Dispersion-Bound Isolated Dimers in the Gas Phase: Observation of the Shortest Intermolecular CH⋅⋅⋅H-C Distance via Stimulated Raman Spectroscopy.

The triphenylmethane and all-meta tert-butyl triphenylmethane dimers, (TPM) and (T  BuPM) , 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  BuPM) 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 Effect of Dispersion on the Structure of Diphenyl Ether Aggregates.

Dispersion interactions can play an important role in understanding unusual binding behaviors. This is illustrated by a systematic study of the structural preferences of diphenyl ether (DPE)-alcohol aggregates, for which OH⋅⋅⋅O-bound or OH⋅⋅⋅π-bound isomers can be formed. The investigation was performed through a multi-spectroscopic approach including IR/UV and microwave methods, combined with a detailed theoretical analysis.

The Structure of Diphenyl Ether-Methanol in the Electronically Excited and Ionic Ground States: A Combined IR/UV Spectroscopic and Theoretical Study.

Diphenyl ether offers competing docking sites for methanol: the ether oxygen acts as a common hydrogen-bond acceptor and the π system of each phenyl ring allows for OH-π interactions driven by electrostatic, induction, and dispersion forces. Based on investigations in the electronic ground state (S ), we present a detailed study of the electronically excited state (S ) and the ionic ground state (D ), in which an impact on the structural preference is expected compared with the S state. Dispersion forces in the electronically excited state were analyzed by comparing the computed binding energies at the coupled-cluster-singles (CCS) and approximate coupled-cluster-singles-doubles levels of theory (CC2 approximation).

Aromatic embedding wins over classical hydrogen bonding - a multi-spectroscopic approach for the diphenyl ether-methanol complex.

Dispersion interactions are omnipresent in intermolecular interactions, but their respective contributions are difficult to predict. Aromatic ethers offer competing docking sites for alcohols: the ether oxygen as a well known hydrogen bond acceptor, but also the aromatic π system. The interaction with two aromatic moieties in diphenyl ether can tip the balance towards π binding.

Investigation of the hydrated 7-hydroxy-4-methylcoumarin dimer by combined IR/UV spectroscopy.

The first molecular beam investigations on a coumarin dimer and clusters of a coumarin dimer with water both in the neutral (S0) and cationic (D0) electronic ground state are performed. The structure and structural changes due to ionization of the isolated 7-hydroxy-4-methylcoumarin dimer (7H4MC)2 as well as its mono- and dihydrate (7H4MC)2(H2O)1-2 are analyzed by applying combined IR/UV spectroscopy compared with density functional theory calculations. In case of the neutral dimer of 7H4MC a doubly hydrogen-bonded structure is formed.

Isolated β-turn model systems investigated by combined IR/UV spectroscopy.

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.

Structural analysis of an isolated cyclic tetrapeptide and its monohydrate by combined IR/UV spectroscopy.

Cyclopeptides are an important class of substances in nature, and their physiological effects are frequently based on the tendency to form bioactive conformations. Therefore the investigation of their structure yields an understanding of their functionalities. Mass-selective combined IR/UV spectroscopy in molecular beam experiments represents an ideal tool for structural analyses on isolated molecules in the gas phase, such as the investigated cyclo[L-Tyr(Me)-D-Pro](2) peptide and its complexes with water.

IR spectroscopy on isolated Co(n)(alcohol)m cluster anions (n=1-4, m=1-3): structures and spin states.

Isolated cobalt-alcohol cluster anions containing n=1-4 cobalt and m=1-3 alcohol molecules (alcohol=methanol, ethanol, propanol) are produced in a supersonic beam by using a laser ablation source. By applying IR photodissociation spectroscopy vibrational spectra in the OH stretching region are obtained. Several structures in different spin states are discussed for the (n,m) clusters.

Investigations of the water clusters of the protected amino acid Ac-Phe-OMe by applying IR/UV double resonance spectroscopy: microsolvation of the backbone.

In order to investigate the influence of hydration on the backbone of a peptide or protected amino acid, the successive aggregation of water to Ac-Phe-OMe is analysed by means of IR/UV double resonance spectroscopy. To achieve meaningful results the spectra have been recorded in the region of the amide A and OH stretching vibrations as well as the amide I/II modes. Comparison with ab initio and DFT calculations leads to size-selective structural assignments.

Structures of isolated Co2(alcohol)1 cluster anions.

Cobalt-alcohol cluster anions of the form Co(2)(alcohol)(1)(-) (alcohol = methanol, ethanol, propanol) are produced in a supersonic beam by using pulsed laser vaporization. The clusters are studied with IR spectroscopy in order to obtain vibrational spectra in the OH stretching region. The mechanisms of photodetachment and photodissociation are discussed.

Structure of isolated xanthone in the T1 state obtained via combined UV/IR spectroscopy.

In order to extend combined UV/IR spectroscopy to triplet states, xanthone has been chosen as a model system due to its efficient intersystem crossing (ISC). The IR/R2PI (resonant two-photon ionisation) spectrum of the electronic ground state (S(0)) as well as the IR spectrum of the T(1) state have been recorded in a supersonic jet. We show that the IR spectrum of a triplet state can be recorded subsequent to an ISC.

IR/UV spectroscopy on jet cooled 3-hydroxyflavone (H2O)n (n = 1,2) clusters along proton transfer coordinates in the electronic ground and excited states.

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.