Gas-Phase Reactivity of Metavanadate [VO3]− towards Methanol and Ethanol: Experiment and Theory

A combined photoelectron-imaging spectroscopic and theoretical investigation on the electronic structure of the VO2H anion

The electronic structure and vibrational spectrum of the VO₂H anion are explored by combining photoelectron imaging spectroscopy and density functional theoretical (DFT) calculations. The electron affinity (EA) of VO₂H is determined to be 1.304 ± 0.030 eV from the vibrationally resolved photoelectron spectrum acquired at 1.52 eV (814 nm). The anisotropy parameter (β) for the EA defined peak is measured to be 1.63 ± 0.10, indicating that it is the 17a' (4s orbital of the vanadium atom) electron attachment leading to the formation of the ground state of the VO₂H anion. The vibrational fundamentals ν ₁, ν ₃, ν ₄ and ν ₅ are obtained for the neutral ground state. Experimental assignments are confirmed by energies from electronic structure calculations and Franck-Condon (FC) spectral simulations. These simulations support assigning the anion ground state as the results obtained from the B3LYP method. In addition, the molecular orbitals and bonding involved in the anionic VO₂H cluster are also examined based on the present theoretical calculations.

Direct hydroxylation of benzene to phenol mediated by nanosized vanadium oxide cluster ions at room temperature

Gas-phase vanadium oxide cluster cations and anions are prepared by laser ablation. The small cluster ions (<1000 amu) are mass-selected using a quadrupole mass filter and reacted with benzene in a linear ion trap reactor; large clusters (>1000 amu) with no mass selection are reacted with C₆H₆ in a fast flow reactor. Rich product variety is encountered in these reactions, and the reaction channels for small cationic and anionic systems are different. For large clusters, the reactivity patterns of (V₂O₅) _n⁺ (n = 6-25) and (V₂O₅) _n O^- (n = 6-24) cluster series are very similar to each other, indicating that the charge state has little influence on the oxidation of benzene. In sharp contrast to the dramatic changes of reactivity of small clusters, a weakly size dependent reaction behavior of large (V₂O₅)_6-25⁺ and (V₂O₅)_6-24O^- clusters is observed. Therefore, the charge state and the size are not the major factors influencing the reactivity of nanosized vanadium oxide cluster ions toward C₆H₆, which is not common in cluster science. In the reactions with benzene, the small and large reactive vanadium oxide cations show similar reactivity of hydroxyl radicals (OH^•) toward C₆H₆ at higher and lower temperatures, respectively; different numbers of vibrational degrees of freedom and the released energy during the formation of adduct complexes can explain this intriguing correlation. The reactions investigated herein might be used as the models of how to realize the partial oxidation of benzene to phenol in a single step, and the observed mechanisms are helpful to understand the corresponding heterogeneous reactions, such as those over vanadium oxide aerosols and vanadium oxide catalysts.

C-F bond activation of trifluoroethanol and trifluoroacetic acid catalysed by the dimolybdate anion, [Mo2O6(F)]- †

Two gas-phase catalytic cycles involving C-F bond activation of trifluoroethanol and trifluoroacetic acid were detected by multistage mass spectrometry experiments. A binuclear dimolybdate centre [Mo₂O₆(F)]^- acts as the catalyst in each cycle. The first cycle, entered via the reaction of [Mo₂O₆(OH)]^- with trifluoroethanol and elimination of water to form [Mo₂O₆(OCH₂CF₃)]^-, proceeds via four steps: (1) oxidation of the alkoxo ligand and its elimination as aldehyde; (2) reaction of [Mo₂O₅(OH)]^- with trifluoroethanol and elimination of water to form [Mo₂O₅(OCH₂CF₃)]; (3) decomposition of the alkoxo ligand via loss of 1,1 difluoroethene; and (4) reaction of [Mo₂O₆(F)]^- with a second equivalent of trifluoroethanol to regenerate Mo₂O₆(OCH₂CF₃)]^-. Steps (2) and (3) do not occur at room temperature and require collisional activation to proceed. The second cycle is entered via the reaction of [Mo₂O₆(OH)]^- with trifluoroacetic acid and elimination of water to form [Mo₂O₆(O₂CCF₃)]^- and involves two steps only: (1) fluoride transfer to a molybdenum centre to form [Mo₂O₆(F)]^-; (2) reaction of [Mo₂O₆(F)]^- with trifluoroacetic acid and loss of water to regenerate [Mo₂O₆(O₂CCF₃)]^-. Comparisons are made with the chemistry of [Mo₂O₆(OH)]^- reacting with acetic acid.

Effect of Alkyl Group on MxOy(-) + ROH (M = Mo, W; R = Me, Et) Reaction Rates

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) clusters form more stable Lewis acid-base complexes than WxOy(-), and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 10(-11) cm(3) s(-1) for Mo2O4(-) + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH(-) addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy(-) + ROH → MxOy+1(-) + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy(-) reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH(-) than for MoxOy+1RH(-). This result is consistent with previous computational studies on analogous MxOy(-) + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy(-) reactions than for WxOy(-).

Unravelling Mechanistic Aspects of the Gas-Phase Ethanol Conversion: An Experimental and Computational Study on the Thermal Reactions of MO2 (+) (M=Mo, W) with Ethanol

The ion/molecule reactions of molybdenum and tungsten dioxide cations with ethanol have been studied by Fourier transform ion-cyclotron resonance mass spectrometry (FT-ICR MS) and density functional theory (DFT) calculations. Dehydration of ethanol has been found as the dominant reaction channel, while generation of the ethyl cation corresponds to a minor product. Cleary, the reactions are mainly governed by the Lewis acidity of the metal center. Computational results, together with isotopic labeling experiments, show that the dehydration of ethanol can proceed either through a conventional concerted [1,2]-elimination mechanism or a step-wise process; the latter occurs via a hydroxyethoxy intermediate. Formation of C2 H5 (+) takes place by transfer of OH(-) from ethanol to the metal center of MO2 (+) . The molybdenum and tungsten dioxide cations exhibit comparable reactivities toward ethanol, and this is reflected in similar reaction rate constants and branching ratios.

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.

Theoretical Investigation of the Methanol Decomposition by Fe+)and Fe(C2H4)+: A π-Type Ligand Effect

Density functional theory has been used to probe the mechanism of gas-phase methanol decomposition by bare Fe(+) and ligated Fe(C(2)H(4))(+) in both quartet and sextet states. For the Fe(+)/methanol system, Fe(+) could directly attach to the O and methyl-H atoms of methanol, respectively, forming two encounter isomers. The methanol reaction with Fe(+) prefers initial C-O bond activation to yield methyl with slight endothermicity, whereas CH(4) elimination is hindered by the strong endothermicity and high-energy barrier of hydroxyl-H shift. For the Fe(C(2)H(4))(+)/methanol system, the major product of H(2)O is formed through six elementary steps: encounter complexation, C-O bond activation, C-C coupling, β-H shift, hydride H shift, and nonreactive dissociation. Both ligand exchange and initial C-O bond activation mechanisms could account for ethylene elimination with the ion products Fe(CH(3)OH)(+) and (CH(3))Fe(OH)(+), respectively. With the assistance of a π-type C(2)H(4) ligand, the metal center in the Fe(C(2)H(4))(+)/CH(3)OH system avoids formation of unfavorable multi-σ-type bonding and thus greatly enhances the reactivity compared to that of bare Fe(+).

Gas-phase reactions of the rhenium oxide anions, [ReOx]- (x = 2 - 4) with the neutral organic substrates methane, ethene, methanol and acetic acid

The ion-molecule reactions of the rhenium oxide anions, [ReOx](-) (x = 2 - 4) with the organic substrates methane, ethene, methanol and acetic acid have been examined in a linear ion trap mass spectrometer. The only reactivity observed was between [ReO(2)](-) and acetic acid. Isotope labelled experiments and high-resolution mass spectrometry measurements were used to assign the formulas of the ionic products. Collision-induced dissociation and ion-molecule reactions with acetic acid were used to probe the structures of the mass-selected primary product ions. Density functional theory calculations [PBE0/LanL2DZ6-311+G(d)] were used to suggest possible structures. The three primary product channels observed are likely to arise from the formation of: the metallalactone [ReO(2)(CH(2)CO(2))](-) (m/z 277) and H(2); [CH(3)ReO(2)(OH)](-) (m/z 251) and CO; and [ReO(3)](-) (m/z 235), H(2) and CH(2)CO.

The competitive O-H versus C-H bond activation of ethanol and methanol by VO2(+) in gas phase: a DFT study

The activation of ethanol and methanol by VO2(+) in gas phase has been theoretically investigated by using density functional theory (DFT). For the VO2(+)/ethanol system, the activation energy (ΔE) is found to follow the order of ΔE(C(β)-H) < ΔE(C(α)-H) ≈ ΔE(O-H). Loss of methyl and glycol occurs respectively via O-H and C(β)-H activation, while acetaldehyde elimination proceeds through two comparable O-H and C(α)-H activations yielding both VO(H2O)(+) and V(OH)2(+). Loss of water not only gives rise to VO(CH3CHO)(+) via both O-H and C(α)-H activation but also forms VO2(C2H4)(+) via C(β)-H activation. The major product of ethylene is formed via both O-H and C(β)-H activation for yielding VO(OH)2(+) and VO2(H2O)(+). In the methanol reaction, both initial O-H and C(α)-H activation accounts for formaldehyde and water elimination, but the former pathway is preferred.

Matrix isolation infrared spectroscopic and theoretical study of the reactions of tantalum oxide molecules with methanol

The reactions of tantalum monoxide (TaO) and dioxide (TaO(2)) molecules with methanol in solid neon were investigated by infrared absorption spectroscopy. The ground-state TaO molecule reacted with CH(3)OH in forming the CH(3)OTa(O)H molecule via the hydroxylic hydrogen atom transfer from methanol to the metal center spontaneously on annealing. The observation of the spontaneous reaction is consistent with theoretical predictions that the OH bond activation process is both thermodynamically exothermic and kinetically facile. In contrast, the TaO(2) molecule reacted with CH(3)OH to give primarily the TaO(2)(CH(3)OH) complex, which further rearranged to the CH(3)OTa(O)OH isomer via the hydroxylic hydrogen atom transfer from methanol to one of oxygen atom of metal dioxide upon visible light excitation.

Chemistry with methane: concepts rather than recipes

Four seemingly simple transformations related to the chemistry of methane will be addressed from mechanistic and conceptual points of view: 1) metal-mediated dehydrogenation to form metal carbene complexes, 2) the hydrogen-atom abstraction step in the oxidative dimerization of methane, 3) the mechanisms of the CH(4)→CH(3)OH conversion, and 4) the initial bond scission (C-H vs. O-H) as well as the rate-limiting step in the selective CH(3)OH→CH(2)O oxidation. State-of-the-art gas-phase experiments, in conjunction with electronic-structure calculations, permit identification of the elementary reactions at a molecular level and thus allow us to unravel detailed mechanistic aspects. Where appropriate, these results are compared with findings from related studies in solution or on surfaces.