Infrared Photodissociation Spectroscopy of Mass-Selected Cluster Ions in the Gas Phase

This habilitation thesis describes my research activities at the Institute for Experimental Physics of the Free University Berlin, which I conducted in the group of Prof. Dr. Ludger Wöste in the period from September 1999 until December 2004. These research activities were the central part of, and financed by, the projects Asmis/Wöste of the Dedicated Research Center „Structure, Dynamics und Reactivity of Transition Metal Oxide Aggregates“ (SFB-546) and the Graduate School „Hydrogen Bonding and Hydrogen Transfer" (GK-788) of the German Research Foundation DFG. The central goal of this work was the development of novel experimental methods to characterize the structure of mass-selected gas phase cluster ions (see Chapter A). Infrared spectroscopy (Chapter B) has been a standard method for structural characterization of condensed phase samples for many decades. Its application to gas phase ions poses mainly two experimental challenges. First, the low number densities of ions attainable in the gas phase, roughly less than one million per cubic centimeter, generally prohibit direct absorption measurements. Second, most of the characteristic infrared transitions lie in the fingerprint region (500 to 2000 cm-1 ) of the electromagnetic spectrum, a region which cannot be continuously covered with the required intensity using commercially available infrared radiation sources. To address these problems a novel, mobile tandem mass spectrometer was constructed (Chapter C.1), which allows trapping, cooling, and probing of mass-selected gas phase ions. The infrared photodissociation experiments (Chapter B.1) were performed at the FOM Institute for Plasma Physics Rijnhuizen (Nieuwegein, The Netherlands) using the free electron laser FELIX (Chapter C.2). In these experiments, FELIX is used as a monochromatic “Bunsen burner”, i.e., the ions are irradiated with intense infrared radiation of a specific wavelength. If the wavelength coincides with an infrared transition, the ion is resonantly heated and eventually breaks apart (Chapter B.3). The absorption is detected indirectly by measuring the fragment ion yield, resulting in a high selectivity and sensitivity. The measured infrared spectrum is a “fingerprint” of the molecular structure and its assignment is generally based on a comparison with the simulated spectra of possible candidates. The developed techniques were applied to two research areas. As part of the SFB-546 we were able to measure the infrared spectra of small vanadium oxide ions for the first time and, based on these, characterize their geometric and electronic structure (Chapter D.1). Unexpectedly, we were able to show a correlation between the spectra of a vanadium oxide surface and cluster ion cages of moderate size (~30 atoms). As part of the GK-788 we measured the first infrared spectra of model systems containing strong hydrogen bonds in the spectral region of the shared proton modes (Chapter D.2). The characterization of the spectral signature of the protonated water dimer H₅O₂⁺, also referred to as the „Zundel cation“, was particularly noteworthy. The failure to accurately model this infrared spectrum impressively demonstrates the difficulties of present day electronic structure theory in describing strongly coupled vibrational modes. The experimental work described in this thesis was a team effort and only possible as such. Parts of this work constitute the Ph.D. and Diploma theses of Mathias Brümmer, Sara Fontanella, Oliver Gause, Cristina Kaposta, Gabriele Santambrogio, and Carlos Cibrián-Uhalte

Epicurus\u27 Scientific Method

I argued that Epicurus sought to infer all truths about what cannot be perceived (what is non-apparent , adêlon) by using what is perceived (the phenomena ) as signs of what is non-apparent. This is an empirical method, based on the claim that sensory perception shows what is true. In the case of his basic physical doctrines, Epicurus argued by a reductio ad absurdum that if the negation of the doctrine were true, there would be a conflict with what we perceive. This applies to Epicurus\u27 first two doctrines, that nothing come to be from non-being and that nothing is destroyed into nonbeing , as much as to succeeding doctrines. Contrary to the usual view (which goes back to Aristotle), Epicurus did not take over these claims from Parmenides. Instead, he proved them by opposing Parmenides\u27 conceptualism with the empirical claim that the phenomena are true. I suggest that Epicurus derived this method from the early atomists

ERRATUM TO: Mass-selective vibrational spectroscopy of vanadium oxide cluster ions [Mass Spect. Rev. 26, 542-562 (2007)]

On p. 558, right column, line 10 from bottom, the reference “(Santambrogio et al., 2007)” should be replaced by “(Santambrogio et al., 2006). On p. 561, the reference “Santambrogio G, Brümmer M, Wöste L, Döbler J, Sierka M, Sauer J, Meijer G, Asmis KR. 2007. Gas Phase Infrared Spectroscopy of Mass-Selected Vanadium Oxide Cluster Anions. Submitted to J Chem Phys.“ Should be replaced by “Santambrogio G, Brümmer M, Wöste L, Döbler J, Sierka M, Sauer J, Meijer G, Asmis KR. 2006. Gas Phase Vibrational Spectroscopy of Mass-Selected Vanadium Oxide Cluster Anions. In preparation.

Cryogenic Ion Vibrational Spectroscopy of Gas-Phase Clusters: Structure, Anharmonicity and Fluxionality

Gas-phase clusters are aggregates of a countable number of particles, which exhibit size-dependent physical and chemical properties that typically lie in the non-scalable size regime. These properties can be systematically characterized at a molecular level with respect to composition, size and charge state. This allows studying how macroscopic properties of condensed matter, e.g. phase transitions or metallic behavior, emerge from the atomic or molecular properties as a function of cluster size. Furthermore, smaller clusters are also amenable to high-level quantum chemical calculations, making them ideal model systems for understanding phenomena in more complex heterogeneous matter. The main advantage here is that clusters can be studied with a very high degree of selectivity and sensitivity, under well-defined conditions and in the absence of perturbing interaction with an environment. The studies presented in this theses focus on the structure characterization of ionic clusters using cryogenic ion vibrational spectroscopy. This technique combines cryogenic ion trapping with mass spectrometric schemes and infrared photodissociation (IRPD) spectroscopy. It makes use of an ion-trap triple mass spectrometer in combination with various light sources that grant access to a wide range of the infrared spectrum (210-4000 cm-1). Structures are typically assigned by comparing experimental IRPD spectra with computed vibrational spectra. The structures of aluminum oxide clusters and their interaction with water are studied in the framework of the collaborative research center CRC1109 "Understanding of Metal Oxide / Water Systems at the Molecular Scale: Structural Evolution, Interfaces, and Dissolution". This project aims at gaining a molecular level understanding of the mechanisms involved in oxide formation and dissolution. Section 4.1 and 4.2 present results of IRPD spectroscopy experiments on small mono and dialuminum oxide anions and on the anionic cluster series (Al2O3)nAlO2- with n = 0 to 6. These studies discuss the effects of the distribution of the excess charge on the cluster structure, analyze how structural properties evolve with size and how these relate to those of nanoparticles and crystal surfaces. The dissociative adsorption of water by Al-oxide clusters is investigated in Section 4.3.2. Boron exhibits a rich variety of polymorphs with the B12 icosahedron as a common building block. This three dimensional (3D) structure is retained in the halogenated closo-dodecaborate dianions (B12X122-). On the other hand, small pure boron clusters are essentially planar. The study presented in Section 5.2 investigates the 3D to 2D structural transition by probing the vibrational spectra of partially deiodinated B12In2- clusters as a function of decreasing n. The results presented in Section 5.1 show that B13+ has a planar structure consisting of two concentric rings. As a result of delocalized aromatic bonding, this structure is particularly stable without being rigid as it permits an almost free rotation of the inner ring. Protonated water clusters are model systems for understanding protons in aqueous solutions. The interpretation of their vibrational spectra is a challenge for state-of-the-art electronic structure calculations and therefore often prone to controversies. The results presented in Chapter 6 clear existing doubts over the assignment of the protonated water pentamer structure and the vibrational fingerprints of the embedded distorted H3O+. This study laid the foundation for a subsequent series of measurements which provided crucial new insights into the proton transfer mechanism in water

Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4].+/C2H4 Couple by Gas‐Phase Vibrational Spectroscopy

The gas‐phase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150 K using cryogenic ion‐trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier‐free thermal C2H4→CH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.DFG, 390540038, EXC 2008: UniSysCatDFG, 234149247, SFB 1109: Molekulare Einblicke in Metalloxid-Wasser-Systeme: Strukturelle Evolution, Grenzflächen und AuflösungTU Berlin, Open-Access-Mittel - 201

10K Ring Electrode Trap - Tandem Mass Spectrometer for Infrared Spectroscopy of Mass Selected Ions

A novel instrumental setup for measuring infrared photodissociation spectra of buffer gas cooled, mass-selected ions is described and tested. It combines a cryogenically cooled, linear radio frequency ion trap with a tandem mass spectrometer, optimally coupling continuous ion sources to pulsed laser experiments. The use of six independently adjustable DC potentials superimposed over the trapping radio frequency field provides control over the ion distribution within, as well as the kinetic energy distribution of the ions extracted from the ion trap. The scheme allows focusing the ions in space and time, such that they can be optimally irradiated by a pulsed, widely tunable infrared photodissociation laser. Ion intensities are monitored with a time-of-flight mass spectrometer mounted orthogonally to the ion trap axis

Wirkungsweise alter und neuer Antikoagulanzien

Zusammenfassung: Die drei häufigsten Indikationen für eine Behandlung mit Hemmern der plasmatischen Gerinnung sind das Vorhofflimmern, die venöse Thromboembolie und valvuläre Kardiopathien. Aufgrund der Häufigkeit dieser Pathologien kann man davon ausgehen, dass rund 1% der Bevölkerung oral antikoaguliert ist. Ungefähr ein Drittel der auf einer chirurgischen Abteilung hospitalisierten Patienten erhält Vitamin-K-Antagonisten. Somit handelt es sich hierbei um eine Behandlungsmodalität, mit der sowohl Grundversorger und Spezialisten in der Praxis als auch Klinikärzte fortwährend konfrontiert sind. Inhibitoren der Gerinnung haben in vielen Situationen einen großen Nutzen gezeigt, welcher allerdings mit einem erhöhten Blutungsrisiko bezahlt sein will. Diese Gratwanderung zwischen einem Zuviel und einem Zuwenig der antikoagulatorischen Wirkung ist eine wichtige Herausforderung in der klinischen Arbeit mit Patienten. Die Kenntnis der der medikamentösen Gerinnungshemmung zugrundeliegenden Mechanismen ist notwendig, um Indikationen für antikoagulatorische Therapien kritisch evaluieren und deren Management effizient steuern zu können. Im Idealfall muss die Sicherstellung einer ausreichenden Hämostase bei gleichzeitiger Vermeidung thrombotischer Ereignisse das Ziel einer adäquaten Gerinnungshemmung sein. Dieser Artikel soll einen Überblick über das Gerinnungssystem und über etablierte, aber auch neue pharmakologische Angriffspunkte biete

Structure Characterization of Metal Oxide Clusters by Vibrational Spectroscopy: Possibilities and Prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field

The Chemical Nature of Ti₄O₁₀^-: Vibrational Predissociation Spectroscopy Combined with Global Structure Optimization

The gas-phase infrared spectrum of Ti4O10- is studied in the spectral range from 400 cm–1 to 1250 cm–1 using cryogenic ion trap vibrational spectroscopy, in combination with density functional theory (DFT). The infrared photodissociation (IRPD) spectrum of D2-tagged Ti4O10- provides evidence for a structure of lower symmetry that contains a superoxo group (1121 cm–1) and two terminal Ti=O moieties. DFT combined with a genetic algorithm for global structure optimization predicts two isomers which feature a superoxo group: the Cs symmetric global minimum-energy structure and a similar isomer (C1) that is slightly higher in energy. Coupled cluster calculations confirm the relative stability. Comparison of the harmonic DFT spectra (different functionals) with the IRPD spectrum suggests that both of these isomers contribute. Earlier assignments to the adamantane-like C3v isomer with three terminal Ti–O• – groups in a quartet state are not confirmed. They were based on the infrared multiple photon photodissociation (IRMPD) spectrum of bare Ti4O10- and local DFT structure optimizations

Gas-Phase Mechanism of O^.-/Ni²⁺-Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl2O4+ with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2O4H+ and NiAl2O3H2+, are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2O4+ contains Ni in the +II oxidation state and features a terminal Al−O.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3⋅) or formaldehyde (CH2O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3⋅ loss or CH3⋅ migration to an O2- site next to the Ni2+ center. The CH3⋅ attaches as CH3+ to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+. The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions