Photochemical reactions are of great interest due to their importance in chemical and biological processes. Highly sensitive IR/UV double and triple resonance spectroscopy in molecular beam experiments in combination with ab initio and DFT calculations yields information on reaction coordinates and Intersystem Crossing (ISC) processes subsequent to photoexcitation. In general, molecular beam experiments enable the investigation of isolated, cold molecules without any influence of the environment. Furthermore, small aggregates can be analyzed in a supersonic jet by gradually adding solvent molecules like water. Conclusions concerning the interactions in solution can be derived by investigating and fully understanding small systems with a defined amount of solvent molecules. In this work the first applications of combined IR/UV spectroscopy on reactive isolated molecules and triplet states in molecular beams without using any messenger molecules are presented. Special focus was on excited state proton transfer reactions, which can also be described as keto enol tautomerisms. Various molecules such as 3-hydroxyflavone, 2-(2-naphthyl)-3-hydroxychromone and 2,5-dihydroxybenzoic acid have been investigated with regard to this question. In the case of 3-hydroxyflavone and 2-(2-naphthyl)-3-hydroxychromone, the IR spectra have been recorded subsequent to an excited state proton transfer. Furthermore the dihydrate of 3-hydroxyflavone has been analyzed concerning a possible proton transfer in the excited state: The proton transfer reaction along the water molecules (proton wire) has to be induced by raising the excitation energy. However, photoinduced reactions involve not only singlet but also triplet states. As an archetype molecule xanthone has been analysed. After excitation to the S2 state, ISC occurs into the triplet manifold leading to a population of the T1 state. The IR spectrum of the T1 state has been recorded for the first time using the UV/IR/UV technique without using any messenger molecules. Altogether it is shown that IR/UV double and triple resonance techniques are suitable tools to analyze reaction coordinates of photochemical processes.
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.