Susanne Hesse

On the low volatility of cyclic esters: an infrared spectroscopy comparison between dimers of γ-butyrolactone and methyl propionate

The dramatically lower volatility of gamma-butyrolactone compared to its open chain analog methyl propionate is analyzed at the molecular dimer level using FTIR spectroscopy in supersonic jets. It is found that the spectral shifts from the monomer to the dimer are about three times more pronounced in the lactone at low temperatures. The spectra are consistent with sandwich-like dimers optimizing their strong dipole-dipole interaction, possibly augmented by specific C-H...O=C hydrogen bond contacts. The spectra show significant evolution from the dimer to the condensed phase, indicative of secondary interactions with the ester oxygen and long range forces. The reduced dipole moment in the open chain ester leads to less specific interactions, unless a trans conformation of the ester group as in the lactones is enforced. The latter is not energetically accessible in open chain esters because it would bring the molecular C=O and C-O-C dipole moments into an unfavorable near-parallel orientation, thus their higher volatility.

Conformation-Changing Aggregation in Hydroxyacetone: A Combined Low-Temperature FTIR, Jet, and Crystallographic Study

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dimers and oligomers. The O-H···O═C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H···O-H hydrogen bonds in competition with isolated O-H···O═C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from ~0° in the gas phase to ~180° in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.

Dimers of cyclic carbonates: chirality recognition in battery solvents and energy storage

Dimers of ethylene carbonate and propylene carbonate are created in supersonic jet expansions and characterized by FTIR spectroscopy. Fermi resonances are switched on and off by dimerization. There is a unique centrosymmetric dimer of ethylene carbonate in a pronounced case of complementary chirality synchronization, contributing to its energy storage capacity at melting. Two chiral propylene carbonate molecules combine in more intricate ways. If they have the same handedness, one of them is forced into an axial conformation and the binding partner stays in the more stable equatorial structure. If they have opposite handedness, centrosymmetric dimers of either axial or equatorial conformations are formed. This suggests the usefulness of chirality control in elucidating ionic transport mechanisms in battery solvents and asymmetric catalysis in such solvents.

Conformation and Aggregation of Proline Esters and Their Aromatic Homologs: Pyramidal vs. Planar RR'N-H in Hydrogen Bonds

Abstract The conformations of proline esters are investigated by infrared spectroscopy in supersonic slit jet expansions. Two easily convertible puckering variants of the pyrrolidine ring with intramolecular N-H···O contacts are shown to be particularly stable. The aggregation tendency of proline esters via intermolecular N-H···O hydrogen bonds is remarkably weak. IR differences between enantiopure and racemic dimers are difficult to quantify. Dehydrogenation of the pyrrolidine ring to pyrrole leads to a stable planar carboxylic ester conformation. Its aggregation tendency is pronounced due to the planar hybridization of the nitrogen atom and leads to a symmetric, β sheet-like dimer with strongly red-shifting hydrogen bonds. The spectroscopic observations underscore the differences between intermolecular interactions of N-terminal and peptide-bound amino acids in peptide chains.

Brightening and locking a weak and floppy N-H chromophore: the case of pyrrolidine.

The N-H stretching signature of the puckering equilibrium between equatorial and axial pyrrolidine is analyzed via FTIR and Raman spectroscopy in supersonic jets as a function of aggregation. Vibrational temperatures along the expansion axis can be extracted from the Raman spectra and allow for a localization of the compression shock waves. While the equatorial conformation is more stable in the ground state monomer, this preference is probably switched in the excited state with one N-H stretching quantum. Furthermore, the dominant dimer involves an axial donor and the trimer and tetramer structures seem to prefer uniform axial conformations. The IR intensity is boosted by up to 3 orders of magnitude upon aggregation, whereas the Raman scattering intensity shows only moderate hydrogen bond effects. B3LYP and MP2 calculations provide a reasonable description of the N-H vibrational dynamics under the influence of self-aggregation. In mixed dimers with pyrrole, pyrrolidine assumes the role of a hydrogen bond acceptor.