Photoelectron Imaging Spectroscopy of AuC₃H^– Anions: Four Isomers

Laser ablation generated AuC₃H^– anions were skimmed into a time-of-flight mass spectrometer (TOF-MS) and selected with a mass gate. Photoelectron spectra of AuC₃H^– were recorded using the velocity map imaging technique at several photon energies. The experimental spectra, quantum chemistry calculations, and Franck–Condon simulations suggest that the AuC₃H^– cluster has four structure isomers, including one unexpected structure of [CCC–Au–H]⁻. When AuC₃H^– is compared with C₃H₂^–, introduction of gold into the hydrocarbon system results in the much lower isomerization barriers

Chemical Ionization of Large Linear Alkanes and Small Oxidized Volatile Organic Compounds by the Reactions with Atomic Gold Cations

Although the use of proton-transfer-reaction mass spectrometry (PTR-MS) in real-time measurements of atmospheric volatile organic compounds (VOCs) has expanded rapidly in recent years, PTR-MS has several serious limitations related to H₃O⁺ reagent ion chemistry, such as considerable fragmentation for large alkanes, low sensitivity to formaldehyde, and inability to separate isomeric aldehydes and ketones. One of the key means to address these limitations is to develop other appropriate reagent ions. In this paper, the reactivity of Au⁺ toward large <i>n</i>-alkanes (C7–C10), formaldehyde, acetaldehyde, propanal, and acetone under high-pressure buffer gas has been explored by mass spectrometry and theoretical calculations. For large <i>n</i>-alkanes, extensive fragmentation was avoided with observations of hydride abstraction products. Formaldehyde could react effectively with the Au⁺ ion by splitting off H₂ or CO. Propanal and acetone behaved with different reaction channels and could be easily distinguished. The hydride transfer for propanal and methyl anion transfer for acetone were observed. These results show that the use of Au⁺ reagent ion chemistry may settle some problems of VOCs detection by H₃O⁺. In addition, reactions between Au⁺ and the tested VOCs were found to take place at the gas collision rate and the detection limit of Au⁺ was estimated to be as low as several parts per trillion by volume. Thus, the gold cation Au⁺ can serve as a useful reagent ion to identify and quantify trace amounts of linear alkanes and several small oxidized VOCs in the atmosphere

Conversion of Methane at Room Temperature Mediated by the Ta–Ta σ‑Bond

Metal–metal bonds constitute an important type of reactive centers for chemical transformation; however, the availability of active metal–metal bonds being capable of converting methane under mild conditions, the holy grail in catalysis, remains a serious challenge. Herein, benefiting from the systematic investigation of 36 metal clusters of tantalum by using mass spectrometric experiments complemented with quantum chemical calculations, the dehydrogenation of methane at room temperature was successfully achieved by 18 cluster species featuring σ-bonding electrons localized in single naked Ta–Ta centers. In sharp contrast, the other 18 remaining clusters, either without naked Ta–Ta σ-bond or with σ-bonding electrons delocalized over multiple Ta–Ta centers only exhibit molecular CH4-adsorption reactivity or inertness. Mechanistic studies revealed that changing cluster geometric configurations and tuning the number of simple inorganic ligands (e.g., oxygen) could flexibly manipulate the presence or absence of such a reactive Ta–Ta σ-bond. The discovery of Ta–Ta σ-type bond being able to exhibit outstanding activity toward methane conversion not only overturns the traditional recognition that only the metal–metal π- or δ-bonds of early transition metals could participate in bond activation but also opens up a new access to design of promising metal catalysts with dual-atom as reactive sites for chemical transformations

Reactivity of Tantalum Carbide Cluster Anions TaC_<i>n</i>^– (<i>n</i> = 1–4) with Dinitrogen

Dinitrogen activation/fixation is one of the most important and challenging subjects in synthetic as well as theoretical chemistry. In this study, the adsorption reactions of N₂ onto TaC_<i>n</i>^– (<i>n</i> = 1–4) cluster anions have been investigated by means of mass spectrometry in conjunction with density functional theory calculations. Following the experimental results that only TaC₄^– was observed to adsorb N₂, theoretical calculations predicted that the TaC₄^– reaction system (TaC₄^– + N₂ → TaC₄N₂^–) has a negligible barrier on the approach of N₂ molecule while insurmountable barriers are located on the reaction pathways of TaC_1–3^–/N₂ reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC_1–4^– would facilitate the initial approach of the nonpolar N₂ molecule, and the appropriate frontier orbital of TaC_1–4^– with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N₂ molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N₂ adsorption

Does Each Atom Count in the Reactivity of Vanadia Nanoclusters?

Vanadium oxide cluster anions (V₂O₅)_<i>n</i>V_<i>x</i>O_<i>y</i>^– (<i>n</i> = 1–31; <i>x</i> = 0, 1; and <i>x</i> + <i>y</i> ≤ 5) with different oxygen deficiencies (Δ = 2<i>y</i>–1–5<i>x</i> = 0, ± 1, and ±2) have been prepared by laser ablation and reacted to abstract hydrogen atoms from alkane molecules (<i>n</i>-butane) in a fast flow reactor. When the cluster size <i>n</i> is less than 25, the Δ = 1 series [(V₂O₅)_<i>n</i>O^– clusters] that can contain atomic oxygen radical anions (O^•–) generally have much higher reactivity than the other four cluster series (Δ = −2, −1, 0, and 2), indicating that <i>each atom counts</i> in the hydrogen-atom abstraction (HAA) reactivity. Unexpectedly, all of the five cluster series have similar HAA reactivity when the cluster size is greater than 25. The critical dimension of vanadia particles separating the cluster behavior (<i>each atom counts</i>) from the bulk behavior (<i>each atom contributes a little part</i>) is thus about 1.6 nm (∼V₅₀O₁₂₅). The strong electron–phonon coupling of the vanadia particles has been proposed to create the O^•– radicals (V⁵⁺ = O^2–+ heat → V⁴⁺–O^•–) for the <i>n</i> > 25 clusters with Δ = −2, −1, 0, and 2. Such a mechanism is supported by a comparative study with the scandium system [(Sc₂O₃)_<i>n</i>Sc_<i>x</i>O_<i>y</i>^– (<i>n</i> = 1–29; <i>x</i> = 0, 1; and <i>x</i> + <i>y</i> ≤ 4)] for which the Δ = 1 series [(Sc₂O₃)_<i>n</i>O^– clusters] always have much higher HAA reactivity than the other cluster series

Methane Activation by Iron-Carbide Cluster Anions FeC₆^–

Laser-ablation-generated and mass-selected iron-carbide cluster anions FeC₆^– were reacted with CH₄ in a linear ion trap reactor under thermal collision conditions. The reactions were characterized by mass spectrometry and density functional theory calculations. Adsorption product of FeC₆CH₄^– was observed in the experiments. The identified large kinetic isotope effect suggests that CH₄ can be activated by FeC₆^– anions with a dissociative adsorption manner, which is further supported by the reaction mechanism calculations. The large dipole moment of FeC₆^– (19.21 D) can induce a polarization of CH₄ and can facilitate the cleavage of C–H bond. This study reports the CH₄ activation by transition-metal carbide anions, which provides insights into mechanistic understanding of iron–carbon centers that are important for condensed-phase catalysis

Thermal Methane Conversion to Syngas Mediated by Rh₁‑Doped Aluminum Oxide Cluster Cations RhAl₃O₄⁺

Laser ablation generated RhAl₃O₄⁺ heteronuclear metal oxide cluster cations have been mass-selected using a quadrupole mass filter and reacted with CH₄ or CD₄ in a linear ion trap reactor under thermal collision conditions. The reactions have been characterized by state-of-the-art mass spectrometry and quantum chemistry calculations. The RhAl₃O₄⁺ cluster can activate four C–H bonds of a methane molecule and convert methane to syngas, an important intermediate product in methane conversion to value-added chemicals. The Rh atom is the active site for activation of the C–H bonds of methane. The high electron-withdrawing capability of Rh atom is the driving force to promote the conversion of methane to syngas. The polarity of Rh oxidation state is changed from positive to negative after the reaction. This study has provided the first example of methane conversion to syngas by heteronuclear metal oxide clusters under thermal collision conditions. Furthermore, the molecular level origin has been revealed for the condensed-phase experimental observation that trace amounts of Rh can promote the participation of lattice oxygen of chemically very inert support (Al₂O₃) to oxidize methane to carbon monoxide

Methane Activation by Tantalum Carbide Cluster Anions Ta₂C₄^–

Methane activation by transition metals is of fundamental interest and practical importance, as this process is extensively involved in the natural gas conversion to fuels and value-added chemicals. While single-metal centers have been well recognized as active sites for methane activation, the active center composed of two or more metal atoms is rarely addressed and the detailed reaction mechanism remains unclear. Here, by using state-of-the-art time-of-flight mass spectrometry, cryogenic anion photoelectron imaging spectroscopy, and quantum-chemical calculations, the cooperation of the two Ta atoms in a dinuclear carbide cluster Ta₂C₄^– for methane activation has been identified. The C–H bond activation takes place predominantly around one Ta atom in the initial stage of the reaction and the second Ta atom accepts the delivered H atom from the C–H bond cleavage. The well-resolved vibrational spectra of the cryogenically cooled anions agree well with theoretical simulations, allowing the clear characterization of the structure of Ta₂C₄^– cluster. The reactivity comparison between Ta₂C₄^– cluster and the carbon-less analogues (Ta₂C₃^– and Ta₂C₂^–) demonstrated that the cooperative effect of the two metal atoms can be well tuned by the carbon ligands in terms of methane activation and transformation