Martin K. Beyer

The mechanical strength of a covalent bond calculated by density functional theory

The rupture forces of covalent bonds in a polymer as a function of bond lifetime are calculated with an Arrhenius kinetics model based on high-level density functional theory calculations. Relaxed potential energy surface scans of small model molecules yield potential functions that account for the deformations and hybridizations caused by the application of force. Morse potentials chosen to exhibit the same well depth and maximum force are used as an analytic representation of an individual bond in an infinitely long one-dimensional polymer. Application of force deforms the potential, and the activation energy for the bond rupture event together with the frequency of an optical phonon in the one-dimensional polymer are the two Arrhenius parameters. Rupture forces of the bonds C–C, C–N, C–O, Si–C, Si–N, Si–O, and Si–Si are reported as a function of the lifetime of the bond.

Diatomic [CuO]+ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane†

The activation of methane and its subsequent conversion into more valuable feedstocks at ambient conditions is regarded as one of the major challenges in contemporary catalysis. In this context, two different transformations are of particular interest. The first one concerns the oxidative coupling of methane (OCM) to the C2 hydrocarbons ethane and ethylene using metal oxide based catalysts in heterogeneous catalysis; 2] the second process is the selective oxidation of methane to methanol, which is performed in nature by the methane monoxygenase (MMO) metalloenzymes. Soluble MMO (sMMO) contains a well-characterized doubly oxygenbridged di-iron cluster; in contrast, the reactivity of particulate MMO (pMMO), after a long controversy about the nature of its active site, has been shown to depend on copper. A useful approach to investigate model systems for oxygen-containing catalysts takes advantage of state-of-theart gas-phase experiments conducted in a mass spectrometer, in conjunction with computational studies; this combined experimental and theoretical approach provides insight into the elementary steps of these reactions at a molecular level and, thus, permits us to unravel detailed mechanistic aspects. For example, the efficient gas-phase activation of methane at room temperature has been demonstrated to be brought about by a variety of systems, including transitionand maingroup-metal oxides as well as some selected nonmetal oxides and mixed metal/nonmetal oxides; based on these studies, a rather detailed understanding of the intriguing mechanistic aspects has been arrived at. With respect to biological relevance, it was demonstrated twenty years ago that bare [FeO] is capable of activating methane at room temperature. The now well-established concept of two-state reactivity (TSR), which also proved important in describing the mechanisms of metalloenzymemediated reactions, is in fact based on a detailed analysis of the gas-phase reactions of this simple, diatomic reagent [FeO]. Yet, only recently has a complete description of the gas-phase conversion of methane to methanol by [FeO] been achieved; this elucidation was based on advanced gas-phase spectroscopy combined with rather high-level calculations. Further, while the detailed nature of the active copper oxide species in pMMO had been under debate for quite some time, 4, 12] bare [CuO] was predicted a decade ago to be a suitable, if not extremely powerful, candidate to mediate the methane to methanol conversion. 14] However, no gasphase experiments with bare [CuO] have been reported to date. The ligated cation [Cu(O)(phen)] (phen = 1,10-phenanthroline) brings about activation of small hydrocarbons, that is, propane or butane, but it is not powerful enough to attack the thermodynamically strong and kinetically inert C H bond of methane. Owing to the relatively low dissociation energy D0(Cu + O) = 130 kJ mol , it proved rather difficult to produce sufficient amounts of [CuO] to probe its reactivity in bond-activation processes, and various attempts to generate this cationic metal oxide by, for example, electrospray ionization mass spectrometry failed. 15] Thus, [CuO] is to date the only bare transition-metal oxide cation of the first row whose reactivity towards methane has not been experimentally investigated. Herein we present our results on 1) the successful formation of gaseous [CuO] and 2) its reactivity towards methane at thermal conditions. Briefly, [CuO] is generated by laser desorption/ionization from isotopically pure copper Cu targets, suitable for the laser-vaporization/ionization source of an FT-ICR mass spectrometer in the presence of a He/N2O plasma (for details about the instrumental setup, see the Experimental Section). As shown in Figure 1, [CuO] brings about efficient activation of methane at room temperature both by hydrogen abstraction [Eq. (1)] and by oxygenatom transfer [Eq. (2)]. Furthermore, the open-shell product cation [CuOH]C itself also homolytically cleaves the C H bond of a second methane molecule, thus giving rise to the formation of a closed-shell water complex [Eq. (3)].

How many molecules make a solution

Water clusters are studied in order to investigate the evolution of solution phase chemistry from the molecular level to bulk. When water clusters are studied in a Fourier transform ion cyclotron resonance (FI-ICR) mass spectrometer, their fragmentation under the influence of infrared black-body radiation must be considered and can be used as a tool to monitor the destabilization of particular arrangements as a function of cluster size. As examples for solution phase chemistry, dissolution of acids, metal ion oxidation and metal halide precipitation, acid-base catalysis and hydrolysis are discussed. All these solution phase reactions proceed on the single-ion level in small water clusters. In accordance with spectroscopic and thermochemical data, gas phase ion chemistry of small water clusters rapidly approaches bulk behaviour, if the ion high concentration and pH value of the cluster are taken into account. Examination of competing reaction pathways as a function of cluster size in hydrated electron clusters reveals the crucial influence of entropy. Recent photodissociation experiments by Metz and Cocrorteers provide experimental evidence for the salt bridge mechanism for charge separation in hydrated dication clusters.

Methane activation by platinum cluster ions in the gas phase: effects of cluster charge on the Pt4 tetramer

Abstract The reactions of cationic and anionic platinum clusters Pt ± n , n =1−9, with methane CH 4 are investigated under single collision conditions in a Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer. The reaction of the platinum clusters proceeds through the activation of C–H bonds of methane and leads to the subsequent elimination of molecular hydrogen H 2 to form the final metal–carbene complex Pt ± n CH 2 . The cation cluster reactions proceed in general with collision rate whereas the anion cluster reactions are more than an order of magnitude slower. The platinum tetramer anion is unique among all the clusters studied, reacting more efficiently than the corresponding cation. Tentative interpretation in terms of electronic and geometric effects is performed.

STABILITY AND REACTIVITY OF HYDRATED MAGNESIUM CATIONS

Abstract Unimolecular fragmentation and bimolecular reactions with HCl of water clusters which nominally contain Mg + cations were studied in an FT-ICR spectrometer. A cluster fragmentation and successive evaporation of single water molecules occurring on a millisecond timescale and driven by ambient black body radiation is triggering interesting intracluster reactions. Below a certain critical size (∼17 water molecules) MgOH + forms, and a hydrogen atom is ejected. Similarly bimolecular reactions of Mg aq + clusters with HCl result in a release of H atom and formation of MgCl aq + . Both findings can be rationalized by assuming that the solvated Mg + cations actually detach an additional electron forming a Mg aq 2+ and e aq − within clusters with more than 17 water molecules. Mg + formed by recombination when not enough solvent is available to stabilize the separate charged species then reacts with a water molecule resulting in H-atom formation. Detailed studies of the ion reactions and fragmentation provide additional insights into the structure and stability of solvated magnesium cations.

Dynamic Strength of the Silicon-Carbon Bond Observed over Three Decades of Force-Loading Rates

The mechanical strength of individual Si-C bonds was determined as a function of the applied force-loading rate by dynamic single-molecule force spectroscopy, using an atomic force microscope. The applied force-loading rates ranged from 0.5 to 267 nN/s, spanning 3 orders of magnitude. As predicted by Arrhenius kinetics models, a logarithmic increase of the bond rupture force with increasing force-loading rate was observed, with average rupture forces ranging from 1.1 nN for 0.5 nN/s to 1.8 nN for 267 nN/s. Three different theoretical models, all based on Arrhenius kinetics and analytic forms of the binding potential, were used to analyze the experimental data and to extract the parameters fmax and D(e) of the binding potential, together with the Arrhenius A-factor. All three models well reproduced the experimental data, including statistical scattering; nevertheless, the three free parameters allow so much flexibility that they cannot be extracted unambiguously from the experimental data. Successful fits with a Morse potential were achieved with fmax = 2.0-4.8 nN and D(e) = 76-87 kJ/mol, with the Arrhenius A-factor covering 2.45 x 10(-10)-3 x 10(-5) s(-1), respectively. The Morse potential parameters and A-factor taken from gas-phase density functional calculations, on the other hand, did not reproduce the experimental forces and force-loading rate dependence.

Methane activation by rhodium cluster argon complexes

Abstract Rhodium cluster argon complexes Rh n + Ar m are produced by laser vaporization followed by supersonic expansion, stored in an FT-ICR mass spectrometer, and their reactions with methane investigated. Ligand exchange reactions are observed, in which up to three argon atoms are replaced by methane. In addition, the solvated rhodium dimer and trimer cations are found to dehydrogenate methane. The efficiency of the dehydrogenation depends on the number of argons, with only the dimer exhibiting this reaction without ligands. This dependence of methane activation on the size of the cluster and number of “solvent” argon atoms is discussed, and compared with heterogenous catalysis on bulk surfaces, where activity and selectivity are controlled by pressure and temperature.

Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Wet electrons and how to dry them

We present the formation of hydrated electrons by laser vaporization, and investigate in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer their destruction by the 300 K black body infrared background radiation. While clusters above n⩾32 decay almost exclusively by fragmentation and loss of ligands, the smaller species also detach electrons, with the relative rates of the two processes exhibiting an interesting alternation. Below n=15 they almost exclusively detach electrons, and for n⩽12 the detachment rate is apparently so fast that no clusters are observed in the ICR-experiment. From n=19 to n=24 a pronounced odd–even alternation between electron detachment and fragmention is observed, which is tentatively attributed to entropic rather than energetic effects.

Density functional calculations of beryllium clusters Ben, n=2–8

Neutral beryllium clusters Ben, na 2‐8, were investigated by density functional techniques. To minimize errors, geometry optimization, frequency and energy calculations were all carried out on the same level of theory, employing a large 6-311++G(3df) basis set. The method reproduces well the experimentally known bond length and vibrational frequency of the dimer, but its binding energy is still significantly overestimated. The computed trends of the vibrational frequencies, bond lengths and binding energies of the clusters as a function of the number of atoms are discussed. The binding energies are found to increase rapidly as a function of size, and approach the binding energy of the bulk metal, 54.1 kJ per bond. ” 2000 Elsevier Science B.V. All rights reserved.