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ABSTRACT
"Spin and orbital contribution to the magnetic moment of transition metal clusters and complexes"
The spin and orbital contributions to the magnetic moments of isolated iron \(Fe_n^+\) \((7 ≤ n ≤ 18)\), cobalt \(Co_n^+\) \((8 ≤ n ≤ 22)\) and nickel \(Ni_n^+\) \((7 ≤ n ≤ 17)\) clusters were investigated. An experimental access to both contributions is possible by the application of x-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectroscopy is based on x-ray absorption spectroscopy (XAS). It exploits the fact that for a magnetic sample the resonant absorption cross sections for negative and positive circular polarized x-rays differ for the transition from a spin orbit split ground state to the valence level. The resulting dichroic effects contain the information about the magnetism of the investigated sample. It can be extracted from the experimental spectrum via application of the so called sum rules. However, only the projections of the magnetic moments onto the quantization axis are experimentally accessible which corresponds to the magnetization of the sample.
We developed a method to apply XMCD spectroscopy to isolated clusters in the gas phase. A modified Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer was used to record the XA spectra in Total Ion Yield (TIY) mode, i.e. by recording the fragmentation intensity of the clusters in dependence of x-ray energy. The clusters can be considered to be a superparamagnetic ensemble. Thus, the magnetization follows a Langevin curve. The intrinsic magnetic moments can be calculated by Langevin correction of the experimental magnetic moments because the cluster temperature and the magnetic field are known.
The spin and the orbital magnetic moments are enhanced compared to the respective bulk values for all three investigated elements. The enhancement of the orbital contribution is more pronounced, by about a factor 3 - 4 compared to the bulk, than for the spin magnetic moment. However, if compared to the atomic value, both contributions are quenched. The orbital magnetic moment only amounts to about 10 - 15 % of the atomic value while the spin retains about 80 % of its atomic value. If the magnetic moments found for the clusters are put into perspective with respect to the atomic and bulk values by means of scaling laws, it becomes evident that both contributions follow different interpolations between the atomic and bulk value. The spin follows the well-known trend
\(n^{-1/3} = 1/(cluster radius)\) (n = number of atoms per cluster, assumption of a spherical particle). This trend relates to the ratio of surface to inner atoms in spherical particle. Hence, our interpretation is that the spin magnetic moment seems to follow the surface area of the cluster. On the other hand, the orbital magnetic moment follows \(1/n = 1/(cluster volume)\).
First XA spectra recorded with circularly polarized x-rays of a Single Molecule Magnet (SMM) \([Fe_4Ln_2(N_3)_4(Htea)_4(piv_6)]\) (Ln = Gd, Tb; \(H_3tea\) = triethanolamine, Hpiv = pivalic acid) are presented.
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.