Using Fourier-transform ion cyclotron resonance mass spectrometry, it was experimentally determined that Sc+ in the highly diluted gas phase reacts with SO2 to form ScO+ and SO. By 18 O labeling, ScO+ was shown to play the role of a catalyst when further reacting with SO2 in a Mars-van Krevelen-like (MvK) oxygen exchange process, where a solid catalyst actively reacts with the substrate but emerges apparently unchanged at the end of the cycle. High-level quantum chemical calculations confirmed that the multi-step process to form ScO+ and SO is exoergic and that all intermediates and transition states in between are located energetically below the entrance level. The reaction starts from the triplet surface; although three spin-crossing points with minimal energy have been identified by computational means, there is no evidence that a two-state scenario is involved in the course of the reaction, by which the reactants could switch from the triplet to the singlet surface and back. Pivotal to the oxygen exchange reaction of ScO+ with SO2 is the occurrence of a highly symmetric four-membered cyclic intermediate by which two oxygen atoms become equivalent.
Oxygen , Catalysis , Oxygen/chemistry
Extensive high-level quantum-chemical calculations reveal that the rod-shaped molecule BeOBeC, which was recently generated in matrix experiments, exists in two nearly isoenergetic states, the 5 Σ quintet (5 6) and the 3 Σ triplet (3 6). Their IR features are hardly distinguishable at finite temperature. The major difference concerns the mode of spin coupling between the terminal beryllium and carbon atoms. Further, the ground-state potential-energy surface of the [2Be,C,O] system at 4â K is presented and differences between the photochemical and thermal behaviors are highlighted. Finally, a previously not considered, so far unknown C2v -symmetric rhombus-like four-membered ring 3 [Be(O)(C)Be] (3 5) is predicted to represent the global minimum on the potential-energy surface.
Presented here is that isolated, long-lived electronic states of ReC+ serve as the root cause for distinctly different reactivities of this diatomic ion in the thermal activation of dihydrogen. Detailed high-level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT-ICR mass spectrometry. The origin for the existence of these long-lived excited electronic states and the resulting implications for the varying mechanisms of dihydrogen splitting are addressed.
[V2 O]+ remains "invisible" in the thermal gas-phase reaction of bare [V2 ]+ with CO2 giving rise to [V2 O2 ]+ ; this is because the [V2 O]+ intermediate is being consumed more than 230â times faster than it is generated. However, the fleeting existence of [V2 O]+ and its involvement in the [V2 ]+ â [V2 O2 ]+ chemistry are demonstrated by a cross-over labeling experiment with a 1:1 mixture of C16 O2 /C18 O2 , generating the product ions [V2 16 O2 ]+ , [V2 16 O18 O]+ , and [V2 18 O2 ]+ in a 1:2:1 ratio. Density functional theory (DFT) calculations help to understand the remarkable and unexpected reactivity differences of [V2 ]+ versus [V2 O]+ towards CO2 .
An unprecedented, spontaneous, and complete cleavage of the triple bond of N2 in the thermal reaction of 15N2 with Ta214N+ was observed experimentally by Fourier transform ion cyclotron resonance mass spectrometry; mechanistic aspects of the degenerate ligand exchange were addressed by high-level quantum chemical calculations. The "hidden" dis- and reassembly of N2, mediated by Ta2N+, constitutes a full catalytic cycle. A frontier orbital analysis reveals that the scission of the N2 triple bond is essentially governed by the donation of d-electrons from the 2 metal centers into antibonding π*-orbitals of N2 and by the concurrent migration of electrons from bonding π- and σ-orbitals of N2 into empty d-orbitals of the metals. This work may contribute to a rational design of catalysts in order to reduce the still enormous energy demand required for an artificial dinitrogen activation.
The reactivity of the cationic metal-carbon cluster FeC4 + towards methane has been studied experimentally using Fourier-transform ion cyclotron resonance mass spectrometry and computationally by high-level quantum chemical calculations. At room temperature, FeC4 H+ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C-H bond activation step proceeds through a radical-based hydrogen-atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC4 + , which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp-orbital of the terminal carbon atom of FeC4 + migrates to the singly occupied π*-orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C-H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed-shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition-metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen-atom transfer.
The mechanisms of the thermal reactions of the two iconic magnesium oxide cations MgO.+ and Mg2 O2.+ with methane have been re-evaluated at the CCSD(T)/CBS//CCSD/def2-TZVP level of theory. For the reaction of MgO.+ with CH4 , only the classical hydrogen-atom transfer (HAT) was found; in contrast, for the Mg2 O2.+ /CH4 couple, both HAT and proton-coupled electron-transfer (PCET) exist as mechanistic variants. In order to evaluate the suitability of density functional theory (DFT) methods, the reactions were computed by using 27 density functionals. The results obtained demonstrate that the various DFT methods often deliver rather different results for both geometric and energetic features. As to the prediction of the apparent barriers, pure functionals give the largest mean absolute errors. BMK, ωB97XD, and the double-hybrid functional mPW2PLYP were confirmed to come closest to the results provided by CCSD(T)/CBS. Thus, mechanistic conclusions based on a single DFT method should be viewed with great caution. In summary, this study may assist in the selection of a suitable quantum chemical method to unravel the mechanistic details of C-H bond activation by charged metal oxides.
Mechanistic aspects of the C-H bond activation of methane by metal-carbide cations MC+ of the 3d transition-metals Sc-Zn were elucidated by NEVPT2//CASSCF quantum-chemical calculations and verified experimentally for M = Ti, V, Fe, and Cu by using Fourier transform ion-cyclotron resonance mass spectrometry. While MC+ species with M = Sc, Ti, V, Cr, Cu, and Zn activate CH4 at ambient temperature, this is prevented with carbide cations of M = Mn, Fe, and Co by high apparent barriers; NiC+ has a small apparent barrier. Hydrogen-atom transfers from methane to metal-carbide cations were found to proceed via a proton-coupled electron transfer mechanism for M = Sc-Co; wherein the doubly occupied πxz/yz-orbitals between metal and carbon at the carbon site serve as electron donors and the corresponding metal-centered vacant π*xz/yz-orbitals as electron acceptors. Classical hydrogen-atom transfer transpires only in the case of NiC+, while ZnC+ follows a mechanistic scenario, in which a formally hydridic hydrogen is transferred. CuC+ reacts by a synchronous activation of two C-H bonds. While spin density is often so crucial for the reactions of numerous MO+/CH4 couples, it is much less important for the C-H bond activation by carbide cations of the 3d transition-metals, in which one notes large changes in bond dissociation energies, spin states, number of d-electrons, and charge distributions. All these factors jointly affect both the reactivity of the metal carbides and their mechanisms of C-H bond activation.
In a full catalytic cycle, bare Ta2+ in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber-Bosch-like process starting from N2 and H2 at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar Ta2N2+, consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of Ta2+ with N2 from the educt side or with two molecules of NH3 from the product side. In the thermal reaction of Ta2+ with N2, the N≡N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N≡N by the ditantalum center. The ability of Ta2+ to serve as a multipurpose tool is further shown by splitting the single bond of H2 in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions.
In the course of combined computational and mass spectrometry-based mechanistic studies, recently we came across rather unusual, if not unprecedented, effects of transition-metal ions and ligands when simple metal oxides or carbides are subjected to thermal gas-phase reactions with methane. Interestingly, "Gedankenexperiments" demonstrate how these effects can be modeled using oriented external electric fields (OEEFs), thus expanding their predicted role as "smart reagents" (Shaik et al., Nat. Chem., 2016, 8, 1091), and further suggesting that the OEEFs may be used in controlling the adsorption/desorption behavior of methane as well as serving as a tool to explore mechanistic features.
Mechanistic insight into the thermal O-H bond activation of water by the cubane-like, prototypical heteronuclear oxide cluster [Al2Mg2O5]â¢+ has been derived from a combined experimental/computational study. Experiments in the highly diluted gas phase using Fourier transform ion-cyclotron resonance mass spectrometry show that hydrogen-atom abstraction from water by the cluster cation [Al2Mg2O5]â¢+ occurs at ambient conditions accompanied by the liberation of an OH⢠radical. Because of a complete randomization of all oxygen atoms prior to fragmentation, about 83% of the oxygen atoms of the hydroxyl radical released originate from the oxide cluster itself. The experimental findings are supported by detailed high-level quantum chemical calculations. The theoretical analysis reveals that the transfer of a formal hydrogen atom from water to the metal-oxide cation can proceed mechanistically via proton- or hydrogen-atom transfer exploiting different active sites of the cluster oxide. In addition to the unprecedented oxygen-atom scrambling, one of the more general and quite unexpected findings concerns the role of spin density at the hydrogen-acceptor oxide atom. While this feature is crucial for [M-O]+/CH4 couples, it is much less important in the O-H bond activation of water.
A mechanistically unique, simultaneous activation of two C-H bonds of methane has been identified during the course of its reaction with the cationic copper carbide, [Cu-C]+. Detailed high-level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT-ICR mass spectrometry. The behavior of [Cu-C]+/CH4 contrasts that of [Au-C]+/CH4, for which a stepwise bond-activation scenario prevails. An explanation for the distinct mechanistic differences of the two coinage metal complexes is given. It is demonstrated that the coupling of [Cu-C]+ with methane to form ethylene and Cu+ is modeled very well by the reaction of a carbon atom with methane mediated by an oriented external electric field of a positive point charge.
The reactivity of the cationic gold carbide [AuC]+ (bearing an electrophilic carbon atom) towards methane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The product pairs generated, that is, Au+ /C2 H4 , [Au(C2 H2 )]+ /H2 , and [C2 H3 ]+ /AuH, point to the breaking and making of C-H, C-C, and H-H bonds under single-collision conditions. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations. As a major result, based on molecular orbital and NBO-based charge analysis, an unprecedented hydride transfer from methane to the carbon atom of [AuC]+ has been identified as a key step. Also, the origin of this novel mechanistic scenario has been addressed. The mechanistic insights derived from this study may provide guidance for the rational design of carbon-based catalysts.
The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](â¢+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](â¢+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.
The number of separations and analyses of molecular species using traveling wave ion-mobility spectrometry-mass spectrometry (TWIMS-MS) is increasing, including those extending the technique to analytes containing metal atoms. A critical aspect of such applications of TWIMS-MS is the validity of the collisional cross sections (CCSs) measured and whether they can be accurately calibrated against other ion-mobility spectrometry (IMS) techniques. Many metal containing species have potential reactivity toward molecular nitrogen, which is present in high concentration in the typical Synapt-G2 TWIMS cell. Here, we analyze the effect of nitrogen on the drift time of a series of cationic 1,10-phenanthroline complexes of the late transition metals, [(phen)M](+), (M = Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg) in order to understand potential deviations from expected drift time behaviors. These metal complexes were chosen for their metal open-coordination site and lack of rotameric species. The target species were generated via electrospray ionization (ESI), analyzed using TWIMS in N2 drift gas, and the observed drift time trends compared. Theoretically derived CCSs for all species (via both the projection approximation and trajectory method) were also compared. The results show that, indeed, for metal containing species in this size regime, reaction with molecular nitrogen has a dramatic effect on measured drift times and must not be ignored when comparing and interpreting TWIMS arrival time distributions. Density-functional theory (DFT) calculations are employed to analyze the periodic differences due to the metal's interaction with nitrogen (and background water) in detail.
The gas-phase reactivity of [V2O5](+) and [Nb2O5](+) towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room-temperature conversions C2H6âC2H5OH or C2H6âCH3CHO are brought about solely by [V2O5](+). In distinct contrast, for the couple [Nb2O5](+)/C2H6, one observes only single and double hydrogen-atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C-H bond activation together with quite different bond-dissociation energies of the M-O bonds cause the rather varying reactivities of [V2O5](+) and [Nb2O5](+) towards ethane. The gas-phase generation of acetaldehyde from ethane by bare [V2O5](+) may provide mechanistic insight in the related vanadium-catalyzed large-scale process.
The heteronuclear transition-metal oxide cluster activates methane: VNbO5(+) reacts with CH4 under ambient conditions via hydrogen-atom transfer (HAT), thus providing an interesting prototype example of room-temperature methane activation by a binary transition-metal oxide cluster.
