Biomedical subjects
Publications and source records attributed to Martin A Suhm.
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.
Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an alpha-hydroxyester. Disruption of the intramolecular OH...O=C contact in favour of a cooperative OH...OH...O=C sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl alpha-hydroxyisobutyrate in seeded supersonic jet expansions. Comparison of the OH stretching modes with quantum-chemical harmonic frequency calculations and 18O labelling of methanol unambiguously prove the insertion of methanol into the intramolecular hydrogen bond. This is in marked contrast to UV/IR hole burning studies of the homologous system methyl lactate: (+/-)-2-naphthyl-1-ethanol, where only addition complexes were found and the intramolecular hydrogen bond was conserved. This switch in hydrogen bond pattern from aliphatic to aromatic heterodimers is thought to reflect not only a kinetic propensity but also a thermodynamic preference for addition complexes when dispersion forces become more important in aromatic systems.
The 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.
Raman active symmetric O-H stretching modes are detected and assigned for the first time in isolated methanol, ethanol and methyl lactate trimers, providing insights into cluster structure, vibrational assignments and hydrogen-bond mediated couplings.
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Hydrogen bonded complexes between two different aliphatic alcohols exhibit donor/acceptor isomerism. In a supersonic jet expansion, the less stable isomer can isomerize to the more stable isomer if the energy difference is sufficiently large and the barrier sufficiently low. We show by FTIR jet spectroscopy that this is progressively the case for methanol/methanol-d1, methanol/ethanol, and methanol/tert-butyl alcohol, until no metastable donor/acceptor isomer persists in the expansion. Collisional relaxation experiments, 18O labeling and quantum chemical calculations are used to assign the spectra. Differences between energetical and spectroscopic acceptor strengths are discussed.
A comprehensive experimental study of the OH stretching vibrations of size-selected clusters of enantiopure and racemic methyl lactate is presented. For the size selection, we measured angular dependent mass spectra and time-of-flight distributions at the different fragment masses. In this way the fragmentation of these clusters upon electron impact ionization is obtained. The largest fragment masses of the neutral (MLac)n clusters are the protonated (MLac)n-1H+ ions. The results of a pressure dependent study in an FTIR jet experiment are compared with completely size-selected experiments based on atomic beam deflection and depletion spectroscopy. The size assignments and spectra agree for dimers and trimers. Structures and spectral information for the trimer and the tetramer at density functional and MP2 level are provided. Selective self-aggregation and chiral recognition was observed for homochiral trimers. They exhibit a ring structure bound by OH...OH hydrogen bonds. A spectacular switch in the hydrogen bonding topology was observed for the tetramer. The homochiral enantiomer exhibits cooperative OH...OH bonding, while the heterochiral version shows isolated OH...O=C bonding in a symmetric SRSR arrangement. The crucial ingredients for this identification are the size-selective IR spectra with their different shifts and line patterns which are reproduced by the calculations.
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Chiral recognition and subsequent selective self-organisation into hydrogen-bonded n-mers is observed in supersonic methyl lactate expansions. The nu(OH) and nu(C=O)-vibrations are investigated by ragout-jet FTIR-spectroscopy and lead to the assignment of homo- and heterochiral clusters of at least three different cluster sizes. Whereas homo- and heterochiral dimers are formed in similar amounts in the racemic mixture, prominent absorptions due to different homochiral and heterochiral lactate trimers and tetramers indicate highly specific chiral self-recognition beyond molecular pairs. Chemical modification of the ester-group (methyl-, ethyl- and isopropyl-lactate) and argon admixture to the helium expansion contribute importantly to an understanding of the cluster spectra and topology.