Formic acid-methanol complexation vs. esterification: elusive pre-reactive species identified by vibrational spectroscopy.

Vibrational spectra of the mixed dimer and the two mixed trimers of methanol and formic acid as well as some of their isotopologues are presented. Out of the eight expected OH stretching fundamentals of these three pre-reactive hydrogen-bonded complexes, the three modes mainly involving an acid OH group bound to the alcohol appear to be missing in the jet-cooled spectra despite a combination of infrared and Raman probing. A possibility of spectral overlap is discussed in the mixed dimer case, but largely discarded. The missing modes correspond to (fractional) concerted elongation of all engaged OH bonds, promoting synchronous degenerate proton transfer between the molecules. One other trimer mode is very tentatively attributed to a broad spectral feature, whereas all OH bonds contacting carbonyl groups can be unambiguously identified by four relatively narrow infrared absorptions. The spectral features are confirmed by vibrational perturbation theory and deviate in a subtle but systematic way from scaled harmonic predictions which were previously validated for the formic acid complex with a more acidic alcohol. Despite being exothermic and exergonic, ester formation can only be detected in the rarefied gas expansions after extended pre-mixture of the gases, which somewhat contrasts the recent microwave spectroscopic evidence of in situ ester formation and in particular the lack of pre-reactive complex signals.

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.

2-Methoxyethanol: harmonic tricks, anharmonic challenges and chirality-sensitive chain aggregation.

The Raman spectrum of jet-cooled, nearly monoconformational 2-methoxyethanol reveals a strong, aggregation-sensitive resonance in the backbone stretching region and pronounced chirality-dependent dimerisation effects in the OH stretching region. These effects are disentangled by a dual-detection scheme which simultaneously probes the two spectral regions. The performance of mass-scaled harmonic wavenumber predictions and of vibrational perturbation theory in reproducing these spectral features is explored.

The Chiral Trimer and a Metastable Chiral Dimer of Achiral Hexafluoroisopropanol: A Multi-Messenger Study.

1,1,1,3,3,3-hexafluoro-propan-2-ol aggregates preferentially into an achiral dimer of achiral monomers, but the trimer is found to prefer three metastable chiral monomer units arranged into a strained OH⋅⋅⋅O hydrogen-bonded ring, which is reinforced by secondary CH⋅⋅⋅FC interactions. This is shown by a combination of infrared, microwave, and Raman spectroscopy in supersonic jet expansions and supported by high-level quantum chemical calculations. It involves an activation of the monomers by >15 kJ mol-1 , clearly driven by the much stronger hydrogen-bond interaction available to the gauche and even more to the cis monomer units.

Matrix-Isolation and Quantum-Chemical Analysis of the C3v 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 C3v structure, thereby strongly supporting the observation of the C3v conformer of the XeF6 molecule in the neon matrix. A C3v transition state connecting the C3v and Oh local minima is located computationally. The calculated barrier of 220 cm-1 between the C3v minima and the transition state corroborates the experimental observation of the C3v conformer and the absence of the Oh 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.