A matter of radical sites and bond energies: Experimental and theoretical studies show that the cationic polynuclear aluminum oxide clusters, which are oligomeric systems of (Al2O3)x+ (x=3, 4, 5), activate methane at room temperature (see picture). In contrast, clusters with an odd number of aluminum atoms are unreactive.

A new variant of nanocalorimetry is proposed for the thermochemical analysis of ion-molecule reactions of hydrated ions in the gas phase. The average number of water molecules evaporating during the reaction is extracted by quantitative modeling of the average number of water molecules in the reactant and product cluster distribution as a function of time, taking into account black-body radiation induced dissociation. The method is tested on reactions of (H2O)n(-) with O2 and CO2, and the core exchange reaction of CO2(-)(H2O)n with O2 to yield O2(-)(H2O)n and CO2. Reproducible results are obtained for the number of water molecules evaporating. Nanocalorimetric analysis reveals a non-ergodic component of DelatE(ne) = 59 +/- 14 kJ mol(-1) in the core exchange reaction, most likely carried away by the neutral CO2 product. Extrapolation to solution phase values suggests hydration enthalpies of DeltaH(hyd) = -375 +/- 30 kJ mol(-1) for O2(-) and DeltaH(hyd) = -268 +/- 27 kJ mol(-1) for CO2(-).

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n(-), n ≈ 60-130, is investigated at temperatures T = 140-300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F(-) anion and the SF5(•) radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S-F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol(-1), in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5(•), EA(SF5(•)) = 4.27 ± 0.25 eV.

Dedicated to Professor Giulia de PetrisRanging from catalytic processes in biology, such as those catalyzed by the P-450 enzymes, [1] to heterogeneous or homogeneous catalysts used in large scale conversions, [2] oxygen-based systems represent a hallmark in contemporary catalysis and continue to attract interest.[3] One approach to address this timely topic takes advantage of the effective interplay of mass spectrometric experiments with computational methods, which allow further insight into the intrinsic properties of these oxygen-containing compounds. [4] A key issue in this context concerns the oxygen-induced homolytic CÀH bond scission of saturated and unsaturated hydrocarbons. For example, hydrogen-atom abstraction from CH 4 to generate CH 3 C is viewed as the decisive step in the oxidative dehydrogenation and dimerization of methane. [5] While the nature of the active metal-oxide surface species is still under debate in heterogeneous catalysis, [6] the crucial role of oxygen-centered radicals to bring about hydrogen abstraction by metal oxides according to reaction (1) has beensuggested in many studies, [7] including well-defined gas-phase experiments with the metal oxides [MgO]C + , [8] [FeO]C + , [9] [PbO]C + , [10] [MoO 3 ]C + , [11] [ReO 3 (OH)]C + , [12] [OsO 4 ]C + , [13] [V 4 O 10 ]C + , [14] and [(Al 2 O 3 ) x ]C + (x = 3-5), [15] as well as the metal-free oxides [SO 2 ]C + and [P 4 O 10 ]C + . [16,17] The latter one represents the first gaseous polynuclear metal-free cluster that is capable of hydrogen-atom abstraction from methane at ambient conditions; thus, it deemed of interest to compare its properties with the isostructural transition-metal cluster [V 4 O 10 ]C + , which had been suggested earlier to serve as an appropriate model for surface-mediated CÀH bond activation processes.[18] These two structural analogues show a comparable behavior in the reaction with methane; however, the reactivity patterns with ethane and ethylene differ dramatically. While [P 4 O 10 ]C + reacts with both substrates predominantly by homolytic C À H bond cleavage, [V 4 O 10 ]C + only gives rise to the oxygen-atom transfer to the hydrocarbons. [19] Therefore, studying the effects caused by the systematic variation of the vanadium/phosphorous ratios, that is generating and probing the reactivity of the cationic clusters [V 3 PO 10 ]C + , [V 2 P 2 O 10 ]C + , and [VP 3 O 10 ]C + is an appropriate approach. In addition, the enormous interest in these systems also derives from their close relations to the so-called vanadium phosphorus oxide (VPO) catalysts, which are used for highly selective oxidation and bond activation processes.[20] Finally, there is the question about the nature of the active site in mixed oxo-clusters. For example, for [AlVO 4 ]C + it is exclusively the radical site that is located at the terminal Al = O group which is responsible for C À H bond scission of methane at room temperature.[21]Herein we present our results on the mixed metal/nonmetal oxide cluster ions [V n P 4Àn O 10 ]C + (n = ...

Eine Frage von Radikalzentren und Bindungsenergien: Experimentelle und theoretische Untersuchungen belegen, dass die kationischen, mehrkernigen Aluminiumoxidcluster, die oligomere Systeme von (Al2O3)x+ (x=3, 4, 5) darstellen, Methan bereits bei Raumtemperatur aktivieren (siehe Bild). Clusterionen mit ungerader Zahl von Aluminiumatomen sind hingegen unreaktiv.

The reactions of hydrated monovalent transition metal ions M(+)(H(2)O)(n), M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward molecular oxygen, nitrous oxide, and carbon dioxide were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Clusters containing monovalent chromium, cobalt, nickel, or zinc were reactive toward O(2), while only hydrated cobalt was reactive toward N(2)O. A strongly size dependent reactivity was observed. Chromium and cobalt react very slowly with carbon dioxide. Nanocalorimetric analysis, (18)O(2) exchange, and collision induced dissociation (CID) experiments were done to learn more about the structure of the O(2) products. The thermochemistry for cobalt, nickel, and zinc is comparable to the formation of O(2)(-) from hydrated electrons. These results suggest that cobalt, nickel, and zinc are forming M(2+)/O(2)(-) ion pairs in the cluster, while chromium rather forms a covalently bound dioxygen complex in large clusters, followed by an exothermic dioxide formation in clusters with n ≤ 5. The results show that hydrated singly charged transition metal ions exhibit highly specific reactivities toward O(2), N(2)O, and CO(2).

Reactions of M(+) (H2 O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH(+) (H2 O)n-1 species, which are formed under the influence of room-temperature black-body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition-metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and (3) NO(-) for M=Fe, Zn, preferentially for n=15-20. For manganese, the hydride in HMnOH(+) (H2 O)n-1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.

Hydrogen‐atom transfer from a thiol group to the CO2 radical anion is a fast and selective downhill process in a gas‐phase water cluster. This suggests that placing a thiol group near the site of electron transfer to CO2 in a nanostructured electrochemical environment allows the selective formation of formic acid (see picture; dark gray C, light gray H, red O, yellow S).