Thomas Häber

FTIR-spectroscopy of molecular clusters in pulsed supersonic slit-jet expansions

A new approach to the Fourier transform infrared (FTIR) absorption spectroscopy of molecular clusters in pulsed supersonic jets is developed to the point where it is competitive with high-sensitivity laser absorption techniques for intermediate and large molecular systems. A combination of rapid spectral acquisition and of a buffered jet chamber enables the use of intense gas pulses which cover complete interferometer scans. Applications to (N2O)n, (CH3OH)n and (HCl)n demonstrate the capabilities of this technique. Investigations of the association of bulky alcohols and of clusters within clusters illustrate some ongoing research.

Ragout-jet FTIR spectroscopy of cluster isomerism and cluster dynamics: from carboxylic acid dimers to N2O nanoparticles

Direct absorption supersonic jet Fourier transform spectroscopy provides a panoramic view of the dynamics of molecular clusters over the entire IR spectral range. The new and generally applicable ragout-jet technique compensates for the sensitivity limits inherent in the incoherent FTIR approach by the use of synchronized giant gas pulses expanding into a large vacuum buffer. A modification based on fragmented interferograms is proposed and demonstrated, by which the spectral resolution can be extended to the limit of the available FTIR spectrometer. The power of the method is illustrated for two classes of compounds. For acetic acid and its isotopomers, the supersonic jet spectra of dimers and oligomers are investigated for the first time, concentrating on the very complex OH/CH stretching domain and on the more regular C=O/C-O stretching range. Issues of cluster isomerism, hydrogen exchange tunneling, anharmonic resonances, intermolecular Franck-Condon sequences, methyl group substitution and cluster coating with argon are explored. For the more weakly interacting nitrous oxide, stretching fundamentals and combination bands of clusters in the 1-3 nm range are studied as a function of composition. Surface vibrations are investigated in detail and modeled quantum mechanically. The semiempirical AM1 approach is found to provide a remarkably accurate description of the cluster structure, energetics and dynamics.

Tautomers and electronic states of jet-cooled 2-aminopurine investigated by double resonance spectroscopy and theory

We present resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectra of jet-cooled 2-aminopurine (2AP) as well as Fourier transform infrared (FTIR) gas phase spectra. 2AP is a fluorescing isomer of the nucleobase adenine. The results show that there is only one tautomer of 2AP which absorbs in the wavelength range 32,300-34,500 cm(-1). The comparison with the calculated IR spectra of 9H- and 7H-2AP points to 9H-2AP as the dominating tautomer in the gas phase but the spectra are too similar to allow an unambiguous assignment to the respective tautomer. Hence, we determined vertical and adiabatic excitation energies of both tautomers employing combined density functional theory and multi-reference configuration interaction techniques. For the 0-0 band of the first 1pipi* transition of 9H-2AP we obtain a theoretical value of 32,328 cm(-1), in excellent agreement with the band origin of our R2PI spectrum at 32,371 cm(-1). The first singlet pipi* transition of the less stable 7H-2AP tautomer is predicted to be red-shifted by about 1700 cm(-1) with respect to the corresponding transition in 9H-2AP. From the absence of experimental bands in the energy region between 30,300 and 32,350 cm(-1) we conclude that 7H-2AP is not present to an appreciable extent in the molecular beam. Our calculations yield nearly equal energies for the 1npi* and 1pipi* minima of isolated 2AP, similar to the situation in adenine. The hitherto existing argument that the energetic order of states is responsible for the different spectroscopic properties of these isomers therefore does not hold. Rather, vibronic levels close to the origin of the 1pipi* transition cannot access the conical intersection between the 1pipi* and S(0) states along a puckering coordinate of the six-membered ring, in contrast to the situation in electronically excited 9H-adenine. As a consequence, a rich vibrational structure can be observed in the R2PI spectrum of 2AP whereas the spectrum of 9H-adenine breaks off at low energies.

Chiral self-recognition in the gas phase: the case of glycidol dimers

Supersonic jet FTIR spectra of the OH-stretching bands of glycidol monomers and clusters are presented. Chiral discrimination leads to marked differences in the absorption patterns of RR (SS) s. RS glycidol dimers. The dominant absorption peaks are located at 3492 (RR, SS) and 3488 cm−1 (RS) within a rich line spectrum with sizeable variations between enantiomerically pure and racemic dimers. A spectral difference technique is used to emphasize the intermolecular diastereomeric effects. Glycidol is possibly the first and likely the smallest molecule for which chiral self-recognition has been experimentally demonstrated in the gas phase. It thus lends itself very well to accurate quantum chemical calculations of the chiral discrimination effect. Qualitative results of exploratory calculations are reported.

Electronic and Vibrational Spectroscopy of 1‐Methylthymine and its Water Clusters: The Dark State Survives Hydration

Electronic and vibrational gas phase spectra of 1-methylthymine (1MT) and 1-methyluracil (1MU) and their clusters with water are presented. Mass selective IR/UV double resonance spectra confirm the formation of pyrimidine-water clusters and are compared to calculated vibrational spectra obtained from ab initio calculations. In contrast to Y. He, C. Wu, W. Kong; J. Phys. Chem. A, 2004, 108, 94 we are able to detect 1MT/1MU and their water clusters via resonant two-photon delayed ionization under careful control of the applied water-vapor pressure. The long-living dark electronic state of 1MT and 1MU detected by delayed ionization, survives hydration and the photostability of 1MT/1MU cannot be attributed solely to hydration. Oxygen coexpansions and crossed-beam experiments indicate that the triplet state population is probably small compared to the (1)n pi* and/or hot electronic ground state population. Ab initio theory shows that solvation of 1MT by water does not lead to a substantial modification of the electronic relaxation and quenching of the (1)n pi* state. Relaxation pathways via (1)pi pi*(1)-n pi*(1) and (1)pi pi*-S(0) conical intersections and barriers have been identified, but are not significantly altered by hydration.

Isomer-selective vibrational spectroscopy of benzene-acetylene aggregates: comparison with the structure of the benzene-acetylene cocrystal.

Cocrystals with defined molecular composition can be synthesized by co-condensation of gaseous compounds in fixed molar ratios followed by multiple heating/cooling cycles. Cocrystals with 1:1 and 1:2 ratios and with different structures have been assembled in this way. 2] The basic structural motifs of the unit cell are, in principle, comparable to nanocrystals (clusters) synthesized in gas jets by adiabatic cooling. However, cooperative effects may lead to isomeric crystal structures that do not necessarily represent the global minimum structures of the clusters. But since supersonic jet cooling is a non-equilibrium process, higher-energy isomers are often formed, thus opening the possibility to study unitcell motifs directly in the form of isolated clusters. Herein, we report the structures of small benzene–acetylene clusters and compare them to the structure of the 1:1 cocrystal. Strong C H···p interactions ( 2.5 kcal mol ) have found broad interest owing to their importance for the stabilization of supramolecular aggregates, crystal packing, molecular recognition, and for the folding of proteins. A typical example of such a strong C H···p interaction is the Tshaped benzene–acetylene (BA) dimer. We decided to investigate benzene–acetylene clusters for direct comparison with the 1:1 cocrystal. Herein, we present infrared spectra of BA2, BA3, and B2A clusters. BA2 forms two isomers in supersonic jets, but only one isomer has been previously characterized by IR spectroscopy. We now report the IR spectrum of the other isomer for the first time. It has a double T-shaped structure that is also found in the 1:1 cocrystal along the c axis (Figure 1). This isomer might be the seed cluster in crystal growth. Experimentally, IR spectra of the acetylenic C H stretching vibration of benzene–acetylene aggregates have been observed in bulk solution, argon matrixes, and supersonic jets. NMR spectroscopy measurements point to a close CH···p contact. The structure of the 1:1 benzene–acetylene cocrystal has a basic packing motif of nearest neighbors consisting of T-shaped BA arrangements (Figure 1). Previous gas-phase studies have concentrated on small B1Am clusters (m = 1, 2, 3). Resonant two-photon ionization (R2PI) spectroscopy 8,12, 13] revealed UV absorption bands at + 137 cm 1 (BA), + 127 cm 1 (BA2, isomer 1), + 123 cm 1 (BA2, isomer 2), and + 116 cm 1 (BA3) relative to the 60 1 band of benzene at 38 606 cm . The blue shift indicates a reduced cluster stability in the electronically excited state owing to a lower p electron density in the benzene ring upon pp* excitation, as in other clusters displaying hydrogen bonds to aromatic p systems. 16] High-level ab initio calculations predict a T-shaped structure of BA. The acetylene molecule lies along the C6 symmetry axis of the benzene ring and forms a p–hydrogen bond with the aromatic p system. IR–UV double-resonance experiments support a T-shaped BA structure, whereas isomer 2 of BA2 forms a ring-like structure. [8] Little is known about the structures of the other benzene–acetylene aggregates, which might correlate with the building blocks of the benzene–acetylene cocrystal. Figure 2 shows the IR–UV ion-dip spectra of the two isomers of BA2 with the UV laser tuned to 60 1 + 127 cm 1 (isomer 1) and + 123 cm 1 (isomer 2). Owing to fragmentation of the cluster ions, the spectra are observable only at the BA mass, but velocity map imaging allowed for an unambiguous assignment to BA2. [9] Also shown are the calculated and scaled stick spectra of the most stable cluster structures, sorted by increasing energy from top to bottom. Figure 1. Solid-state structure of the benzene–acetylene 1:1 cocrystal displaying the packing motif of T-shaped BA units, as determined by X-ray crystallography. Each unit cell is formed by three BA dimers.

Structural assignment of adenine aggregates in CDCl3

We reinvestigated the self-association of 9-substituted adenine derivatives in CDCl3 solutions and present the infrared spectra of 9-ethyladenine and N-methyl-9-ethyladenine and its aggregates in the spectral regions between 1500 and 1800 cm(-1) and between 2700 and 3600 cm(-1). Wavelength dependent absolute extinction coefficients of the monomer and dimers are presented on the basis of a simple deconvolution method. Comparison of the deconvoluted dimer spectra with quantum chemical calculations allows for a structural assignment of the two dimer structures that coexist in 9-ethyladenine/CDCl3 solutions. In contrast, the dimer spectrum of N-methyl-9-ethyladenine is dominated by a single isomer.

Quantitative chirality synchronization in trifluoroethanol dimers

2,2,2-Trifluoroethanol molecules synchronize their transiently chiral gauche configurations upon dimerization in supersonic jet expansions, while they avoid an energetically competitive heteroconfigurational hydrogen bonded dimer topology predicted by extensive quantum chemical calculations.

Fourier transform infrared spectroscopy of 1-cyclohexyluracil aggregates in CDCl3 solutions

We reinvestigated the self-aggregation of 1-cyclohexyluracil (1CHU) in CDCl(3) solutions. Wavelength dependent absolute extinction coefficients of the monomer and of dimers are presented on the basis of a simple deconvolution method. Two isomeric dimer structures coexist in a solution with similar abundance. Our results are supported by RI-B3LYP/TZVP calculations within the conductorlike screening model framework to account for solvent effects in the ab initio calculations.

The performance of the semi-empirical AM1 method on small and nanometre-sized N2O clusters

Large N2O clusters containing up to 177 molecules were simulated using the semiempirical AM1 method. Simulated spectra of different cluster sizes show excellent agreement with experimental spectra. The vibrational band shape of the strong N–N stretching vibration is more strongly influenced by shape and size than that of the N–O stretching vibration. Stabilization energies and spectral band shapes confirm a crystal like structure of N2O clusters generated in supersonic jet expansions. The AM1 results are carefully checked against experiment and high-level ab initio methods. Overall, AM1 reproduces experimental results (structure and vibrational frequencies) and MP2 dissociation energies of small N2O clusters far better than any other quantum mechanical method studied here, although AM1 is not explicitly calibrated for the (N2O)n van der Waals system. The good performance of AM1 allows us to simulate N2O clusters built from hundreds of molecules, a size range neither accessible by ab initio nor by continuum methods.