Detlef Schröder

Selective activation of alkanes by gas-phase metal ions.

The importance of the selective activation of alkanes for science and technology in the forthcoming decades does not need to be explicitly pointed out in this thematic issue of Chemical ReViews. Instead, we would like to summarize the development and the state-of-art of experimental and theoretical methods for the investigation of model reactions for alkane activation in the gas phase.1 However, before doing so let us address the question, how other scientists, both in academia as well as industry, can profit from such model studies in the gas phase, usually involving very small, charged species under conditions in a mass spectrometer which are very far from real catalysis. To this end, we refer to the synthesis of HCN as a reasonably simple example of how large-scale technical processes can be investigated in microscopic models. † In memoriam of Dr. Andreas Fiedler. * To whom correspondence should be addressed. E-mail: detlef.schroeder@ uochb.cas.cz, E-mail: roithova@natur.cuni.cz. ‡ Charles University in Prague. § Academy of Sciences of the Czech Republic. Jana Roithová (right), Ph.D., leads a group on the topic of reaction mechanisms at the Faculty of Science at the Charles University in Prague. Her research is based on the synergy of gas-phase experiments and theoretical calculations. She received her Ph.D. degree in the group of Prof. Zdenek Herman (J. Heyrovsky Institute of Physical Chemistry, Prague) on the topic of reactivity of small molecular dications in 2003. Since then she has made key contributions to superelectrophilic chemistry in the gas phase, with particular attention toward the activation of nonreactive substrates such as rare gases, nitrogen, and methane. Recently, she concentrates her research on the properties of redox-active molecules, their interactions with transition metals, and mechanisms of organometallic reactions. She is author of more than 80 papers and received, among others, a “L’Oréal for Women in Science” stipend and the Hlavka prize.

Activation of hydrogen and methane by thermalized FeO+ in the gas phase as studied by multiple mass spectrometric techniques

Abstract The ion-molecule reactions of thermalized iron-oxide cation FeO + with dihydrogen and methane have been studied by three different experimental techniques: Fourier transform ion cyclotron resonance (ICR), guided ion beam (GIB), and selected-ion flow tube (SIFT) mass spectrometry. Although these studies agree in a qualitative sense, i.e., FeO + brings about activation of H 2 and CH 4 with quite low efficiencies, there exists a considerable quantitative divergence as far as rate constants and branching ratios are concerned. The sources of error in these three related, but yet different experimental techniques are analyzed and critically reviewed. This error analysis brings the data to internal consistency with each other, once an accurate reference is used for calibration. In general, the rate constants obtained with the SIFT apparatus appear as the most accurate ones, while those obtained under ICR conditions are slightly too large, and the rate constants determined with the GIB instrument are somewhat lower than SIFT. However, the branching ratios for the formation of Fe + and FeOH + in the reaction of FeO + with methane are subject to more subtle effects. In the SIFT apparatus, termolecular stabilization of the intermediates causes differences from the ICR and GIB measurements, which were obtained under single-collision conditions.

Gas-phase activation of methane by ligated transition-metal cations

Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C–H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or σ-bond metathesis.

Dissociation behavior of Cu(urea)+ complexes generated by electrospray ionization

Abstract Electrospray ionization of aqueous solutions of Cu(II) salts in the presence of urea is used to generate the monoligated copper(I) cation Cu(urea) + . To this end, the cone voltage is appropriately adjusted, thereby affording extensive collision-induced fragmentations of the multiply ligated ions evolving from solution. Among the wide range of transition-metal complexes studied during the two last decades, the dissociation behavior of Cu(urea) + is exceptional, because the product distributions significantly deviate from expectation based on thermochemical criteria only. While fully confirming previous experimental and theoretical studies of Luna et al. [J. Phys. Chem. A 104 (2000) 3132], the present results add a note of caution to the uncritical application of kinetic methods to the competitive dissociation of transition-metal complexes.

Mass spectrometric approaches to the reactivity of transient neutrals

During the past few years, Neutralisation–Reionisation Mass Spectrometry (NRMS) has developed from a method for the generation and structural characterisation of elusive and highly reactive neutral molecules to a useful tool for probing their chemical reactivity. Three major principles can be distinguished: (i) peak shape analysis, (ii) activation of the neutrals by collisions or light, and (iii) variation of the neutrals’ lifetimes. Several methodological approaches are discussed in conjunction with illustrating examples for the chemical reactivity of transient neutrals.

Applications of electrospray ionization mass spectrometry in mechanistic studies and catalysis research.

Mechanistic studies form the basis for a better understanding of chemical processes, helping researchers develop more sustainable reactions by increasing the yields of the desired products, reducing waste production, and lowering the consumption of resources and energy overall. Conventional methods for the investigation of reaction mechanisms in solution include kinetic studies, isotope labeling, trapping of reactive intermediates, and advanced spectroscopic techniques. Within the past decade, electrospray ionization mass spectrometry (ESI-MS) has provided an additional tool for mechanistic studies because researchers can directly probe liquid samples by mass spectrometry under gentle conditions. Specifically, ESI-MS allows researchers to identify the molecular entities present in solution over the course of a chemical transformation. ESI-MS is particularly useful for investigations of organic reactions or metal catalysis that involve ionic intermediates. Accordingly, researchers are increasingly using ESI-MS in mechanistic studies and catalyst development. However, a further understanding of the ESI process and how it can facilitate mechanistic studies has not accompanied this increased use of the technique. Therefore, at least in part the ESI-MS method not only has offered great promise for the elucidation of reaction mechanisms but also became a black box with the occasional risk of misinterpretation. In this Account, we summarize applications of ESI-MS for synthetic and mechanistic research. Recently researchers have established direct linkages between gas-phase data obtained via ESI-MS and processes occurring in solution, and these results reveal qualitative and quantitative correlations between ESI-MS measurements and solution properties. In this context, time dependences, concentration series, and counterion effects can serve as criteria that allow researchers assess if the gas-phase measurements correlate with the situation in the solution. Furthermore, we report developments that bridge the gap between gas-phase and solution-phase studies. We also describe predictions derived from ESI-MS that have been verified with solution-phase chemistry experiments.

Concepts of metal-mediated methane functionalization. An intersection of experiment and theory

Concepts for the activation of methane are derived from ion-molecule reactions of mass-selected, ground-state transition-metal cations M+. Elementary steps of industrially important processes are uncovered (e.g., oxygenation of methane or its coupling with carbon dioxide). In addition, the implications of the electronic structures of M(CH2)+ complexes for their reactions with nucleophiles are discussed, and the crucial role of contemporary quantum mechanical calculations in the elucidation of mechanistic details is emphasized.

Ethylenedione: An intrinsically short-lived molecule

Ethylenedione C2O2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques, This is surprising, since theoretical studies predicted the triplet state of C2O2 to be stable towards spin-allowed dissociation and hence long-lived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization -reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive, Both the singlet and triplet states of C2O2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

Energy‐dependent dissociation of benzylpyridinium ions in an ion‐trap mass spectrometer

Benzylpyridinium ions, generated via electrospray ionization of dilute solutions of their salts in acetonitrile/water, are probed by collisional activation in an ion-trap mass spectrometer. From the breakdown diagrams obtained, phenomenological appearance energies of the fragment ions are derived. Comparison of the appearance energies with calculated reaction endothermicities shows a reasonably good correlation for this particular class of compounds. In addition, the data indirectly indicate that at threshold the dissociation of almost all of the benzylpyridinium ions under study leads to the corresponding benzylium ions, rather than the tropylium isomers. Substituent effects on the fragmentation for a series of benzylpyridinium ions demonstrate that neither mass effects nor differences in density of states seriously affect the energetics derived from the ion-trap experiments.

Kinetic-energy dependence of competitive spin-allowed and spin-forbidden reactions: V++CS2

The kinetic-energy dependence of the V++CS2 reaction is examined using guided ion-beam mass spectrometry. Several different ion sources are used to systematically vary the V+ electronic state distributions and elucidate the reactivities of both the ground and excited state V+ cation. The cross section for VS+ formation from ground state V+(5D) exhibits two endothermic features corresponding to the formation of ground state VS+(3Σ−) and excited state VS+(5Π). The thresholds for these two processes are in good agreement with theoretically determined excitation energies. The cross section for spin-forbidden formation of ground state VS+(3Σ−) exhibits an unusual variation with kinetic energy that is attributed to the energy dependence of the surface-crossing probability. From the thresholds associated with the formation of VS+ and V(CS)+, D0(V+–S)=3.72±0.09 eV and D0(V+–CS)=1.70±0.08 eV are derived. Further, circumstantial evidence for formation of a high-energy isomer of V(CS)+ is obtained.