Competing Reaction Pathways in Gas-Phase Oxidation of CH by Protonated HO.

Reactions between protonated hydrogen peroxide and benzene (and benzene-) have been studied in the gas phase using an FT-ICR mass spectrometer. Four competing paths for the bimolecular system were identified, namely, proton transfer, hydride abstraction, dissociative single-electron transfer, and an electrophilic addition of HO to give the Wheland intermediate [CH, OH] followed by a subsequent elimination of water. The three latter pathways correspond to three different ways to oxidize benzene.

Gas phase models of hydride transfer from divalent alkaline earth metals to CO and CHO.

The reactivity of HMg, HMgCl, and HMgCl in hydride transfer reactions with CO and CHO were studied by means of the reverse reactions-decarboxylation of HCOMgCl and deformylation of CHOMgCl ( = 0-2)-by a combination of quantum chemical computations and mass spectrometry experiments. HCOMg, HCOMgCl and HCOMgCl display similar energetics for unimolecular carbon dioxide loss; for CHOMg, CHOMgCl and CHOMgCl, 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 CO and CHO systems, the intermediate reaction barriers are higher for the CO eliminations due to a more complex and demanding reaction mechanism.

Mechanisms for sonochemical oxidation of nitrogen.

NO, and mixtures of N and O, 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.

The unimolecular dissociation of magnesium chloride squarate (ClMgCO) 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.

Characterization of the alkali metal oxalates (MCO) and their formation by CO reduction via the alkali metal carbonites (MCO).

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).

Investigation of mineralization products of lead soaps in a late medieval panel painting.

Metal soaps pose significant concerns in the preservation of paintings made with oil as a binding medium. In highly alkaline environments, metal soap aggregates may undergo mineralization processes with the formation of new phases, such as carbonates, chlorides, oxides and sulfates of the metal cations that are present in pigments or paint additives. In this work, we report new examples of the mineralization of lead soaps in paint-cross sections taken from a late-medieval panel painting owned by the Museum of Cultural History at the University of Oslo.

Unimolecular Dissociation of Hydrogen Squarate (HCO) and the Squarate Radical Anion (CO) 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 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.

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

Gas-phase activation of CO by chloride tagged metal atoms, [ClM] (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO with significant bending of the CO moiety to form complexes with the general formula [ClM,CO ] . The structure of the metal-CO 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.

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

Electrospray ionization of mixtures of succinic acid (here denoted HSu) 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.

Low-Energy Collisions of Protonated Enantiopure Amino Acids with Chiral Target Gases.

Here we report on the gas-phase interactions between protonated enantiopure amino acids (L- and D-enantiomers of Met, Phe, and Trp) and chiral target gases [(R)- and (S)-2-butanol, and (S)-1-phenylethanol] in 0.1-10.0 eV low-energy collisions.

Reactions between microhydrated superoxide anions and formic acid.

Reactions between water clusters containing the superoxide anion, O˙(HO) (n = 0-4), and formic acid, HCOH, 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) H-elimination (affording microhydrated CO˙), and (d) dihydrogen transfer (affording HO and microhydrated CO˙). The effect of reactant-ion hydration on reaction rates was investigated and the involved reaction mechanisms were elucidated.

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

Three deprotonated 2-oxocarboxylic acids, glyoxylate, pyruvate, and 2-oxobutyrate (RCOCO, R = H, CH, CHCH) have been associated with MgCl and ZnCl to generate [RCOCOMCl] (M = Mg, Zn) complexes. Upon collision-induced dissociation these complexes all undergo efficient eliminations of CO and CO, via an intermediate [RCOMCl] product, to ultimately give [RMCl] products. The pyruvate and 2-oxobutyrate complexes also undergo efficient elimination of HCl to produce the enolate-metal complexes [HC[double bond, length as m-dash]COCOMCl] and [HCHC[double bond, length as m-dash]COCOMCl].

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.

Quantum chemical modeling of the reaction path of chorismate mutase based on the experimental substrate/product complex.

Chorismate mutase is a well-known model enzyme, catalyzing the Claisen rearrangement of chorismate to prephenate. Recent high-resolution crystal structures along the reaction coordinate of this enzyme enabled computational analyses at unprecedented detail. Using quantum chemical simulations, we investigated how the catalytic reaction mechanism is affected by electrostatic and hydrogen-bond interactions.

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.

Unimolecular dissociation of anions derived from maleic acid (MaH2) in the gas phase: MaH- and MaMgCl--- relationship to Grignard chemistry and reductive CO2 fixation.

We have conducted collision induced dissociation experiments on the hydrogen maleate anion (MaH(-), m/z = 115) and the anionic maleate MgCl complex (MaMgC(-), 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 27 for MaH(-), and at m/z = 129 and 85 for MaMgCl(-).

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.

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.

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.

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.

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.