Thomas Weiske

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.

Generation and characterization of neutral and cationic 3-sila-cyclopropenylidene in the gas phase: Description of a new BEBE tandem mass spectrometer

Abstract Collision-induced dissociation of mass-selected SiC 2 X + 2 species (X = H, D), generated by electron impact ionization of ClSi(CX 3 ) 3 (X = H, D), gives rise to spectra which are consistent with the structure of the theoretically predicted cation radical of 3-silacyclopropenylidene ( 1 + ). This species can be successfully neutralized and reionized under the conditions of neutralization-reionization mass spectrometric experiments, thus supporting previous theoretical predictions that 3-silacyclopropenylidene ( 1 ) is a stable molecule in the gas phase. A description of the BEBE tandem mass spectrometer is given.

Structure of the oxygen-rich cluster cation Al2O7+ and its reactivity toward methane and water.

The oxygen-rich cluster Al(2)O(7)(+) is generated in the gas phase and investigated with respect to both its structure and its reactivity toward small, inert molecules using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry and DFT-based calculations. Al(2)O(7)(+) reacts with CH(4) under ambient conditions via hydrogen atom transfer (HAT), and with H(2)O a ligand exchange occurs which gives rise to the evaporation of two O(2) molecules. The resulting product ion Al(2)O(4)H(2)(+) is also capable of abstracting a hydrogen atom from both H(2)O and CH(4). As indicated in the H(2)O/2O(2) ligand exchange and supported by collision-induced dissociation (CID) experiments, two O(2) units constitute structural elements of Al(2)O(7)(+). Further insight is provided by DFT calculations, performed at the unrestricted B3LYP/TZVP level, and reaction mechanisms are suggested on the basis of both the experimental and theoretical results.

Formation of endohedral carbon-cluster noble-gas compounds with high-energy bimolecular reactions: C60Hen+ (n=1,2)

Abstract Results are reported for high-energy beam studies of the formation ions in the reactions of C+.60 and C2+60 with He and D2 in a four-sector mass spectrometer. Studies of the addition of He to C+.60 at translational energies of 2,3,4,5,6 and 8 keV showed optimal adduct formation fro 5 to 6 keV. The C60He2+ adduct was observed in collisions between C2+60 and He at 6 keV translational energy. No adduct formation was observed between D2 and C+ 60 or C2+60 at 6 keV translational energy.

Catalytic Redox Reactions in the CO/N2O System Mediated by the Bimetallic Oxide-Cluster Couple AlVO3+/AlVO4+†

Exhaustive studies: The exact reaction pathway of catalytic conversion of automobile exhaust gases, such as N 2O and CO, into N 2 and CO 2 is still not completely understood. Studying this reaction at room temperature using the bimetallic oxide cluster couple AlVO 3 +/AlVO 4 + in the gas phase shows that the active M-O t . site is located at the Al-bound and not the V-bound oxygen atom (see scheme, Al pink).

Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2]+ (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer

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](•+). 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](•+), 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.

Ab initio MO calculation on the energy barrier for the penetration of a benzene ring by a helium atom. Model studies for the formation of endohedral He@C60+ complexes by high-energy bimolecular reactions

Abstract Ab initio MO calculations are reported for the C 3v -symmetric penetration of C 6 H 6 and C 6 H 6 + by a helium atom. At the highest level of theory (MP2/6-31G**//MP2/3-21G*) barriers of 10.7 and 9.4 eV are obtained. The calculated binding energy of a helium atom to benzene is negligible. These computational results are in agreement with recent experiments on the successful formation of endohedral He@C 60 + complexes and may contribute to the understanding of the penetration mechanism of fullerenes in high-energy bimolecular reactions in the gas phase.

Intrinsic alkyl radical properties inferred from the study of unimolecular dissociations of gaseous carboxylic acid cation radicals

The study of metastable carboxylic acid cation radicals (lifetime 10-5s) in the gas phase provides a detailed insight into a chemistry which is best described in terms of “free radical chemistry”. In addition to the well-known 1, n-hydrogen migrations (n = 4,5) of radicals evidence is presented that even the longsought 1,2-hydrogen migration may take place in appropriate cases. Examples for 1, n-migrations of protonated carboxyl groups (n = 2,4,5) are also discussed, while (l,n) alkyl migrations do not occur. For all n-alkyl elimination processes studied, ene-1,1-diol cation radicals were identified as the essential intermediates ; the mechanism of dissociation is the exact counterpart to the addition of nucleophilic alkyl radicals to deactivated double bonds. The study of metastable carboxylic acids in the gas phase provides the following ordering of relative leaving group abilities for n-alkyl radicals: C2H2 ≫ > C3H7 > C4H9 > C5H11 > C6H13 > C7H15 > C8H17 ⋙> CH3

Experiments aimed at generating the long-sought-after ethylenedione (OCCO) by neutralizationreionization mass spectrometry

While OCCO·+ is easily accessible from different sources and can be structurally characterized by means of collisional activation, all attempts failed to successfully neutralize C2O·+2 to the long-sought-after ethylenedione OCO. A comparison with previously published theoretical studies of the geometries of C2O·+2, C2O·-2 and C2O2 clearly indicates that vertical electron transfer is not likely to result in the formation of an observable C2O2 neutral, in keeping with our neutralization-reionization results.

Multi-step reaction pathways in the decomposition of ionised n-hexanoic acid

Abstract Metastable molecular ions of n-hexanoic acid ( 1 ) decompose unimolecularly to C2H5 and protonated methacrylic acid ( 5 -H+). Investigation of the mechanism reveals that 1) the branched cation radical 11 must be regarded as the essential intermediate in the course of the rearrangement/dissociation reaction and 2) the process commences with intramolecular hydrogen transfer from either C-3 or C-5 to the ionised carbonyl oxygen. Hydrogen transfer from C-4, which would correspond to the well-known McLafferty rearrangement, is of no importance in the C2 H5 elimination from 1 . The same conclusion applies for various alternative mechanisms, as for example a SR i type reaction, e.g. 1 + → 2 -H+. The gas phase chemistry of the cation radical of 1 , and in particular the hydrogen exchange processes between the methylene groups C -2 C -3 and C -5 C -6 , is in surprisingly close correspondence to the chemistry of alkyl radicals.