Corinna Emmeluth

A monomers-in-dimers model for carboxylic acid dimers

The OH stretching fundamental band system of carboxylic acid dimers is studied using acetic acid and its isotopomers as a model system. Comparing experimental jet spectra with multidimensional quantum mechanical calculations the origin of the extremely broad vibrational band structure (Δν≈800 cm−1) is found in strong anharmonic resonances involving the OH stretching vibration. Within an adiabatic picture of hydrogen bonding a new monomers-in-dimers model allows to analyze the observed vibrational band structure in terms of the anharmonic quantum dynamics of the CO2H functional group. The results are discussed in terms of the time-dependent population dynamics and its implications for the mode-specificity of the vibrational predissociation of the hydrogen bonds. On a subpicosecond time scale the intramolecular vibrational energy redistribution of the dimer remains effectively localized within the six-dimensional manifold of the internal vibrations of the carboxyl group, conserving its local CS symmetry.

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.

Combined jet relaxation and quantum chemical study of the pairing preferences of ethanol

The subtle trans-gauche equilibrium in the ethanol molecule is affected by hydrogen bonding. The resulting conformational complexity in ethanol dimer manifests itself in three hydrogen-bonded OH stretching bands of comparable infrared intensity in supersonic helium expansions. Admixture of argon or nitrogen promotes collisional relaxation and is shown to enhance the lowest frequency transition. Global and local harmonic frequency shift calculations at MP2 level indicate that this transition is due to a gauche-gauche dimer, but the predictions are sensitive to basis set and correlation level. Energetically, the homochiral gauche-gauche dimer is predicted to be the most stable ethanol dimer conformation. The harmonic MP2 predictions are corroborated by perturbative anharmonicity contributions and CCSD(T) energies. Thus, a consistent picture of the subtle hydrogen bond energetics and vibrational dynamics of the ethanol dimer is starting to emerge for the first time.

A peptide co-solvent under scrutiny: self-aggregation of 2,2,2-trifluoroethanol

Trifluoroethanol (TFE) and its aggregates are studied via supersonic jet FTIR and Raman spectroscopy as well as by quantum chemistry and simple force field approaches. A multi-slit nozzle is introduced to study collisionally excited clusters. Efforts are made to extract harmonic frequencies from experiment for better comparison to theory. Based on deuteration, the OH stretching anharmonicity changes weakly upon dimerization, but increases for trimers. Among the possible dimer conformations, only an all-gauche, homoconfigurational, compact, OH-F connected structure is observed in an extreme case of chiral discrimination. Quantum tunneling assisted pathways for this surprising helicity synchronization are postulated. The oscillator coupling in hydrogen-bonded trimers is analyzed. Trans conformations of TFE start to become important for trimers and probably persist in the liquid state. Simple force fields can be refined to capture some molecular recognition features of TFE dimer, but their limitations are emphasized.

Infrared spectra of the Li+–(H2)n (n=1–3) cation complexes

The Li+-(H2)n n=1-3 complexes are investigated through infrared spectra recorded in the H-H stretch region (3980-4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H-H stretch band of Li+-H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 A increasing by 0.004 A when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+-H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Spirko, Chem. Phys. 330, 190 (2006)]. The H-H stretch band of Li+-(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2-Li+-H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+-H2. The Li+-(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1.

A chemical approach towards the spectroscopy of carboxylic acid dimer isomerism

The vibrational dynamics and hydrogen bond topology of excited isomers of carboxylic acid dimers is elucidated by an FTIR study of mixed acetic acid-methyl acetate clusters in supersonic jet expansions. The partial esterification prevents a second OH–O hydrogen bond in the dimer and replaces it by a weak CH–O contact. Vibrational transitions due to mixed acid-ester dimers are observed in the O–H, CO, and C–O stretching range. Similarities between the mixed dimer spectrum and weak bands in the spectrum of pure acetic acid clusters suggest a common hydrogen bond pattern for both species. It is the hydrogen bond pattern observed between two adjacent monomers in solid acetic acid. The conclusions are supported by quantum-chemical calculations.

Rotationally resolved infrared spectrum of the Li+-D2 cation complex.

The infrared spectrum of mass selected Li+–D2 cations is recorded in the D–D stretch region (2860–2950cm−1) in a tandem mass spectrometer by monitoring Li+ photofragments. The D–D stretch vibration of Li+–D2 is shifted by −79cm−1 from that of the free D2 molecule indicating that the vibrational excitation of the D2 subunit strengthens the effective Li+⋯D2 intermolecular interaction. Around 100 rovibrational transitions, belonging to parallel Ka=0-0, 1-1, and 2-2 subbands, are fitted to a Watson A-reduced Hamiltonian to yield effective molecular parameters. The infrared spectrum shows that the complex consists of a Li+ ion attached to a slightly perturbed D2 molecule with a T-shaped equilibrium configuration and a 2.035A vibrationally averaged intermolecular separation. Comparisons are made between the spectroscopic data and data obtained from rovibrational calculations using a recent three dimensional Li+–D2 potential energy surface [R. Martinazzo, G. Tantardini, E. Bodo, and F. Gianturco, J. Chem. Phys. ...