Molecular beam investigations in combination with IR/UV spectroscopy offer the possibility to obtain structural information on isolated molecules and clusters. One of the demanding tasks is the discrimination of different isomers, e.g., by the use of isomer specific UV excitations. If this discrimination fails due to overlaying UV spectra of different isomers, IR/IR methods offer another possibility. Here, we present a new IR/UV/IR/UV quadruple resonance technique to distinguish between different isomers especially in the electronically excited state. Due to the IR spectra, structural changes and photochemical pathways in excited states can be assigned and identified. The method is applied to the dihydrated cluster of 3-hydroxyflavone which has been investigated as photochemically relevant system and proton wire model in the S(1) state. By applying the new IR/UV/IR/UV technique, we are able to show experimentally that both in the electronic ground (S(0)) and the electronically excited state (S(1)) two isomers have to be assigned.

The functionality of bioactive molecules sensitively depends on their structure. For the investigation of intrinsic structural properties, molecular beam experiments combined with laser spectroscopy have proven to be a suitable tool. Herein we present an analysis of the two isolated tripeptide model systems Ac-Phe-Tyr(Me)-NHMe and Boc-Phe-Tyr(Me)-NHMe. For this purpose, mass-selective combined IR/UV spectroscopy is applied to both substances in a molecular beam experiment. The comparison of the experimental data with DFT calculations, including different functionals as well as dispersion corrections, allows an assignment of both tripeptide models to β-turns formed independently from the protection groups and supported by the interaction of the two aromatic chromophores.

The structure and reactivity of isolated 3-hydroxyflavone (3-HF) aggregates with one and two water molecules has been investigated by applying combined infrared/ultraviolet (IR/UV) spectroscopy in a supersonic jet both for the electronic ground and excited states. In combination with density functional theory (DFT) calculations, the IR spectra of the S(0) states recorded from the upper fingerprint region to the OH stretching vibrations are assigned to the most stable isomers of the clusters. For the first electronically excited (pi pi(*)) state of the 3-HF(H(2)O)(2) cluster structural information are obtained from the IR spectra in combination with time-dependent-DFT (TD-DFT) calculations. No proton transfer structure is observed in the case of a low excess energy within the electronically excited state. By raising the excitation energy, additional bands appear that can be assigned to a tautomeric form indicating the induction of a proton transfer reaction.

IR/R2PI-spectroscopy has been applied to the electronic ground and electronically excited states of 3-hydroxyflavone (3-HF) and 2-(2-naphthyl)-3-hydroxychromone (2-NHC) in a supersonic jet yielding direct structural information on the educt and product of a proton transfer reaction. We show that IR spectra of the electronically excited states can be recorded subsequent to a photoinduced chemical reaction, in this case a proton transfer. In combination with DFT and TDDFT calculations structural assignments are performed.