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.

The first HyDRA challenge for computational vibrational spectroscopy.

Vibrational spectroscopy in supersonic jet expansions is a powerful tool to assess molecular aggregates in close to ideal conditions for the benchmarking of quantum chemical approaches. The low temperatures achieved as well as the absence of environment effects allow for a direct comparison between computed and experimental spectra. This provides potential benchmarking data which can be revisited to hone different computational techniques, and it allows for the critical analysis of procedures under the setting of a blind challenge. In the latter case, the final result is unknown to modellers, providing an unbiased testing opportunity for quantum chemical models. In this work, we present the spectroscopic and computational results for the first HyDRA blind challenge. The latter deals with the prediction of water donor stretching vibrations in monohydrates of organic molecules. This edition features a test set of 10 systems. Experimental water donor OH vibrational wavenumbers for the vacuum-isolated monohydrates of formaldehyde, tetrahydrofuran, pyridine, tetrahydrothiophene, trifluoroethanol, methyl lactate, dimethylimidazolidinone, cyclooctanone, trifluoroacetophenone and 1-phenylcyclohexane-cis-1,2-diol are provided. The results of the challenge show promising predictive properties in both purely quantum mechanical approaches as well as regression and other machine learning strategies.

Coupled proton vibrations between two weak acids: the hinge complex between formic acid and trifluoroethanol.

When formic acid and 2,2,2-trifluoroethanol are co-expanded through a slit nozzle into vacuum, a single dominant, hinge-like 1 : 1 complex is formed in significant amounts and its two OH stretching fundamentals separated by 100 cm-1 can be unambiguously assigned by a combination of infrared absorption and Raman scattering. Quantum chemical calculations at different levels reproduce this finding in a satisfactory way and suggest that in-phase (Raman-sensitive and lower wavenumber) OH stretch excitation more or less along the concerted degenerate proton transfer coordinate in the hydrogen-bonded ring stays below the barrier for this concerted exchange. Anharmonic calculations indicate only weak intensity sharing with dark states coming into reach due to the hydrogen bond downshift of the OH stretching vibration. This well-behaved system sets the stage for acid combinations with more basic alcohols, where the in-phase OH stretching vibration is more difficult to detect, possibly due to fast intra-complex vibrational dynamics. It thus provides a benchmark point from which one can explore the evolution of vibrational resonances when the acidic proton meets a more electron-rich alcoholic oxygen.