Size-selected methyl lactate clusters: fragmentation and spectroscopic fingerprints of chiral recognition

Abstract

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.

Chirality‐Induced Switch in Hydrogen‐Bond Topology: Tetrameric Methyl Lactate Clusters in the Gas Phase

“…Chiral recognition in neutral dimers has also been studied for many years5–11 and can even lead to chirality‐dependent switches in the hydrogen‐bond pattern 10. Our recent spectroscopic observations of neutral methyl lactate clusters and related aggregates beyond dimers12–16 confirm that a net charge is not necessary for specific and spontaneous self‐assembly in the gas phase beyond dimerization. In supersonic jets of methyl lactate, the spectral signatures of a unique homoconfigurational trimer and—even more surprisingly—that of a unique heteroconfigurational tetramer were found 12.…”

“…In supersonic jets of methyl lactate, the spectral signatures of a unique homoconfigurational trimer and—even more surprisingly—that of a unique heteroconfigurational tetramer were found 12. Extensive studies of homologous compounds,12 achiral variants,13, 15 argon‐coated clusters,12, 13 and the stagnation pressure dependence,12, 16 as well as size‐selective scattering experiments16 unambiguously confirmed the phenomenon, but the underlying hydrogen‐bond topology has remained elusive. This is due to the complexity of the intermolecular potential‐energy (hyper)surface with competing hydrogen‐bond patterns, conformational flexibility, and subtle secondary interactions beyond the classical hydrogen‐bond contacts 12…”

Free jet rotational spectrum of the most stable conformer of 1-(2-fluorophenyl)-1-ethanol

“…Clusters of neutral methyl lactate, glycidol, and of related aggregates beyond dimers were observed in the gas phase by supersonic-jet Fourier-transform infrared (FTIR) spectroscopy of the OH-stretching bands [7][8][9][10]. The interpretation of the experimental results obtained with this technique needs, similarly to the one described above, to rely on theoretical tools, such as, e.g., RI-BP86 density functional calculations, supplemented by RI-MP2 optimizations and energy calculations, as utilized in the study of methyl lactate tetramer structures [7].…”

Mass spectrometry of aerosol particle analogues in molecular beam experiments

“…This method has been used mainly to investigate the fragmentation after electron ionization of small rare gas clusters, and some molecular clusters, for example, water, ammonia, hydrocarbon clusters, etc . Besides, the same scattering principle enabled vibrational IR spectroscopy of size selected neutral clusters in molecular beams . Another, more recent, method for the neutral cluster size selection was based on the cluster beam deflection in an electrostatic deflector: this way a spatial separation of indole‐water complexes from other clusters generated in the molecular beams was demonstrated .…”

“…77,78 Besides, the same scattering principle enabled vibrational IR spectroscopy of size selected neutral clusters in molecular beams. [79][80][81][82] Another, more recent, method for the neutral cluster size selection was based on the cluster beam deflection in an electrostatic deflector: this way a spatial separation of indole-water complexes from other clusters generated in the molecular beams was demonstrated. 83,84 Both mentioned size-selection methods are relatively simple from the physics principles, nevertheless, experimentally very difficult and applicable only to small clusters.…”