Dimerization of Acetic Acid in the Gas Phase—NMR Experiments and Quantum-Chemical Calculations

Abstract

Due to the nature of the carboxylic group, acetic acid can serve as both a donor and acceptor of a hydrogen bond. Gaseous acetic acid is known to form cyclic dimers with two strong hydrogen bonds. However, trimeric and various oligomeric structures have also been hypothesized to exist in both the gas and liquid phases of acetic acid. In this work, a combination of gas-phase NMR experiments and advanced computational approaches were employed in order to validate the basic dimerization model of gaseous acetic acid. The gas-phase experiments performed in a glass tube revealed interactions of acetic acid with the glass surface. On the other hand, variable-temperature and variable-pressure NMR parameters obtained for acetic acid in a polymer insert provided thermodynamic parameters that were in excellent agreement with the MP2 (the second order Møller–Plesset perturbation theory) and CCSD(T) (coupled cluster with single, double and perturbative triple excitation) calculations based on the basic dimerization model. A slight disparity between the theoretical dimerization model and the experimental data was revealed only at low temperatures. This observation might indicate the presence of other, entropically disfavored, supramolecular structures at low temperatures.

Lactic Acid Spectroscopy: Intra- and Intermolecular Interactions

“…At room temperature, the calculated equilibrium constant for this process is K = 8 × 10 3 at 18 °C, which is comparable to that of formic acid ( K ≈ 3 × 10 2 ). − However, this calculated equilibrium constant is exponentially affected by the accuracy of the underlying calculated binding energy, which is notoriously difficult to calculate accurately with computational chemistry methods, so the number reported here is best considered as an estimate of the true equilibrium constant . Other examples of strongly bonded carboxylic acid dimers include acetic acid ( K ≈ (1 to 2) × 10 3 ), − propionic acid ( K ≈ 1 × 10 3 ), , and octo-, nona-, and decanoic acid ( K ≈ 5 × 10 5 ) . The reason that dimer formation is favorable stems from the orientation of the carboxylic acid group being able to form two very strong intermolecular hydrogen bonds in the cyclic dimer. ,− This stability of the dimer is, in part, due to the hydrogen bonds being “resonance-assisted hydrogen bonds”. , This also explains why double proton transfer, also known as chattering, can occur in the cyclic dimer of carboxylic acids .…”

“…Other examples of strongly bonded carboxylic acid dimers include acetic acid ( K ≈ (1 to 2) × 10 3 ), − propionic acid ( K ≈ 1 × 10 3 ), , and octo-, nona-, and decanoic acid ( K ≈ 5 × 10 5 ) . The reason that dimer formation is favorable stems from the orientation of the carboxylic acid group being able to form two very strong intermolecular hydrogen bonds in the cyclic dimer. ,− This stability of the dimer is, in part, due to the hydrogen bonds being “resonance-assisted hydrogen bonds”. , This also explains why double proton transfer, also known as chattering, can occur in the cyclic dimer of carboxylic acids . In a molecule such as pyruvic acid, in which the lowest energy monomer has an orientation of the carboxylic acid group like that of the SsT and AaT lactic acid conformers, which is due to internal hydrogen bonding, the formation of the hydrogen-bonded dimer is much less favorable. , The equilibrium constant for pyruvic acid dimerization is several orders of magnitude lower than it is for formic acid and lactic acid. − …”

Thin Films of an Ultrastable Metal–Organic Framework for Formic Acid Sensing with High Selectivity and Excellent Reproducibility

“…The impedance change arises mainly from the adsorption of a certain chemical vapor on the surface of MOF-802 thin film. Accordingly, three main factors can affect the impedance change, i.e., (1) the adsorption capability of the chemical vapor on the surface of MOF-802 thin film, which will directly affect the coverage of chemical vapor on the surface of film; (2) the hydrogen-bonding capability of chemical vapor molecules toward the moieties on the surfaces of MOF-802 crystal grains, such as μ 3 -O and μ 3 -OH groups in Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 nodes, pyrazole rings in linkers, coordinated −COO – groups of linkers and formates, coordinated and lattice H 2 O molecules; (3) the molecular aggregation form of chemical vapors, (e.g., carboxylic acid molecules commonly adopt in stable dimeric form at the high temperature above its boiling point, − and alcohol molecules have monomeric, dimeric, and hexameric forms in vapor , ), and the different aggregation forms can affect the hydrogen-bonding configuration.…”

“…Interestingly, MOF-802 thin-film sensor also presented an increased impedance to AcOH vapor, even though AcOH is a carboxylic acid that is capable of providing mobile protons. It is probably because the special aggregation models of AcOH vapor, , influencing the hydrogen-bonding formation on the film surfaces. Indeed, the abnormal response to AcOH vapor was previously reported for other chemiresistive sensors .…”

An Adjustable Cleft Based on an 8‐Sulfonamide‐2‐Naphthoic Acid with Oxyanion Hole Geometry

“…The shortest H-bond corresponds, nevertheless, to the carboxyl proton and the basic pyridine nitrogen, with a distance of 2.662 Å. Interestingly, two more acetic acid molecules show up in this crystalline structure (Figure S35), with H-bonds between the oxygen atoms (2.658 Å and 2.642 Å) similar to the already reported for acetic acid dimers. [14] To study the stability of the acetic acid complex, an NMR titration was carried out. Adding acetic acid to a 1.5 • 10 À 3 M solution of receptor 6 in deuterochloroform induces large shifts in the receptor 6 signals; in particular, H-1 shifts from 9.126 ppm to 9.329 ppm showing its proximity with the acetic acid carboxyl in the complex.…”