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