Observation of Distinct Carboxylic Acid Conformers in Aqueous Solution

Abstract

We investigate the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of ∼60° with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature.

Increasing the weights in the molecular work-out of cis- and trans-formic acid: extension of the vibrational database via deuteration

“…Here, we present the first perturbation-free band positions of partially and fully deuterated cis-formic acid, extending the available cis-database by more than 200% from five to sixteen fundamentals. In 2019, cis-formic acid has also been detected as a distinct species in solution, 25 where it has a much higher abundance than in the gas phase. The O-D and CQO stretches were shown to be higher in frequency than those in the trans-form, as observed here under vacuum isolation, but only in weakly hydrogen-bonded solvents.…”

De Novo Determination of Molecular Orientation at Interfaces through Multimode Polarization-Dependent Sum-Frequency Generation Spectroscopy

“…The spectrum shows a 1710 cm -1 positive feature and a 1750 cm -1 negative feature which can be assigned to the trans-and cis-conformations of a formic acid molecule at the air-water interface, respectively. 49 The signs of the peaks in the Im𝜒 , ( ) spectra provide information on the absolute orientation of the C→H and C→O groups. The negative C-H peaks indicate that the C→H groups of both trans-and cis-conformations point up towards the air.…”

Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case

“…It has not yet explored the power of isotope substitution 80 , which provides valuable independent checks from the experimental side and sometimes even surprising phenomena such as counterintuitive isotope effects 14 which have to be modelled by quantum treatments of the nuclear dynamics. Alternatives to the relatively insensitive linear FTIR and Raman spectroscopy for jet-cooled species 81 have to be further developed in different spectral ranges, either as linear 15,82,83 or as action spectroscopy 84,85 tools, also extending into different environments [85][86][87][88][89] .…”