metal complexes [H2CCOCO2MCl]− and [H3CHCCOCO2MCl]−. These enolate complexes have binding motifs reminiscent of the active centres in some CO2-fixating enzymes and the CO2 reactivity of these enolate complexes was therefore investigated, but only adduct formation could be observed. Quantum chemical calculations predict the magnesium complexes to decarboxylate without reverse barriers to carboxylation, and the zinc complexes to decarboxylate with considerable reverse barriers. The subsequent CO loss occurs with reverse barriers in all cases. The HCl loss is also predicted to occur overall without reverse barriers for both metals.

Gas-phase activation of CO2 by chloride tagged metal atoms, [ClM]- (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO2 with significant bending of the CO2 moiety to form complexes with the general formula [ClM,CO2 ]- . The structure of the metal-CO2 complex depends on the method of formation, and the energy landscapes and reaction dynamics have been probed by collisional induced dissociation and thermal ion molecule reactions with isotopically labeled species. Having established these structural relationships, the gas-phase reactivity of [ClM(κ2 -O2 C)]- with acetaldehyde (here considered a carbohydrate mimic) was then studied. Formation of lactate and enolate-pyruvate complexes are observed, showing that CO2 fixation by C-C bond formation takes place. For M=Zn, even formation of free pyruvate ([C3 H3 O3 ]- ) is observed. Implications of the observed CO2 reactivity for the electrochemical conversion of carbon dioxide, and to biochemical and artificial photosynthesis is briefly discussed. Detailed potential energy diagrams obtained by the quantum chemical calculations offer models consistent with experimental observation.

Three deprotonated 2-oxocarboxylic acids, glyoxylate, pyruvate, and 2-oxobutyrate (RCOCO

International audience; A method for spectroscopic characterization of free ionic clusters and nanoparticles utilizing X-ray synchrotron radiation is presented. We demonstrate that size-selected ammonium bisulphate cluster ions, NH4+(NH4HSO4)(n), captured in a linear ion trap, exhibit well-defined core-level absorption edges in the reconstructed fragment-ion abundance spectra. In addition to the specific photo-fragmentation pathways observed at the N1s-, O1s- and S2p-edges, dissociation also occurs as a consequence of clusters colliding with helium present as buffer gas in the ion trap. Separate off-beam experiments were conducted to establish the activation kinetics of these collision induced dissociation processes. Furthermore, it is demonstrated that the electrons released upon photoionization of background helium are too few in number to produce multiply charged cluster ions, and thereby induce fragmentation of the salt clusters, to any significant degree. The mechanisms for photon absorption and subsequent cluster fragmentation are analysed and discussed. In addition to its inherent element specificity, the method holds promise for cluster structure elucidation resulting from the sensitivity of the near edge absorption structure to the local chemical environment of the excited atom.

On the basis of mass spectrometric experiments and quantum chemical calculations, including detailed kinetic and dynamics calculations, we report the unimolecular dissociation of an isolated glycolate anion. The dominating processes are: loss of formaldehyde; loss of carbon monoxide; loss of carbon dioxide; and loss of a hydrogen molecule, with the latter having the lowest energetic threshold. At higher energies, CO loss is the dominating reaction. The loss of CO may be followed by a second CO loss, leading to the H(-)H2O complex in close mechanistic relationship to the Nibbering reaction. The results provide valuable insights into possible mechanisms for interstellar and prebiotic formation of glycolate via the reverse of the unimolecular dissociation reactions. We propose that the addition of the complex of OH(-) and CO to CH2O is the most feasible route to gas phase synthesis of glycolate, since all species are abundant in interstellar space.

We have conducted collision-induced dissociation experiments on the hydrogen maleate anion (MaH−, m/z = 115) and the anionic maleate–MgCl complex (MaMgCl−, m/z = 173). In addition, we have computationally investigated the observed fragmentation reactions. We find that both anions readily undergo two consecutive decarboxylations resulting in product ions at m/z = 71 and m/z 27 for MaH−, and at m/z = 129 and m/z 85 for MaMgCl−. The first decarboxylation is more facile for MaMgCl− than for MaH−, while loss of CO2 from Ma(–CO2) H− is more facile than that for Ma(–CO2)MgCl−. We also find that MaH− loses water, and we propose a mechanism for this loss. No firstgeneration fragmentation product other than Ma(–CO2)MgCl− is seen for MaMgCl−. Based on the observed unimolecular chemistry, we discuss some of its implications on reductive CO2-fixation and Grignard chemistry.

A magnesium complex incorporating a novel metal-CO2 binding motif is spectroscopically identified. Here we show with the help of infrared photodissociation spectroscopy that the complex exists solely in the [ClMg(η(2) -O2 C)](-) form. This bidentate double oxygen metal-CO2 coordination has previously not been observed in neutral nor in charged unimetallic complexes. The antisymmetric CO2 stretching mode in [ClMg(η(2) -O2 C)](-) is found at 1128 cm(-1) , which is considerably redshifted from the corresponding mode in bare CO2 at 2349 cm(-1) , suggesting that the CO2 moiety has a considerable negative charge (∼1.8 e(-) ). We also employed electronic structure calculations and kinetic analysis to support the interpretation of the experimental results.