Raman spectroscopic investigaton of the folding of esters in supersonic expansions, Or: While looking for a hairpin in an esterstack, how to find five at once

The conformational preference of linear alkyl chains with an ester group in the center was investigated using Raman spectroscopy in supersonic expansions. Like alkanes, short esters prefer a stretched conformation, but beyond a critical chain length a hairpin-like conformation dominates due to intramolecular dispersive attraction. This turnover point was determined by combining experimental results with quantum chemical calculations. As part of the analysis, a rigorous characterisation of experimental Raman intensities for the setup was performed. For better comparability between calculated and experimental vibrational spectra a simulation of the rotational broadening was developed.2022-07-0

Quantifying Conformational Isomerism in Chain Molecules by Linear Raman Spectroscopy: The Case of Methyl Esters

The conformational preferences of the ester group have the potential to facilitate the large amplitude folding of long alkyl chains in the gas phase. They are monitored by Raman spectroscopy in supersonic jet expansions for the model system methyl butanoate, after establishing a quantitative relationship with quantum–chemical predictions for methyl methanoate. This requires a careful analysis of experimental details, and a simulation of the rovibrational contours for near-symmetric top molecules. The technique is shown to be complementary to microwave spectroscopy in quantifying coexisting conformations. It confirms that a C−O−C(=O)–C–C chain segment can be collapsed into a single all-trans conformation by collisional cooling, whereas alkyl chain isomerism beyond this five-membered chain largely survives the jet expansion. This sets the stage for the investigation of linear alkyl alkanoates in terms of dispersion-induced stretched-chain to hairpin transitions by Raman spectroscopy

Matrix-Isolation and Quantum-Chemical Analysis of the <i>C</i>_3<i>v</i> Conformer of XeF₆, XeOF₄, and Their Acetonitrile Adducts

A joint experimental-computational study of the molecular structure and vibrational spectra of the XeF₆ molecule is reported. The vibrational frequencies, intensities, and in particular the isotopic frequency shifts of the vibrational spectra for ¹²⁹XeF₆ and ¹³⁶XeF₆ isotopologues recorded in the neon matrix agree very well with those obtained from relativistic coupled-cluster calculations for XeF₆ in the <i>C</i>_3<i>v</i> structure, thereby strongly supporting the observation of the <i>C</i>_3<i>v</i> conformer of the XeF₆ molecule in the neon matrix. A <i>C</i>_3<i>v</i> transition state connecting the <i>C</i>_3<i>v</i> and <i>O</i>_<i>h</i> local minima is located computationally. The calculated barrier of 220 cm^–1 between the <i>C</i>_3<i>v</i> minima and the transition state corroborates the experimental observation of the <i>C</i>_3<i>v</i> conformer and the absence of the <i>O</i>_<i>h</i> conformer in solid noble gas matrices. For comparison matrix-isolation spectra have also been recorded and analyzed for the ¹²⁹XeOF₄ and the ¹³⁶XeOF₄ isotopologues. The matrix-isolation complexation shifts obtained for the XeF₆·NCCH₃ relative to those of free matrix isolated XeF₆ and CH₃CN are in good agreement with those reported for crystalline XeF₆·NCCH₃