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.

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.

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.

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.

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.

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.

Concerted proton migration along short hydrogen bonded water bridges in bipyridine-water clusters.

The outcome of reactions between D(2)O and size-selected ionic clusters of the type MH(+)(H(2)O)(n) (M = bipyridine, n = 1-30) shows that H-D-exchange is significantly higher for 2,2'-bipyridines than for 4,4'-bipyridines. This gives strong support for the idea that the existence of short water wires between the two nitrogen sites is essential to proton migration in water clusters containing basic sites.

Proton mobility in water clusters.

Proton mobility in water occurs quickly according to the so-called Grotthuss mechanism. This process and its elementary reaction steps can be studied in great detail by applying suitable mass spectrometric methods to ionic water clusters. Careful choice of suitable core ions in combination with analysis of cluster size trends in hydrogen/deuterium isotope exchange rates allows for detailed insights into fascinating dynamical systems.

Structural rearrangements and magic numbers in reactions between pyridine-containing water clusters and ammonia.

Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster.

Proton mobility and stability of water clusters containing the bisulfate anion, HSO4(-)(H2O)n.

Bisulfate water clusters, HSO(4)(-)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. For the cluster distributions studied, there are some possible "magic number" peaks, although the increase in abundance compared to their neighbours is small. Experiments with size-selected clusters with n = 0-25, reacting with D(2)O at a center-of-mass energy of 0.

Isotope exchange in reactions between D2O and size-selected ionic water clusters containing pyridine, H+ (pyridine)m(H2O)n.

Pyridine containing water clusters, H(+)(pyridine)(m)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. In the experiments, H(+)(pyridine)(m)(H(2)O)(n) with m = 1-4 and n = 0-80 are observed. For the cluster distributions observed, there are no magic numbers, neither in the abundance spectra, nor in the evaporation spectra from size selected clusters.

Stability and structure of protonated clusters of ammonia and water, H+(NH3)m (H2O)n.

Mass spectrometric experiments show that protonated mixed ammonia/water clusters predominant exist in three forms namely H(+)(NH(3))(4)(H(2)O)(n), H(+)(NH(3))(5)(H(2)O)(n), and H(+)(NH(3))(6)(H(2)O)(n) (n = 1-25). For the first two series the collisional activation mass spectra are dominated by loss of water, whereas ions of the latter series preferably lose ammonia. The quantitative characteristics of these observations are reproduced by quantum chemical calculations that also provide insight into the geometrical structures of the clusters.

Isotope exchange and structural rearrangements in reactions between size-selected ionic water clusters, H3O(+)(H2O)(n) and NH4(+)(H2O)(n), and D2O.

Hydrogen/deuterium exchange in reactions of H3O(+)(H2O)n and NH4(+)(H2O)n (1 < or = n < or = 30) with D2O has been studied experimentally at center-of-mass collisions energies of < or = 0.2 eV. For a given cluster size, the cross-sections for H3O(+)(H2O)n and NH4(+)(H2O)n are similar, indicating a structural resemblance and energetics of binding.