1,720,968 research outputs found
N-H center dot center dot center dot pi interactions in pyrroles: systematic trends from the vibrational spectroscopy of clusters
No full textPyrrole and some of its methylated derivatives are aggregated in a controlled way in pulsed supersonic jet expansions. The cluster N-H stretching dynamics is studied using FTIR and Raman spectroscopy. Dimers, trimers and tetramers can be differentiated. Systematic trends in the dimer N-H center dot center dot center dot pi interaction as a function of methyl substitution are identified and explored for predictions. Overtone jet absorption spectroscopy is used to extract anharmonicities for the N-H bond in different environments. The N-H anharmonicity constant increases by 10% upon dimerization. Bulk matrix shifts can be emulated by the formation of Ar-decorated clusters. The experimental results are expected to serve as benchmarks for an accurate ab initio characterization of the N-H center dot center dot center dot pi hydrogen bond
Variations of bite angle and coupling patterns in double hydrogen bonds: The case of oxime dimers
No full textstretching excitation. It is faster than in alpha-hydroxyester or alcohol dimers but slower than in carboxylic acid dimers. For this purpose, supersonic jet FTIR and Raman spectra of free and Ar-nanocoated acetoxime monomers, dimers and trimers are presented. By comparison to heated nozzle cyclohexanone oxime spectra, to condensed phase data and to quantum chemical calculations, a Fermi resonance description of the O-H stretching dynamics in the symmetric, doubly hydrogen-bonded dimers is obtained. C=N stretching and librational O-H bending modes are predicted to participate in the coupling. Hydrogen bond topologies and cooperativity issues in larger oxime clusters are discussed. (c) 2007 Elsevier B.V. All rights reserved
Elementary peptide motifs in the gas phase: FTIR aggregation study of formamide, acetamide, N-methylformamide, and N-methylacetamide
No full textCold, isolated peptide model compounds and their aggregates are generated in pulsed supersonic jet expansions and detected by FTIR spectroscopy in the amide-A region, complemented by amide-I spectra. The most stable, symmetric dimer of formamide is unambiguously assigned in the gas phase for the first time, also by comparison to the analogous acetamide dimer. Efficient quenching of a hot-state Fermi resonance by cooling of the dimers is invoked. As the preferred relative orientation of the C=O and N-H groups in N-methylated formamide and acetamide is trans, these compounds show a fundamentally different dimerization pattern. Their most stable dimers, which would be analogous to those of formamide and acetamide, remain undetected as a consequence of kinetic control in the jet. Accurate benchmark quantities for multidimensional vibrational treatments of these peptide models are derived, and the influence of methyl groups on the N-H stretching dynamics is discussed.Fonds der Chemischen Industrie; DFG research training group [782
OH-stretching red shifts in bulky hydrogen-bonded alcohols: Jet spectroscopy and modeling
No full textThe available database for OH-stretching bands of jet-cooled aliphatic alcohol dimers is extended to systems including 1-adamantanol and 2-adamantanol, using a heated pulsed nozzle coupled to an FTIR spectrometer. This database is used to simplify and parametrize the standard Wang et al. AMBER/parm99.dat force field for the prediction of hydrogen-bond-induced red shifts, as it avoids complications due to mode coupling and cooperativity. Apart from subtle chiral recognition effects, the performance of the simple model in describing steric, electronic, and conformational influences on the red shifts is remarkable, as exemplified by predictions for mixed-alcohol dimers. The resulting semiempirical approach can complement quantum chemical calculations, in particular for larger systems, although the good performance is rather specific to red shift predictions
Dimerization of pyrazole in slit jet expansions
No full textPyrazole dimer is observed for the first time in a free jet expansion. Its IR-active N-H stretching vibration is shifted by -269cm(-1) relative to the monomer. Along the 600 mm slit jet expansion, the average number density of pyrazole dimers is approximate to 10(11) cm(-3). Exploratory quantum chemical calculations including electron correlation are in good agreement with the observed frequency shift and confirm reciprocal hydrogen bonding with bent hydrogen bonds in a planar C-2h structure, as postulated by W. Hockel 65 years ago in this journal. Nanomatrix isolation spectra can be obtained by using Ar as the carrier gas. The more strongly coupled vibrational dynamics in pyrazole trimer is illustrated
Torsional isomers in methylated aminoethanols - A jet-FT-IR study
No full textJet-FT-IR spectra of aminoethanol (AE), N-methylaminoethanol (MAE), and N,N-dimethylaminoethanol (DMAE) are presented and compared to gas-phase and matrix isolation IR, microwave, and quantum chemical data. The intramolecular O-H...N hydrogen bond gives rise to characteristically shifted O-H stretching bands and torsional difference bands in the monomer spectra. It stabilizes one out of 14 diastereomeric monomer conformations of AE and DMAE and two out of 27 diastereomeric MAE conformations. The latter two correspond to torsional or N-inversional isomers, the interconversion of which is hindered by the intramolecular hydrogen bond. Their energy difference is found to be <1.2 kJ/mol. The intermolecular aggregation patterns of the three aminoethanols are quite distinct and non-monotonic, as suggested by their O-H stretching fingerprints
Infrared spectroscopy of pyrrole-2-carboxaldehyde and its dimer: A planar β-sheet peptide model?
No full textIntermolecular interactions relevant for antiparallel beta-sheet formation between peptide strands are studied by Fourier transform infrared spectroscopy of the low temperature, vacuum-isolated model compound pyrrole-2-carboxaldehyde and its dimer in the N-H and C = O stretching range. Comparison to quantum chemical predictions shows that even for some triple-zeta quality basis sets, hybrid density functionals and Moller-Plesset perturbation calculations fail to provide a consistent and fully satisfactory description of hydrogen bond induced frequency shifts and intensity ratios in the double-harmonic approximation. The latter approach even shows problems in reproducing the planar structure of the dimer and the correct sign of the C = O stretching shift for standard basis sets. The effect of matrix isolation is modeled by condensing layers of Ar atoms on the isolated monomer and dimer. The dimer structure is discussed in the context of the peptide beta-sheet motif. (c) 2007 American Institute of Physics
Adaptive Aggregation of Peptide Model Systems
No full textJet-cooled infrared spectra of acetylated glycine, alanine, and dialanine esters and their dimers are reported in the amide A and amide I-III regions. They serve as particularly simple peptide aggregation models and are found to prefer a single backbone conformation in the dimer that is different from the most stable monomer backbone conformation. In the case of alanine, evidence for topology-changing chirality discrimination upon dimer formation is found. The jet spectroscopic results are compared to gas phase spectra and quantum chemical calculations. They provide reliable benchmarks for the evaluation of the latter in the field of peptide interactions.Fonds der Chemischen Industrie; DFG research training group 78
Hydrogen bonding lights up overtones in pyrazoles
No full textThe spectral complexity in the NH stretching mode of hydrogen-bonded pyrazoles is traced back to an extensive Fermi resonance system involving combinations and overtones of at least four aromatic ring modes with significant in plane hydride bending character. The couplings are shown to be inherent in the monomer, but hydrogen bonding is required to bring them into resonance with the NH stretching chromophore. A cost-efficient variational "monomers-in-clusters" model is presented and applied to a five-dimensional subspace of pyrazole. Spectra of substituted pyrazoles confirm the robustness of the coupling, which remains dark in strained dimers, but lights up in linearly hydrogen-bonded trimers. (c) 2007 American Institute of Physics
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