Clusters bridge the gap between single atoms or molecules and the condensed phase and it is the challenge of cluster science to obtain a deeper understanding of the molecular foundation of the observed cluster specific properties/reactivities and their dependence on size. The electronic structure of hydrated magnesium monocations [Mg,nH2O]+, n<20, exhibits a strong cluster size dependency. With increasing number of H2O ligands the SOMO evolves from a quasi-valence state (n=3-5), in which the singly occupied molecular orbital (SOMO) is not yet detached from the metal atom and has distinct sp-hybrid character, to a contact ion pair state. For larger clusters (n=17,19) these ion pair states are best described as solvent separated ion pair states, which are formed by a hydrated dication and a hydrated electron. With growing cluster size the SOMO moves away from the magnesium ion to the cluster surface, where it is localized through mutual attractive interactions between the electron density and dangling H-atoms of H2O ligands forming "molecular tweezers" HO-H (e-) H-OH. In case of the hydrated aluminum monocations [Al,nH2O]+,n=20, different isomers of the formal stoichiometry [Al,20H2O]+ were investigated by using gradient-corrected DFT (BLYP) and three different basic structures for [Al,20H2O]+ were identified: (a) [AlI(H2O)20]+ with a threefold coordinated AlI; (b) [HAlIII(OH)(H2O)19]+ with a fourfold coordinated AlIII; (c) [HAlIII(OH)(H2O)19]+ with a fivefold coordinated AlIII. In ground state [AlI(H2O)20]+ (a) which contains aluminum in oxidation state +1 the 3s2 valence electrons remain located at the aluminium monocation. Different than for open shell magnesium monocations no electron transfer into the hydration shell is observed for closed shell AlI. However, clusters of type (a) are high energy isomers (DE»+190 kJ mol-1) and the activation barrier for reaction into cluster type (b) or (c) is only approximately 14 kJ mol-1. The performed ab initio calculations reveal that unlike in [Mg,nH2O]+, n=7-17, for which H atom eliminiation is found to be the result of an intracluster redoxreaction, in [Al,nH2O]+,n=20, H2 is formed in an intracluster acid-base reaction. In [Mg,nH2O]+, n>17, the magnesium dication was found to coexist with a hydrated electron in larger cluster sizes. This proves that intermolecular electron delocalization - previously almost exclusively studied in (H2O)n- and (NH3)n- clusters - can also be an important issue for water clusters doped with an open shell metal cation or a metal anion. Structures and stabilities of hydrated magnesium water cluster anions with the formal stoichiometry [Mg,nH2O]-, n=1-11, were investigated by application of various correlated ab initio methods (MP2, CCSD, CCSD(T)). Metal cations surely have high relevance in numerous biological processes, and as most biological processes take place in aqueous solution hydrated metal ions will be involved. However, in biological systems solvent molecules (i.e. water) compete with different solvated chelate ligands for coordination sites at the metal ion and the solvent and chelate ligands are in mutual interactions with each other and the metal ion. These interactions were investigated for the hydration of ZnII/carnosine complexes by application of FT-ICR-MS, gas-phase H/D exchange experiments and supporting ab initio calculations. In the last chapter of this work the Free Electron Laser IR Multi Photon Dissocition (FEL-IR-MPD) spectra of mass selected cationic niobium acetonitrile complexes with the formal stoichiometry [Nb,nCH3CN]+, n=4-5, in the spectral range 780 – 2500 cm-1 are reported. In case of n=4 the recorded vibrational bands are close to those of the free CH3CN molecule and the experimental spectra do not contain any evident indication of a potential reaction beyond complex formation. By comparison with B3LYP calculated IR absorption spectra the recorded spectra are assigned to high spin (quintet, S=2), planar [NbI(NCCH3)4]+. In [Nb,nCH3CN]+, n=5, new vibrational bands shifted away from those of the acetonitrile monomer are observed between 1300 – 1550 cm-1. These bands are evidence of a chemical modification due to an intramolecular reaction. Screening on the basis of B3LYP calculated IR absorption spectra allow for an assignment of the recorded spectra to the metallacyclic species [NbIII(NCCH3)3(N=C(CH3)C(CH3)=N)]+ (triplet, S=1), which has formed in a internal reductive nitrile coupling reaction from [NbI(NCCH3)5]+. Calculated reaction coordinates explain the experimentally observed differences in reactivity between ground state [NbI(NCCH3)4]+ and [NbI(NCCH3)5]+. The reductive nitrile coupling reaction is exothermic and accessible (Ea=49 kJ mol-1) only in [NbI(NCCH3)5]+, whereas in [NbI(NCCH3)4]+ the reaction is found to be endothermic and retarded by significantly higher activation barriers (Ea>116 kJ mol-1).
Within this thesis a series of molecular species has been studied, with focus on hydrogen bonded species and on (solvated) transition metal complexes. Experimental techniques such as FT-ICR-MS and IRMPD were combined with ab initio calculations for the determination of structure and reactivity of the aforementioned types of systems. On the basis of high level electronic structure calculations of neutral water clusters (H2O)n with n = 17-21 a transitional size regime has been determined, where a structural stabilization between all-surface and interior configurations alternates with the addition or removal of a single water molecule. Electronic structure calculations suggested that for n = 17 and 19 the interior configuration would be energetically more stable than the all-surface one. The gas phase infrared spectrum of the singly hydrated ammonium ion, NH4+(H2O), had previously been recorded by photodissociation spectroscopy of mass selected ions and interpreted by means of ab initio calculations. The present work provides additional information on the shape of the potential energy curves of NH4+(H2O) along the N-H distance on MP2/aug-cc-pVDZ level of theory yielding an anharmonic potential shape. Calculation of potential energy curves of the O-H mode of the intramolecular hydrogen bond of various dicarboxylic acids (oxalic to adipic acid) revealed that the shapes of the potentials directly correlate to the size of the system and the resulting ring strain The shape of the potential is also influenced by the charge of the system. Calculation of anharmonic frequencies based on the VPT2 approach lead to reasonable results in all systems with narrow potentials. IRMPD spectra of complexes in the gas phase have been recorded for a series of cationic vanadium oxide complexes when reacted with acetonitrile, methanol and ethanol. The experimental spectra are compared to calculated absorption spectra. The systematic DFT study identifies potential candidates for reductive nitrile coupling in cationic transition metal acetonitrile complexes. On the basis of the calculations, the formation of metallacyclic structures in group 3 through 7 complexes can be ruled out. Solvation of the transition metal cation by five acetonitrile ligands leads to a reductive nitrile coupling reaction in three types of complexes, namely those containing either niobium, tantalum or tungsten.