Energy-resolved Collision-induced Dissociation Experiments Research Articles

Fragmentation reactions of thiourea- and urea-compounds examined by tandem MS-, energy-resolved CID experiments, and theory

Fragmentation reactions of thiourea- and urea-compounds, which are promising reagents for chemical crosslinking (XL), are investigated in detail by collision-induced dissociation (CID) experiments in a quadrupole ion trap (QIT), energy-resolved CID experiments, and computational modeling. For this study, an array of six labeled and unsymmetrical substituted thiourea- and urea-derivatives were synthesized, which allow unambiguous characterization of competing fragmentation pathways. The results of the QIT-CID-experiments are explored in detail for two compounds and confirmed by results for the other four. These results document the subtle competition of characteristic fragmentation pathways of this class of compounds. The multi dimensional investigations of the characteristic fragmentation reactions allow reliable structure proposals of prominent product ions. Energy-dependent CID experiments on two of the six compounds lead to breakdown curves that show similar relative threshold energies for the formation of the product ions on which the functioning of the XL application relies. The experimental results are in full consistency with the results from in-depth computations. For the dominant fragmentations observed, the transition states for moving the proton from the most basic site of the precursor ion (the thiourea-sulfur or the urea carbonyl oxygen) to less basic heteroatoms in the protonated molecular ion is the necessary and decisive step that determines the extent of the charge-driven fragmentation processes that follow.

Transmetalation of Methyl Groups Supported by PtII–AuI Bonds in the Gas Phase, in Silico, and in Solution

We report Pt(II)-to-Au(I) methyl transfer reactions that occur in the gas phase and in solution. The heterobimetallic Pt(II)/Au(I) complexes {[(dmpe)PtMe(2)][AuPR(3)]}(+) (R = Me (2a), Ph (2b), (t)Bu (2c)), observed in the gas phase by means of electrospray ionization, were subjected to collision induced dissociation (CID) from which we could observe Pt-to-Au transmetalation along two reaction pathways involving formation of a Au-Me bond, analogous to those observed for the Pt(II)/Cu(I) complex recently reported. In the first pathway, neutral AuMe is generated with concomitant migration of PR(3) from Au(I) to the Pt(II) center, forming cation [(dmpe)PtMe(PR(3))](+) (R = Me (5a) or Ph (5b)). In the second pathway, the monophosphine stays attached to the gold center, yielding cation [(dmpe)PtMe](+) (7) and R(3)PAuMe. Quantitative energy-resolved collision induced dissociation experiments as well as density functional theory (DFT) calculations were used to investigate the potential surface involved in the transmetalation processes. Energy barriers of 22.3 and 47.9 kcal mol(-1) for the two reaction processes of 2b and of 45.4 kcal mol(-1) for the single reaction process of 2c were obtained. Parallel reactivity is observed in THF solution, allowing for a comparison of the product distributions with those observed in the gas phase, and the postulation of simple steric control of the branching ratio between the two pathways. DFT calculations at the M06-2X//BP86/TZP level were in good agreement with the experiments.

Structure and Bonding of Isoleptic Coinage Metal (Cu, Ag, Au) Dimethylaminonitrenes in the Gas Phase

Dimethylaminonitrene complexes of IMesM(+) (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; M = Cu, Ag, Au) were prepared in the gas phase and structurally characterized by high-resolution infrared spectroscopy of the cold species, ion-molecule reactions, and DFT computations. We measured the binding energies of the nitrene fragment to the IMesM(+) moiety by energy-resolved collision-induced dissociation experiments in the gas phase, affording a trend in bond strength of M = Cu ≈ Au > Ag. This trend is explained in terms of a detailed metal-nitrogen bonding analysis, from which relativistic effects on the bonding were assessed. Various density functionals were evaluated for reproducing the observed thermochemical data and Truhlar's M06 functional was found to give the best agreement.

The dissociation energy of Cl 2O 2+

Crystal structures containing a trapezoidal Cl 2O 2 + cation, one of several structural isomers of Cl 2O 2 +, have been recently obtained by the Seppelt group. There is no experimental thermochemistry for this isomer, partly because the corresponding neutral structure has no significant interdiatomic bonding. We have measured the 0 K dissociation energy D(Cl 2 + O 2) = 57 ± 7 kJ/mol using energy resolved collision-induced dissociation experiments in a flowing afterglow-tandem mass spectrometer. This leads to Δ f H 298(Cl 2O 2 +) = 1056 ± 7 kJ/mol. The calculated B3LYP/aug-cc-pVTZ dissociation energy is 13 kJ/mol higher than experiment, while CASPT2 calculations by Seppelt and coworkers give a result 8 kJ/mol below experiment. The agreement between experiment and theory is better than for the related X 4 + (X = Cl, Br, and I) cations.

Competitive fragmentation and electron loss kinetics of photoactivated silver cluster anions: Dissociation energies of Agn− (n=7–11)

The photodecomposition kinetics of silver cluster anions, Agn− (n=7–11), has been investigated. The time-resolved intensities of Agn− parent ions and Agn−1− and Agn−2− photofragment product ions are measured following excitation with visible laser radiation, 415–750 nm. The atom-loss and dimer-loss product yields and reactant cluster ion depletion are compared to elucidate the decomposition kinetics of photoexcited silver cluster anions, including electron loss channels. Both prompt, direct electron photodetachment and delayed, statistical electron emission are observed in competition with cluster fragmentation product channels for some clusters. Dissociation threshold energies are determined by fitting the measured time profiles for fragmentation products using a statistical unimolecular dissociation model. The photodissociation lifetime method for measuring cluster dissociation energies is compared with previous energy-resolved collision-induced dissociation experiments on silver cluster anions.