Cyclic peptides represent a large class of substances that occur in nature with important biological and medical functions. Synthetic cyclic peptides are used as artificial receptors due to a series of advantages over conventional receptors. In order to optimize their binding abilities, investigations of their intrinsic structural properties especially with regard to the influence of different amino acid residues are fundamental. Here we report the structural analysis of two synthetic cyclic tetrapeptides cyclo[l-Tyr(Me)-d-Pro-l-Ala-d-Pro] (CPAla) and cyclo[l-Tyr(Me)-d-Pro-l-Glu(Me)-d-Pro] (CPGlu) in a molecular beam by means of combined IR/UV spectroscopic techniques. Structural assignments were achieved by comparing experimentally obtained vibrations and harmonically calculated frequencies including dispersion corrections (B3LYP-D3/TZVP). The investigated cyclic peptides contain an arrangement of an amino acid sequence which is no longer symmetric compared to the former investigations of the cyclo[l-Tyr(Me)-d-Pro] peptide. It turns out that all investigated compounds prefer conformations stabilized by two internal hydrogen bonds. In the case of CPGlu containing a flexible side chain with a terminal hydrogen bond acceptor an additional structure was observed in which a hydrogen bond between the terminal carboxylate group and a ring NH group is formed.

We report on a detailed multi-spectroscopic analysis of the structures and internal dynamics of diphenylether and its aggregates with up to three water molecules by employing molecular beam experiments.

The triphenylmethane and all-meta tert-butyl triphenylmethane dimers, (TPM) 2 and (T t BuPM) 2 , respectively, were studied with ionization loss stimulated Raman spectroscopy in molecular beam experiments to resolve structure sensitive vibrations. This answers the question whether the recently reported linear head-to-head arrangement in (T t BuPM) 2 results from crystal packing or prevails also in the gas phase, and therefore must result from extraordinarily strong London dispersion (LD) interactions. Our study clearly demonstrates that the head-to-head arrangement is maintained even under isolated molecular beam conditions in the absence of crystal packing effects. The central Raman-active aliphatic CÀD vibration of appropriately deuterated (T t BuPM) 2 associated with an unusually short CÀD•••DÀC distance exhibits a strong blue-shift compared to the undisturbed case. As the LD stabilizing tert-butyl groups are absent in (TPM) 2 , it displays an approximately S 6 -symmetric tail-to-tail arrangement.

Isomer-specific, IR-induced reactions in the electronic ground state (S) can be of great interest to control reaction pathways. Here we show a first example of these reactions with isomer-specific excitation and formation of a new isomer under isolated conditions in a molecular beam experiment. The investigated dihydrated cluster of 3-hydroxyflavone forms two isomers, I and D, in the S state. We show that only a mode-specific excitation of isomer I leads to a structural rearrangement yielding an isomer that has not been identified so far. This isomer is assigned in comparison to quantum chemical calculations. The experiments are performed by applying an IR/IR method in combination with a mass-selective resonant two-photon ionization (R2PI) process. Usually these kinds of IR/IR/R2PI methods are chosen to discriminate isomers; here it is demonstrated that this powerful method can also be applied for analysis of IR-induced reactions probed by an IR/R2PI process.

Triphenylmethan und seine (substituierten) all-metatert-Butyl-Triphenylmethan-Dimere, (TPM) 2 und (T t BuPM) 2 , wurden mit stimulierter Raman-Spektroskopie in Molekularstrahlexperimenten (Ionization loss stimulated Raman spectroscopy, ILSRS) untersucht, um struktursensitive Schwingungen zu analysieren. Damit konnte die Frage beantwortet werden, ob die kürzlich beschriebene lineare Kopf-an-Kopf-Anordnung in (T t BuPM) 2 aufgrund von Kristallpackungseffekten resultiert oder auch in der Gasphase vorherrscht und folglich durch außerordentlich starke London-Dispersionswechselwirkungen (LD) bedingt ist. Unsere Studie belegt eindeutig, dass die Kopf-an-Kopf-Anordnung auch unter isolierten Molekularstrahlbedingungen ohne Kristallpackungseffekte beobachtet wird. Bei entsprechend deuteriertem (T t BuPM) 2 , welches in der hier zugeordneten Struktur einen ungewçhnlich kurzen C-D•••D-C-Abstand aufweist, zeigt die zentrale Raman-aktive aliphatische C-D-Streckschwingung eine charakteristische Blauverschiebung im Vergleich zu anderen Dimeranordnungen. Da im Fall von (TPM) 2 die LD-stabilisierenden tert-Butylgruppen fehlen, zeigt es eine annähernd S 6symmetrische Rücken-an-Rücken-Anordnung.