ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
The effect of solvation by water molecules on the nucleophilicity of the superoxide anion, O2(â¢-), has been investigated in detail by mass spectrometric experiments and quantum chemical calculations, including direct dynamics trajectory calculations. Specifically, the SN2 reactions of O2(â¢-)(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.
ABSTRACT
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.
ABSTRACT
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. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.
