Multistate Dynamics and Kinetics of CO2 Activation by Ta+ in the Gas Phase: Insights into Single-Atom Catalysis.

The activation of carbon dioxide (CO2) by a transition-metal cation in the gas phase is a unique model system for understanding single-atom catalysis. The mechanism of such reactions is often attributed to a "two-state reactivity" model in which the high-energy barrier of a spin state correlating with ground-state reactants is avoided by intersystem crossing (ISC) to a different spin state with a lower barrier. However, such a "spin-forbidden" mechanism, along with the corresponding dynamics, has seldom been rigorously examined theoretically, due to the lack of global potential energy surfaces (PESs). In this work, we report full-dimensional PESs of the lowest-lying quintet, triplet, and singlet states of the TaCO2+ system, machine-learned from first-principles data. These PESs and the corresponding spin-orbit couplings enable us to provide an extensive theoretical characterization of the dynamics and kinetics of the reaction between the tantalum cation (Ta+) and CO2, which have recently been investigated experimentally at high collision energies using crossed beams and velocity map imaging, as well as at thermal energies using a selected-ion flow tube apparatus. The multistate quasi-classical trajectory simulations with surface hopping reproduce most of the measured product translational and angular distributions, shedding valuable light on the nonadiabatic reaction dynamics. The calculated rate coefficients from 200 to 600 K are also in good agreement with the latest experimental measurements. More importantly, these calculations revealed that the reaction is controlled by intersystem crossing, rather than potential barriers.

Master equation modeling of blackbody infrared radiative dissociation (BIRD) of hydrated peroxycarbonate radical anions.

Molecular cluster ions, which are stored in an electromagnetic trap under ultra-high vacuum conditions, undergo blackbody infrared radiative dissociation (BIRD). This process can be simulated with master equation modeling (MEM), predicting temperature-dependent dissociation rate constants, which are very sensitive to the dissociation energy. We have recently introduced a multiple-well approach for master equation modeling, where several low-lying isomers are taken into account. Here, we experimentally measure the BIRD of CO4●-(H2O)1,2 and model the results with a slightly modified multiple-well MEM. In the experiment, we exclusively observe loss of water from CO4●-(H2O), while the BIRD of CO4●-(H2O)2 leads predominantly to loss of carbon dioxide, with water loss occurring to a lesser extent. The MEM of two competing reactions requires empirical scaling factors for infrared intensities and the sum of states of the loose transition states employed in the calculation of unimolecular rate constants so that the simulated branching ratio matches the experiment. The experimentally derived binding energies are ΔH0(CO4●--H2O) = 45 ± 3 kJ/mol, ΔH0(CO4●-(H2O)-H2O) = 41 ± 3 kJ/mol, and ΔH0(CO2-O2●-(H2O)2) = 37 ± 3 kJ/mol. Quantum chemical calculations on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ level, corrected for the basis set superposition error, yield binding energies that are 2-5 kJ/mol higher than experiment, within error limits of both experiment and theory. The relative activation energies for the two competing loss channels are as well fully consistent with theory.

Doubly charged dimers and trimers of heavy noble gases.

Many doubly charged heteronuclear dimers are metastable or even thermodynamically stable with respect to charge separation. Homonuclear dicationic dimers, however, are more difficult to form. He22+ was the first noble gas dimer predicted to be metastable and, decades later, observed. Ne22+ is the only other dicationic noble gas dimer that has been detected so far. Here, we present a novel approach to form fragile dicationic species, by post-ionization of singly charged ions that are embedded in helium nanodroplets (HNDs). Bare ions are then extracted by colliding the HNDs with helium gas. We detect homonuclear doubly charged dimers and trimers of krypton and xenon, but not argon. Our multi-reference ab initio calculations confirm the stability of Kr22+, Kr32+, Xe22+, Xe32+, and Ar22+, but put the stability of Ar32+ towards dissociation to Ar+ + Ar2+ into question.

Ta+ and Nb+ + CO2: intersystem crossing in ion-molecule reactions.

The reactions of Ta+ and Nb+ with CO2 proceed only by a highly efficient oxygen atom transfer reaction to the respective oxide at room temperature in the gas phase. Although the product spin states are not determined, thermochemistry dictates that they must be different from ground state quintet Ta+ and Nb+, implying that intersystem crossing (ISC) has occurred. Recent reactive scattering experiments found dominant indirect dynamics for the reaction with Ta+ hinting at a bottleneck along the reaction path. The question on the nature of the bottleneck, whether it involves a crossing point or a transition state, could not be finally answered because theory located both close to each other. Here, we aim at shedding further light onto the impact of intersystem crossing on the reaction dynamics and ultimately the reactivity of transition metal ion reactions in the gas phase. We employ a combination of thermal kinetics for Ta+ and Nb+ with CO2 using a selected-ion flow tube (SIFT) apparatus and differential scattering cross sections for Nb+ + CO2 from crossed-beam velocity map imaging. The reaction with niobium again shows dominant indirect dynamics and in general very similar dynamics compared to Ta+ + CO2. At thermal energies, both reactions show sub-collisional rate constants with small negative temperature dependencies. Experiments are complemented by high level quantum chemical calculations of the minimum energy pathway. Statistical modelling well-reproduces the experimental thermal rate constants, and suggests that the Nb+ reaction is rate-limited by the intersystem crossing at thermal energies.

Magic cluster sizes of cationic and anionic sodium chloride clusters explained by statistical modeling of the complete phase space.

As one of the main components of sea salt aerosols, sodium chloride is involved in numerous atmospheric processes. Gas-phase clusters are ideal models to study fundamental physical and chemical properties of sodium chloride, which are significantly affected by the cluster size. Of particular interest are magic cluster sizes, which exhibit high intensities in mass spectra. In order to understand the origin of these magic cluster sizes, quantum chemical calculations at the CCSD(T)//DFT level are performed, yielding structures and binding energies of neutral (NaCl)x, anionic (NaCl)xCl- and cationic (NaCl)xNa+ clusters up to x = 8. Our calculations show that the clusters can easily isomerize, enabling dissociation into the lowest-energy isomers of the fragments. Energetics can explain the special stability of (NaCl)4Cl-, but (NaCl)4Na+ actually offers low-lying dissociation channels, despite being a magic cluster size. Collision-induced dissociation experiments reveal that the loss of neutral clusters (NaCl)x, x = 2, 4, is in most cases more favorable than the loss of NaCl or the atomic ion, i.e. sodium chloride clusters actually fragment via the cleavage of the entire cluster, not by evaporating small cluster building blocks. This is rationalized by the calculated high stability of even-numbered neutral clusters (NaCl)x, especially x = 2, 4. Analysis of the density of states and rate constants calculated with a modified Rice-Ramsperger-Kassel-Marcus (RRKM) equation called AWATAR - considering all energetically accessible isomers of reactants and fragments - shows that entropic effects are responsible for the magic cluster character of (NaCl)4Na+. In particular, low-lying vibrational modes provide a high density of states of the near-planar cluster. Together with the small contribution of an atomic ion to the sum of states in a loose transition state for dissociation, this leads to a very small unimolecular rate constant for dissociation into (NaCl)4 and Na+, which is the lowest energy fragmentation pathway. Thus, entropic effects may override energetics for certain magic cluster sizes.

Hydrated Formic Acid Clusters and their Interaction with Electrons.

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low-energy (1.5-15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FAM ⋅ WN clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FAm +H]+ ions, and in FA-rich clusters, the stable ring structures surrounding H3 O+ ions are formed. In contrast, EA leads to a competition between the formation of intact [FAm ⋅ Wn ]- and dissociated [FAm ⋅ Wn -H]- fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of low-energy EA towards forming [FAm ⋅ Wn ]- , while higher electron energies tend to favor the formation of [FAm ⋅ Wn -H]- due to intracluster ion-molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA-rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.

Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe+(H2O) at Optical Wavelengths.

The fullerene ion C60+ is the only carrier of diffuse interstellar bands (DIBs) identified so far. Transition-metal compounds feature electronic transitions in the visible and near-infrared regions, making them potential DIB carriers. Since iron is the most abundant transition metal in the cosmos, we here test this idea with Fe+(H2O). Laboratory spectra were obtained by photodissociation spectroscopy at 80 K. Spectra were modeled with the reflection principle. A high-resolution spectrum of the DIB standard star HD 183143 served as an observational reference. Two broad bands were observed from 4120 to 6800 Å. The 4120-4800 Å band has sharp features emerging from the background, which have the width of DIBs but do not match the band positions of the reference spectrum. Calculations show that the spectrum arises from a d-d transition at the iron center. While no match was found for Fe+(H2O) with known DIBs, the observation of structured bands with line widths typical for DIBs shows that small molecules or molecular ions containing iron are promising candidates for DIB carriers.

Gas-Phase Electronic Structure of Phthalocyanine Ions: A Study of Symmetry and Solvation Effects.

Research into and applications of phthalocyanines (Pc) are mostly connected to their intriguing electronic properties. Here, messenger-type UV-vis spectroscopy of two metal-free ions from the phthalocyanine family, cationic H2Pc+ and H2PcD+, along with their hydrates is performed. They show that the electronic properties of both ions can be traced to those in the conjugate base, Pc2-, however, they are affected by state splitting due to the reduced symmetry; in the H2Pc+ radical cation, a new band appears due to excitations into the singly-occupied molecular orbital. Quantum chemical spectra modeling reproduces all important features of the measured spectra and provides insight into the nature of electronic transitions. Hydration of the ions has only a mild effect on the electronic spectra, showing the stability of the electronic structure with respect to solvation effects.

Electron-triggered processes in halogenated carboxylates: dissociation pathways in CF3COCl and its clusters.

Trifluoroacetyl chloride, CF3COCl, is produced in the Earth's atmosphere by photooxidative degradation of hydrochlorofluorocarbons, and represents a potential source of highly reactive halogen radicals. Despite considerable insight into photochemistry of CF3COCl, its reactivity towards electrons has not been addressed so far. We investigate the electron ionization and attachment in isolated CF3COCl molecules and (CF3COCl)N, max. N ≥ 10, clusters using a molecular beam experiment in combination with quantum chemical calculations. The ionization of the molecule at 70 eV electron energy leads to strong fragmentation: weakening of the C-C bond yields the CF3+ and COCl+ ions, while the fission of the C-Cl bond produces the major CF3CO+ fragment ion. The cluster spectra are dominated by Mn·COCl+ and Mn·CF3CO+ ions (M = CF3COCl). The electron attachment at energies between 1.5 and 11 eV also leads to the dissociation of the molecule breaking either the C-Cl bond at low energies below 3 eV yielding mainly Cl- ions, or dissociating the C-C bond at higher energies above 4 eV leading mainly to CF3- ions. In the clusters, the intact Mn- ions are stabilized after electron attachment at low energies with contribution of Mn·Cl- fragment ions. At higher energies, the Mn·Cl- fragments dominate the spectra, and C-C bond dissociation occurs as well yielding Mn·CF3-. Interestingly, Mn·Cl2- ions appear in the spectra at higher energies. We briefly discuss possible atmospheric implications.

Oxygen Radicals Entrapped between MgO Nanocrystals: Formation, Spectroscopic Fingerprints, and Reactivity toward Water.

Compaction of dehydroxylated MgO nanocrystal powders produces adsorbed oxygen radicals with characteristic UV-vis spectroscopic fingerprints. Identical absorption bands arise upon UV excitation in an oxygen atmosphere but in the absence of uniaxial pressure. Photophysical calculations on MgO gas-phase clusters reveal that the observed optical transitions at 4.4 and 3.0 eV are consistent with adsorbed superoxide (O2·-) and ozonide (O3·-) species, respectively. The presence of these oxygen radicals is corroborated by electron paramagnetic resonance spectroscopy. Upon reaction with interfacial water, oxygen radicals convert into diamagnetic products with no absorptions in the UV-vis range. Since superoxide O2·- and ozonide anions O3·- play a key role in a variety of processes in heterogeneous catalysis, sensing, or as transient species in cold sintering, their UV-vis spectroscopic detection will enable in situ monitoring of transient oxygen radicals inside metal oxide powders.

Spectroscopy of cluster aerosol models: IR and UV spectra of hydrated glyoxylate with and without sea salt.

Glyoxylic acid is formed in the troposphere by oxidation of organic molecules. In sea salt aerosols, it is expected to be present as glyoxylate, integrated into the salt environment and strongly interacting with water molecules. In water, glyoxylate is in equilibrium with its gem-diol form. To understand the influence of water and salt on the photophysics and photochemistry of glyoxylate, we generate small model clusters containing glyoxylate by electrospray ionization and study them by Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. We used infrared multiple photon dissociation spectroscopy and UV/vis photodissociation spectroscopy for structural characterization as well as quantum chemical calculations to model the spectra and dissociation patterns. Resonant absorption of infrared radiation leads to water evaporation, which indicates that water and glyoxylate are separate molecular entities in a significant fraction of the clusters, in line with the observed absorption of UV light in the actinic region. Hydration of glyoxylate leads to a change of the dihedral angle in the CHOCOO-·H2O complex, causing a slight redshift of the S1 ← S0 transition. However, the barriers for internal rotation are below 5 kJ mol-1, which explains the broad S1 ← S0 absorption extending from about 320 to 380 nm. Most importantly, hydration hinders dissociation in the S1 state, thus enhancing the quantum yield of fluorescence combined with water evaporation. No C-C bond photolysis is observed, but due to the limited signal-to-noise ratio, it cannot be ruled out. The quantum yield, however, will be relatively low. Fluorescence dominates the photophysics of glyoxylate embedded in the dry salt cluster, but the quantum yield shifts towards internal conversion upon addition of one or two water molecules.

Spectroscopy of C60+ and C120+ in the mid-infrared.

Infrared spectra of C60+ and C120+, obtained via helium messenger spectroscopy, are reported. For C60+, new absorption features have been found just above the discrete vibrational spectrum of the ion. The absorption profile, which is broad and contains little structure, is assigned to one or more electronic absorption transitions and is in good agreement with predictions from time-dependent density functional theory. It seems likely that the transitions observed correspond to excitation from the 2A1u electronic ground state to one or both of the low-lying 2E1u and 2E2u electronic states previously identified as dark states of C60+. These states presumably become optically bright through vibronic coupling and specifically the Jahn-Teller effect. In the case of C120+, the simplest positively charged oligomer of C60, we present the first vibrational spectrum of this ion. Through a comparison with theory, vibrational features are best explained by a peanut-shaped structure for C120+, maintained by covalent bonding between the two C60 units. We have also discovered electronic transitions for C120+, which, similar to C60+, lie just above the vibrational spectrum.

Dynamics of the Oxygen Atom Transfer Reaction between Carbon Dioxide and the Tantalum Cation.

The understanding of fundamental atomic-level processes often requires well-defined model systems. The oxygen atom transfer from CO2 to a transition metal cation in the gas phase presents such a model system. We investigate the reaction of Ta+ + CO2 for which the formation of TaO+ is highly efficient and attributed to multistate reactivity. Here, we study the atomistic dynamics of the oxygen atom transfer reaction by recording experimental energy and angle differential cross sections by crossed beam velocity map imaging supported by ab initio quantum chemical calculations. Product ion velocity distributions are dominated by signatures for indirect dynamics, despite the reaction being highly exothermic. Product kinetic energy distributions show little dependence on additional collision energy even with only four atoms involved, which hints at dynamical trapping behind a submerged barrier.

Imaging Frontside and Backside Attack in Radical Ion-Molecule Reactive Scattering.

We report on the reactive scattering of methyl iodide, CH3I, with atomic oxygen anions O-. This radical ion-molecule reaction can produce different ionic products depending on the angle of attack of the nucleophile O- on the target molecule. We present results on the backside and frontside attack of O- on CH3I, which can lead to I- and IO- products, respectively. We combine crossed-beam velocity map imaging with quantum chemical calculations to unravel the chemical reaction dynamics. Energy-dependent scattering experiments in the range of 0.3-2.0 eV relative collision energy revealed that three different reaction pathways can lead to I- products, making it the predominant observed product. Backside attack occurs via a hydrogen-bonded complex with observed indirect, forward, and sideways scattered iodide products. Halide abstraction via frontside attack produces IO-, which mainly shows isotropic and backward scattered products at low energies. IO- is observed to dissociate further to I- + O at a certain energy threshold and favors more direct dynamics at higher collision energies.

Decomposition of triazole and 3-nitrotriazole upon low-energy electron attachment.

Compounds based on nitrotriazole have been studied for their application as potential radiosensitizers for the treatment of tumors and as energetic materials. In the former application, the initial reduction of the compounds may serve as a mechanism which leads to the formation of tumor-active species. In this study, we investigated the fundamental properties of anion formation in isolated 3-nitro-1,2,4-triazole (3NTR) molecules upon attachment of low-energy electrons. The resulting product anions formed were detected via mass spectrometry. Quantum chemical calculations were performed to study the dissociation pathways and to derive the threshold energies. We also studied the attachment of electrons to the native 1H-1,2,4-triazole (TR) molecule, revealing the influence of the nitro group on anion formation. Comparing the results for these two systems, we computationally observed a considerable more stable parent anion for 3NTR, which results in significantly more effective degradation of the molecule at lower electron energies. Although characteristic fragmentation reactions in the presence of the nitro group were observed (like formation of NO2- or the release of an OH radical), the main dissociation channel for the 3NTR anion turned out to be the direct dissociation of a hydrogen radical by a single bond cleavage, which we also observed for TR as the main channel. Thus, the triazole ring shows a pronounced stability against electron attachment-induced cleavage compared, for example, to the imidazole ring, which is found in common nitroimidazolic radiosensitizers.

Carbon Dioxide and Water Activation by Niobium Trioxide Anions in the Gas Phase.

Transition metals are important in various industrial applications including catalysis. Due to the current concentration of CO2 in the atmosphere, various ways for its capture and utilization are investigated. Here, we study the activation of CO2 and H2O at [NbO3]- in the gas phase using a combination of infrared multiple photon dissociation spectroscopy and density functional theory calculations. In the experiments, Fourier-transform ion cyclotron resonance mass spectrometry is combined with tunable IR laser light provided by the intracavity free-electron laser FELICE or optical parametric oscillator-based table-top laser systems. We present spectra of [NbO3]-, [NbO2(OH)2]-, [NbO2(OH)2]-(H2O) and [NbO(OH)2(CO3)]- in the 240-4000 cm-1 range. The measured spectra and observed dissociation channels together with quantum chemical calculations confirm that upon interaction with a water molecule, [NbO3]- is transformed to [NbO2(OH)2]- via a barrierless reaction. Reaction of this product with CO2 leads to [NbO(OH)2(CO3)]- with the formation of a [CO3] moiety.

Surface or Internal Hydration - Does It Really Matter?

The precise location of an ion or electron, whether it is internally solvated or residing on the surface of a water cluster, remains an intriguing question. Subtle differences in the hydrogen bonding network may lead to a preference for one or the other. Here we discuss spectroscopic probes of the structure of gas-phase hydrated ions in combination with quantum chemistry, as well as H/D exchange as a means of structure elucidation. With the help of nanocalorimetry, we look for thermochemical signatures of surface vs internal solvation. Examples of strongly size-dependent reactivity are reviewed which illustrate the influence of surface vs internal solvation on unimolecular rearrangements of the cluster, as well as on the rate and product distribution of ion-molecule reactions.

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions-A Tale of Three Metals.

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature black-body radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations.

Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH4 and CO2 , two abundant greenhouse gases, mediated by Ta1,4 + ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH4 dehydrogenation [Ta1,4 CH2 ]+ to CO2 in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C-O coupling with a conversion to products of CH2 O, whereas atomic cations enable C-C coupling yielding CH2 CO. Selected experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta1,4 + compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.

Imaging the Atomistic Dynamics of Single Proton Transfer and Combined Hydrogen/Proton Transfer in the O- + CH3I Reaction.

We report on reactive scattering studies of the proton transfer and combined hydrogen/proton transfer in the O- + CH3I reaction. We combine state-of-the-art crossed-beam velocity map imaging and quantum chemistry calculations to understand the dynamics for the formations of the CH2I- + OH and CHI- + H2O products. The experimental velocity- and angle-differential cross section show for both products and at all collision energies (between 0.3 and 2.0 eV) that the product ions are predominantly forward scattered. For the CHI- + H2O channel, the data show lower product velocities, indicative of higher internal excitation, than in the case of single proton transfer. Furthermore, our results suggest that the combined hydrogen/proton transfer proceeds via a two-step process: In the first step, O- abstracts one H atom to form OH-, and then the transient OH- removes an additional proton from CH2I to form the energetically stable H2O coproduct.