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• W2564446364 abstract "This thesis combined gas phase mass spectrometric investigations of ionic transition metal clusters that are either homogeneous ((Nb_n^{+/-}, Co_n^{+/-})) or heterogeneous (([Co_nPt_m]^{+/-})), of their organo metallic reaction products, and of organic molecules (aspartame and Asp-Phe) and their alkali metal ion adducts.At the Paris FEL facility CLIO a newly installed FT-ICR mass spectrometer has been modified by inclusion of an ion bender that allows for the usage of additional ion sources beyond the installed ESI source. The installation of an LVAP metal cluster source served to produce metal cluster adsorbate complex ions of the type ([Nb_n(C_6H_6)]^{+/-}). IR-MPD of the complexes ([Nb_n(C_6H_6)]^{+/-} (n = 18, 19)) resulted in ([Nb_n(C_6)]^{+/-} (n = 18, 19)) fragments. Spectra are broad, possibly because of vibronic / electronic transitions. In Kaiserslautern the capabilities of the LVAP source were extended by adding a gas pick up unit. Complex gases containing C-H bonds otherwise break within the cluster forming plasma. More stable gases like CO seem to attach at least partially intact. Metal cluster production with argon tagged onto the cluster failed when introducing argon through the pick up source, but succeeded when using argon as expansion gas. A new mass spectrometer concept of an additional multipole collision cell for metal cluster adsorbate formation is currently under construction. Subsequent cooling shall achieve high resolution IR-MPD spectra of transition metal cluster adsorbate complexes.Prior work on reaction of transition metal clusters with benzene was extended by investigating the reaction with benzene and benzene-d6 of size selected cationic cobalt clusters (Co_n^+) and of anionic cobalt clusters (Co_n^-) in the size range (n = 3 - 28) and of bimetallic cobalt platinum clusters ([Co_nPt_m]^{+/-}) in the size range (n + m le 8). Dehydrogenation by cationic cobalt clusters (Co_n^+) is sparse, it is effective in small bimetallic clusters ([Co_nPt_m]^+ (n + m le 3)). Thus single platinum atoms promote benzene dehydrogenation while further cobalt atoms quench it. Dehydrogenation is ubiquitous in reactions of anionic cobalt clusters. Mixed triatomic clusters ([Co_2Pt_1]^-) and ([Co_1Pt_2]^-) are special in causing effective reactions and single dehydrogenation through some kind of cooperativity while ([Co_nPt_{1,2}]^- (n ge 3)) do not react at all. Kinetic isotope effects KIE(n) in total reaction rates are inverse and - in part - large, dehydrogenation isotope effects DIE(n) are normal. A multistep model of adsorption and stepwise dehydrogenation from the precursor adsorbate proves suitable to rationalize the found KIEs and DIEs in principle. Particular insights into the effects of charge and of cluster size are largely beyond this model. Some DFT calculations - though preliminary - lend strong support to the otherwise assumed structures and enthalpies. More insights into the cause of the found effects of charge, size and composition of both pure and mixed clusters shall arise from ongoing high level ab initio modeling (of especially the (n + m = 3) case for mixed clusters).The influence of the methylester group in the molecules aspartame (Asp-PheOMe) and Asp-Phe has been explored. Therefore, their protonated and deprotonated species and their complexes with alkali metal ions attached were investigated with different techniques utilizing mass spectrometry.Gas phase H-/D-exchange with (ND_3) has proven that in both molecules all acidic NH and OH binding motifs do exchange their hydrogen atom and that simultaneous multi exchange is present. Kinetic studies revealed that with alkali metal ions attached the speed of the first exchange step decreases with increasing ion size. The additional OH of the carboxylic COOHPhe group in Asp-Phe increases the exchange speed by a constant value. CID experiments yielded water and the protonated Asp-Phe anhydride as main fragments out of the protonated molecules, neutral Asp anhydride and ([Phe M]^+ / [PheOMe M]^+) for (Li^+) and (Na^+) attached, and neutral aspartame / Asp-Phe and ionic (M^+) for (K^+), (Rb^+) and (Cs^+) attached. The threshold energy (E_{CID}), indicating ion stability, decreases with increasing ion size. For aspartame fragmentation occurs at lower (E_{CID}) values for complexes with (H^+), (Li^+) and (Na^+) than for the Asp-Phe analoga. Complexes with (K^+), (Rb^+) and (Cs^+) give the same (E_{CID}) value for aspartame and Asp-Phe. IR-MPD investigations lead to the same fragments as the CID experiments. In combination with quantum mechanical calculations a change in the preferred structure from charge-solvated, tridentate type for complexes with small alkali metal ions ((Li^+)) to salt-bridge type structure for large alkali metal ions ((Cs^+)) could be confirmed. Calculations thereby reveal nearly no structural differences between aspartame and Asp-Phe for cationized species. The deprotonation of the additional COOHPhe group in Asp-Phe is preferred against other acidic positions. A better experimental distinction between possible (calculated) structure types would arise from additional FEL IR-MPD measurements in the energy range of 600 to 1800 (cm^{-1}). The comparison of the (E_{CID}) values with calculated fragmentation energy values proves that not only for alkali metal complexes with (K^+), (Rb^+) and (Cs^+), but also for (Li^+) and (Na^+) the bond breaking of all metal atom bonds is part of the transition state. The lower (E_{CID}) values for aspartame with small cations may be explained in terms of internal energy. Aspartame is a larger molecule, possesses more internal energy and can be recognized as the larger heat bath. Less energy is needed for fragmentation, if the Phe part with the additional methylester group is involved in the fragmentation process." @default.
• W2564446364 created "2017-01-06" @default.
• W2564446364 creator A5060898464 @default.
• W2564446364 date "2013-01-01" @default.
• W2564446364 modified "2023-09-27" @default.
• W2564446364 title "Metal cluster aggregates of the composition Nbn+/-, Con+/- and [ConPtm]+/- as model systems for catalytic C-H activation and structural determination of selected metal peptide complexes of the molecules aspartame (Asp-PheOMe) and Asp-Phe" @default.
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