M. Gerhards

Cobalt–benzene cluster anions: Mass spectrometry and negative ion photoelectron spectroscopy

(Cobalt)n(benzene)m− cluster anions, (n,m) were generated by laser vaporization and studied by both mass spectrometry and anion photoelectron spectroscopy. Our assignment of the photoelectron spectrum of the (1,2) cluster anion suggests that it possesses a sandwich structure with the cobalt atom located between two parallel benzene rings, that the ground state of this anion is a singlet, and that the ground state of its corresponding neutral is a doublet. The photoelectron spectra of cobalt-rich cluster anions of the form (n,1) are interpreted as cobalt metal cluster anions which have been solvent-stabilized by their interaction with, in each case, a single benzene molecule. The photoelectron spectra of the benzene-rich cluster anions, (2,3), (2,2), and (3,3), are tentatively interpreted as suggesting extended sandwich structures for these anion complexes.

Structure of a β-sheet model system in the gas phase: Analysis of the CO stretching vibrations

In this communication the first IR/R2PI (infrared/resonant two-photon ionisation) spectra of CO stretching vibrations are presented applied to a β-sheet model system. CO vibrations are extremely important to describe the structure of isolated peptides and especially the hydrogen bonding in β-sheet related structures in the gas phase.

Structure and vibrations of phenol(H2O)2

Extensive ab initio calculations at the Hartree–Fock (HF) level using different basis sets have been performed in order to obtain the minimum energy structure of the phenol(H2O)2‐cluster. Several hydrogen bonding arrangements and a van der Waals structure are discussed. The most stable structure turns out to be cyclic with nonlinear hydrogen bonds. This structure is similar to the one calculated for the water trimer. In contrast with the water trimer the average binding energy of a hydrogen bond decreases with increasing cluster size of Ph(H2O)n (n=1,2). This is a result of non equal hydrogen bonds. A normal coordinate analysis has been carried out for the fully optimized minimum energy structure of phenol(H2O)2 and its deuterated isotopomer d‐phenol(D2O)2. The calculated harmonic intramolecular vibrational modes are compared with experimental values and the intermolecular stretching vibrations are assigned.

β-sheet model systems in the gas phase: Structures and vibrations of Ac–Phe–NHMe and its dimer (Ac–Phe–NHMe)2

A combined experimental and theoretical study on the structure of the dipeptide model Ac–Phe–NHMe and its dimer (Ac–Phe–NHMe)2 is presented. In order to get a detailed vibrational analysis of all functional groups which are relevant to analyse the different structural arrangements, IR/R2PI spectra are recorded in the regions of the NH and the CO stretching vibrations. Force field calculations are used to scan the complex conformational landscape both of Ac–Phe–NHMe and the dimer (Ac–Phe–NHMe)2. Subsequent ab initio and DFT calculations on the most stable structures lead to predictions of the cluster geometries and their vibrational frequencies. Three isomers of the Ac–Phe–NHMe monomer have been assigned which contain either a β-sheet related configuration or hydrogen-bonded structures. The most prominent species has a β-sheet related conformation. The observed dimer contains a doubly hydrogen-bonded arrangement and turns out to be a β-sheet model system. In contrast to the β-sheet model (Ac–Phe–OMe)2 a different structural arrangement is found, connecting “the outer” CO and NH groups.

The structure of phenol(H2O) obtained by microwave spectroscopy

The microwave spectrum of phenol(H2O) has been recorded using a pulsed molecular beam Fourier transform microwave spectrometer. Twenty a‐type transitions have been observed and assigned. All a‐type transitions are doublets with splittings varying from 1.13 to 4.01 MHz. These splittings are interpreted to result from the internal rotation of the water molecule. The ground state of this torsional motion is split into two levels with different spin statistical weights. Both torsional levels are treated independently as asymmetric rotor states. The resulting rotational constants are A=4291.486 MHz, B=1092.1484 MHz, and C=873.7263 MHz for the lower torsional level (σ=0) and A=4281.748 MHz, B=1092.329 55 MHz, and C=873.906 81 MHz for the upper level (σ=1). A fit of the molecular structure is performed by optimizing the intermolecular OO distance and two angles, describing the H‐bonding arrangement. The resulting trans‐linear structure is in reasonable agreement with the ab initio calculations

Ultraviolet/infrared-double resonance spectroscopy and ab initio calculations on the indole+ and indole(H2O)1+ cations

In this paper we report on the application of infrared/photoinduced Rydberg ionization (IR/PIRI) and IR-photodissociation spectroscopy to investigate the CH, NH or OH stretching vibrations of indole+ and the indole(H2O)1+ cluster cation. All vibrational frequencies of indole+ and indole(H2O)1+ are compared with the values obtained from ab initio calculations. In the case of the indole+ cation the NH vibration is observed. This is the first observation of a NH vibration in a bare cation. For indole(H2O)1+ a hydrogen-bonded structure with a nearly linear hydrogen bond can be derived both from ab initio calculations and the IR-spectra. By applying the state selective IR/PIRI spectroscopy to indole(H2O)1+, no vibrational couplings between the intermolecular O–H⋯N stretching vibration and the intramolecular OH stretching modes of the water moiety are observed. In the IR-photodissociation spectra of indole(H2O)1+ the NH, OH, and CH stretching vibrations as well as overtones of bending modes are observed. In agr...

Structure and vibrations of catechol and catechol⋅H2O(D2O) in the S0 and S1 state

The inter‐ and intramolecular vibrations in the S0 and S1 state of catechol, d2‐catechol, catechol(H2O)1, and d2‐catechol (D2O)1 have been investigated experimentally by resonant two photon ionization (R2PI), spectral hole burning (SHB), and dispersed fluorescence spectroscopy (DF). The experimental frequencies are compared to the vibrational frequencies obtained from ab initio normal mode calculations using the 6‐31G(d,p) basis set. In order to get a complete interpretation of the S0 state spectra of d2‐catechol the strong coupling of the two OD torsional motions has been taken into account. A two‐dimensional calculation of the torsional eigenvalues based on an ab initio potential [6‐31G(d,p) basis] obtained from single point calculations is presented. Due to these calculations all vibrations in the S0 state can be assigned. Furthermore a new assignment of the vibrations in the S1 state of d2‐catechol is given. In the case of catechol (H2O)1 [d2‐catechol(D2O)1] different structural isomers are discussed....

Structure of the protected dipeptide Ac-Val-Phe-OMe in the gas phase: Towards a β-sheet model system

In this paper we report on the structure of the isolated dipeptide Ac–Val–Phe–OMe (Val=valine, Phe=phenylalanine) which is protected at the terminal positions by introducing an acetyl and a methyl group. Both resonant two-photon ionization (R2PI) and IR/R2PI spectroscopy are applied. This is the first application of IR/R2PI spectroscopy to a dipeptide. Both the region of the C–H and N–H stretching vibrations as well as the region of the C=O stretching vibrations are investigated. The chosen dipeptide exhibits only one prominent conformer in the gas phase containing a “linear” non-hydrogen-bonded structure which is an ideal candidate for a β-sheet model.

OH stretching vibrations in aromatic cations: IR/PIRI spectroscopy

Abstract This Letter presents a new technique (IR/PIRI) to investigate OH stretching vibrations of cations. In the IR/PIRI technique the MATI signals obtained for the electronic origin or a vibrational state of the ion are depleted by an IR laser if the frequency of the IR laser is in resonance with an OH stretching vibration of the cation. As an example, the resorcinol molecule is chosen. Two different isomers of this molecule are investigated. In combination with ab initio calculations and IR depletion spectroscopy performed for the electronic ground (S 0 ) and electronically excited state (S 1 ) of the neutral molecule all OH stretching vibrations of the different isomers in the S 0 , S 1 , and D 0 states are interpreted.

Investigation of Secondary Structure Elements by IR/UV Double Resonance Spectroscopy : Analysis of an Isolated β-Sheet Model System

An isolated beta-sheet model system is investigated in a molecular beam experiment by means of mass- and isomer-selective IR/R2PI double resonance spectroscopy as well as ab initio and DFT calculations. As the exclusive intermolecular assembly, a beta-sheet motif is formed by spontaneous dimerization of two isolated peptide molecules. This secondary structure is produced from the tripeptide model Ac-Val-Tyr(Me)-NHMe without any further environment to form the binding motif which is analyzed by both the characteristic amide A and I vibrations. The experimental and theoretical investigations yield the assignment to an antiparallel beta-sheet model. The result of this detailed spectroscopic analysis on an isolated beta-sheet model indicates that there are intrinsic properties of a beta-sheet structure which can be formed without a solvent or a peptidic environment.