Gas phase models of hydride transfer from divalent alkaline earth metals to CO2 and CH2O.

The reactivity of HMg+, HMgCl, and HMgCl2- in hydride transfer reactions with CO2 and CH2O were studied by means of the reverse reactions-decarboxylation of HCO2MgCln+/0/- and deformylation of CH3OMgCln+/0/- (n = 0-2)-by a combination of quantum chemical computations and mass spectrometry experiments. HCO2Mg+, HCO2MgCl and HCO2MgCl2- display similar energetics for unimolecular carbon dioxide loss; for CH3OMg+, CH3OMgCl and CH3OMgCl2-, formaldehyde loss is more favourable for the cationic species than for the anionic one, with the neutral species found in-between. Despite very similar overall thermochemistry for each of the charge states of the CO2 and CH2O systems, the intermediate reaction barriers are higher for the CO2 eliminations due to a more complex and demanding reaction mechanism. Exothermic ligand exchange is observed for CH3OMg+ reacting with CO2, forming HCO2Mg+ and CH2O. Both the thermochemistry and the presence of intermediate energy barriers slightly disfavour this type of ligand exchange for CH3OMgCl and CH3OMgCl2-. It was also found that CH3OMg+ reacts readily with H2O to eliminate H2, whereas quantum chemical computations predict that the corresponding neutral and anionic species suffer unfavourable reaction thermochemistry. A corresponding reaction for the magnesium formate compounds was not observed. Quantum chemical computations were performed to investigate periodic trends in reactivity. The energetic requirements for decarboxylation of HCO2M+, HCO2MCl and HCO2MCl2- increase in the order M = Be, Mg, Ca, Sr and Ba; the only exception is the cationic Be species for which the reaction is more endothermic than for the corresponding Mg species. For deformylation of CH3OM+, CH3OMCl and CH3OMCl2- the trends are more irregular and less pronounced than for the decarboxylation reactions; however, the Be species was found to have higher reaction energies than the Mg species for all the methoxy-compounds irrespective of charge state. The effect on decarboxylation and deformylation upon replacing Cl with F, Br or I was found to be minimal for all aforementioned species. The consequences of these observations on the reverse reactions, hydride transfer to CO2 and CH2O, are discussed, and the effects of systematic structural changes on reactivity are rationalized on the basis of thermochemical cycles and well-known periodic trends.

Mechanisms for sonochemical oxidation of nitrogen.

N2O, and mixtures of N2 and O2, dissolved in water-both in the presence and absence of added noble gases-have been subjected to ultrasonication with quantification of nitrite and nitrate products. Significant increase in product formation upon adding noble gas for both reactant systems is observed, with the reactivity order Ne < Ar < Kr < Xe. These observations lend support to the idea that extraordinarily high electronic and vibrational temperatures arise under these conditions. This is based on recent observations of sonoluminescence in the presence of noble gases and is inconsistent with the classical picture of adiabatic bubble collapse upon acoustic cavitation. The reaction mechanisms of the first few reaction steps necessary for the critical formation of NO are discussed, illustrated by quantum chemical calculations. The role of intermediate N2O in this series of elementary steps is also discussed to better understand the difference between the two reactant sources (N2O and 2 : 1 N2 : O2; same stoichiometry).

The unimolecular dissociation of magnesium chloride squarate (ClMgC4O4-) and reductive cyclooligomerisation of CO on magnesium.

In this paper, we present an investigation of the unimolecular dissociation of an anionic magnesium chloride squarate complex, ClMgC4O4- using mass spectrometry supported by theoretical reaction models based on quantum chemical calculations. Sequential decarbonylation is the main fragmentation pathway leading to the deltate and ethenedione complexes, ClMgC3O3- and ClMgC2O2-, and MgCl--yet the monomer, ClMgCO-, is not observed. Calculations using the G4 composite method show that the latter is unstable with respect to further dissociation. The implications for the reverse reaction sequence, cyclooligomerisation of CO on MgCl-, are discussed in detail and also compared with recent results from synthetic efforts in finding benign and efficient metal catalysed pathways to squaric acid from CO by reduction. It appears that the first step in these reactions, the formation of the first C-C bond by coupling of two CO molecules on MgCl-, is the most critical. The role of electron transfer in step-by-step stabilising the nascent CnOn centre is highlighted.

Characterization of the alkali metal oxalates (MC2O4-) and their formation by CO2 reduction via the alkali metal carbonites (MCO2-).

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations. The critical step in the reductive process is the coordination of CO2 to an alkali metal anion, forming a metal carbonite MCO2- able to subsequently receive a second CO2 molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO2 in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li-Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO2, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C2O42- moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.

Unimolecular Dissociation of Hydrogen Squarate (HC4O4-) and the Squarate Radical Anion (C4O4•-) in the Gas Phase and the Relationship to CO Cyclooligomerization.

The unimolecular dissociation of hydrogen squarate and the squarate radical anion has been studied by electrospray ionization mass spectrometry (including collisionally induced dissociation) and quantum chemical calculations, providing consistent reaction models. In both cases, consecutive decarbonylations are observed as the dominating fragmentations. The reverse of these reactions corresponds to the successive cyclooligomerization of CO, which constitutes the most atom-efficient route to the cyclic oxocarbons. The reaction models indicate moderate barriers for CO addition to HCnOn- and CnOn•-, respectively, being larger for the former than for the latter. Cyclooligomerization leading to a neutral product is endothermic, while the analogous one-electron reductive coupling is exothermic. The analysis shows that the addition of an electron is essential for cyclooligomerization to give the cyclic four-CO squarate structure.

C-C Bond Formation of Mg- and Zn-Activated Carbon Dioxide.

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.

The activation of carbon dioxide by first row transition metals (Sc-Zn).

The activation of CO2 by chloride-tagged first-row transition metal anions [ClM]- (M = Sc-Zn), was examined by mass spectrometry, quantum chemical calculations, and statistical analysis. The direct formation of [ClM(CO2)]- complexes was demonstrated in the reaction between [ClM]- and neutral CO2. In addition, the reverse reaction was investigated by energy-variable collisionally induced dissociation (CID) of the corresponding [ClM(CO2)]- anions generated in-source. Five different mono- and bi-dentate binding motifs present in the ion/CO2 complexes were identified by quantum chemical calculations and the relative stability of each of these isomers was established and analyzed for all first-row transition metals based on the experimental and theoretical ion/molecule binding energies. It was found that the early first row transition metals form strong covalent bonds with the neutral CO2 molecule, while the late ones and in particular copper and zinc are weakly bonded. Using simple valence bond Lewis diagrams, the different binding motifs and their relative stabilities across the first row were described using multi-configurational self consistent field (MSCSCF) wavefunctions in a quantitative manner based on the electronic structure of the individual metals. This analysis provides an explanation for the change of the most favorite bonding motif of the transition metals with CO2 along the 1st transition metal row. The nature of the activated CO2 complex and the relationship between its stability and other structural and spectral properties was also analyzed by Principal Component Analysis (PCA) and artificial neural networks.

Unimolecular dissociation of anions derived from succinic acid (H2Su) in the gas phase: HSu- and ClMgSu-. Relationship to CO2 fixation.

Electrospray ionization of mixtures of succinic acid (here denoted H2Su) and magnesium chloride in water/methanol give rise to ions of the type ESu- (E = H or ClMg). The unimolecular dissociation of these ions was studied by collisionally induced dissociation mass spectrometry and interpreted by quantum chemical calculations (density functional theory and the composite Gaussian-4 method) of relevant parts of the potential energy surfaces. The major dissociation pathways from HSu- were seen to be dehydration and decarboxylation, while ClMgSu- mainly undergoes decarboxylation. The latter reaction proceeds without barrier for the reverse reaction; addition of CO2 to a Grignard type structure ClMg(CH2CH2CO2)-. In contrast, addition of CO2 to the analogous H(CH2CH2CO2)- ion has a substantial barrier. Dehydration of HSu- gives rise to deprotonated succinic anhydride via a transition state for the key intramolecular proton transfer having an entropically favorable seven-member ring structure. The succinate systems studied here are compared to the previously reported analogous maleate systems, providing further insight to the structure-reactivity relationship.

Dissociation of Mg(ii) and Zn(ii) complexes of simple 2-oxocarboxylates - relationship to CO2 fixation, and the Grignard and Barbier reactions.

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 [H2C[double bond, length as m-dash]COCO2MCl]- and [H3CHC[double bond, length as m-dash]COCO2MCl]-. 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.

Reactions between microhydrated superoxide anions and formic acid.

Reactions between water clusters containing the superoxide anion, O2˙-(H2O)n (n = 0-4), and formic acid, HCO2H, were studied experimentally in vacuo and modelled using quantum chemical methods. Encounters between microhydrated superoxide and formic acid were found to result in a number of different reactions, including (a) proton transfer, (b) ligand exchange, (c) H2-elimination (affording microhydrated CO4˙-), and (d) dihydrogen transfer (affording H2O2 and microhydrated CO2˙-). The effect of reactant-ion hydration on reaction rates was investigated and the involved reaction mechanisms were elucidated.

Changing role of carrier gas in formation of ethanol clusters by adiabatic expansion.

Adiabatic expansion of molecular vapors is a celebrated method for producing pure and mixed clusters of relevance in both applied and fundamental studies. The present understanding of the relationship between experimental conditions and the structure of the clusters formed is incomplete. We explore the role of the backing/carrier gas during adiabatic expansion of ethanol vapors with regard to cluster production and composition. Single-component clusters of ethanol were produced over a wide size-range by varying the rare gas (He, Ar) backing pressure, with Ar being more efficient than He in promoting the formation of pure ethanol clusters. However, at stagnation pressures Ps>1.34(4) bar and temperature 49(2) °C, synchrotron-based valence and inner-shell photoelectron spectroscopy reveals condensation of Ar carrier gas on the clusters. Theoretical calculations of cluster geometries as well as chemical shifts in carbon 1s ionization energies confirm that the experimental observations are consistent with an ethanol core covered by an outer shell of argon. Experiments on the 1-propanol/Ar system display a similar pattern as described for ethanol/Ar, indicating a broader range of validity of the results.

Chirally sensitive collision induced dissociation of proton-bound diastereomeric complexes of tryptophan and 2-butanol.

In this work we report the stereo-dependent collision-induced dissociation (CID) of proton-bound complexes of tryptophan and 2-butanol. The dissociation efficiency was measured as a function of collision energy in single collision mode. The homochiral complex was found to be less stable against CID than a heterochiral one. Additional gas dependence measurements were performed with diastereomeric complexes that confirm the findings.

Oxidation of NO˙ by small oxygen species HO2(-) and O2˙(-): the role of negative charge, electronic spin and water solvation.

The reactions of HO2(-)(H2O)n and O2˙(-)(H2O)n clusters (n = 0-4) with NO˙ were studied experimentally using mass spectrometry; the experimental work was supported by quantum chemical computations for the case n = 0, 1. It was found that HO2(-)(H2O)n clusters were efficient in oxidizing NO˙ into NO2(-), although the reaction rate decreases rapidly with hydration above n = 1. Superoxide-water clusters did not oxidize NO˙ into NO2(-) under the present experimental conditions (low pressure): instead a reaction occurred in which peroxynitrite, ONOO(-), was formed as a new cluster core ion. The latter reaction was found to need at least one water molecule present on the reactant cluster in order to enable the product to stabilize itself by evaporation of H2O.

Solution and gas-phase acidities of all-trans (all-E) retinoic acid: an experimental and computational study.

Retinoic acid is of fundamental biological importance. Its acidity was determined in the gas phase and in acetonitrile solution by means of mass spectrometry and UV/Vis spectrophotometry, respectively. The intrinsic acidity is slightly higher than that of benzoic acid. In solution, the situation is opposite. The experimental systems were described theoretically applying quantum chemical methods (wave function theory and density functional theory). This allowed the determination of the molecular structure of the acid and its conjugate base, both in vacuo and in solution, and for computational estimates of its acidity in both phases.

Nucleophilic Substitution in Reactions between Partially Hydrated Superoxide Anions and Alkyl Halides.

The effect of solvation by water molecules on the nucleophilicity of the superoxide anion, O2(•-), has been investigated in detail by mass spectrometric experiments and quantum chemical calculations, including direct dynamics trajectory calculations. Specifically, the SN2 reactions of O2(•-)(H2O)n clusters (n = 0-5) with CH3Cl and CH3Br were studied. It was found that the reaction rate decreases when the number of water molecules in the cluster increases; furthermore, reaction with CH3Br is in general faster than reaction with CH3Cl for clusters of the same size. In addition, key transition-state geometries were identified and probed by Born-Oppenheimer molecular dynamics, showing how a water molecule may be transferred from the nucleophile to the leaving group during the reaction. The computational models are in good agreement with the experimental observations.

Geometry of the magic number H(+)(H2O)21 water cluster by proxy.

Abundance mass spectra, obtained upon carefully electrospraying solutions of tert-butanol (TB) in water into a mass spectrometer, display a systematic series of peaks due to mixed H(+)(TB)m(H2O)n clusters. Clusters with m + n = 21 have higher abundance (magic number peaks) than their neighbours when m ≤ 9, while for m > 9 they have lower abundance. This indicates that the mixed TB-H2O clusters retain a core hydrogen bonded network analogous to that in the famous all-water H(+)(H2O)21 cluster up to the limit m = 9. The limiting value corresponds to the number of dangling O-H groups pointing out from the surface of the degenerated pentagonal dodecahedral, considered to be the lowest energy form of H(+)(H2O)21; the experimental findings therefore support this geometry. The experimental findings are supported by ab initio quantum chemical calculations to provide a consistent framework for the interpretation of these kinds of experiments.

CO(2) incorporation in hydroxide and hydroperoxide containing water clusters--a unifying mechanism for hydrolysis and protolysis.

The reactions of CO2 with anionic water clusters containing hydroxide, OH(-)(H2O)n, and hydroperoxide, HO2(-)(H2O)n, have been studied in the isolated state using a mass spectrometric technique. The OH(-)(H2O)n clusters were found to react faster for n = 2,3, while for n >3 the HO2(-)(H2O)n clusters are more reactive. Insights from quantum chemical calculations revealed a common mechanism in which the decisive bicarbonate-forming step starts from a pre-reaction complex where OH(-) and CO2 are separated by one water molecule. Proton transfer from the water molecule to OH(-) then effectively moves the hydroxide ion motif next to the CO2 molecule. A new covalent bond is formed between CO2 and the emerging OH(-) in concert with the proton transfer. For larger clusters, successive proton transfers from H2O molecules to neighbouring OH(-) are required to effectively bring about the formation of the pre-reaction complex, upon which bicarbonate formation is accomplished according to the concerted mechanism. In this manner, a general mechanism is suggested, also applicable to bulk water and thereby to CO2 uptake in oceans. Furthermore, this mechanism avoids the intermediate H2CO3 by combining the CO2 hydrolysis step and the protolysis step into one. The general mechanistic picture is consistent with low enthalpy barriers and that the limiting factors are largely of entropic nature.

Mechanochemistry: the effect of dynamics.

Dynamical effects on the mechanochemistry of linear alkane chains, mimicking polyethylene, are studied by means of molecular dynamics simulations. Butane and octane are studied using density-functional theory (DFT), whereas higher homologues are studied using a simple one-dimensional model in which the molecules are represented by a linear chain of Morse potentials (LCM). The application of a fixed external force to a thermodynamically pre-equilibrated molecule leads to a preference for cleavage of the terminal C-C bonds, whereas a sudden application of the force favors bond breaking in the central part of the chain. In all cases, transition-state theory predicts higher bond-breaking rates than found from the more realistic molecular dynamics simulations. The event of bond dissociation is related to dynamic states involving symmetric vibrational modes. Such modes do in general have lower frequencies of vibration than antisymmetric modes, which explains the deviation between the statistical theory and the dynamics simulations. The good qualitative agreement between the DFT and LCM models makes the latter a useful tool to investigate the mechanochemistry of long polymer chains.

Spectroscopic identification of a bidentate binding motif in the anionic magnesium-CO2 complex ([ClMgCO2 ](-)).

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.