Aggregation of acetic and propionic acid in argon matrices--a matrix isolation and computational study

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Carboxylic acids exist as monomers and dimers in various environments.
• Understanding dimer stability is crucial for chemical processes.
• Matrix isolation spectroscopy is a powerful tool for studying unstable species.

Purpose of the Study:

• To investigate the dimerization of monomeric acetic acid (MA) and propionic acid (MP) in argon matrices.
• To determine the most stable dimer structures and identify factors influencing dimer formation.
• To compare the dimerization behavior of acetic acid and propionic acid.

Main Methods:

• Isolation of MA and MP in argon matrices at 10K using pulse deposition.
• Induction of dimerization by warming matrices to 40K.
• Monitoring dimerization via infrared (IR) spectroscopy.
• Rationalization of observations using Density Functional Theory (DFT) and ab initio calculations.

Main Results:

• Both acetic acid and propionic acid form thermodynamically stable symmetrical dimers (B) with strong C=O...HO hydrogen bonds.
• Acetic acid forms a less stable dimer (AA) at high concentrations, which rearranges to the stable dimer (BA) upon annealing.
• Propionic acid does not form a similar less stable dimer under any experimental conditions.

Conclusions:

• The stability of carboxylic acid dimers is influenced by molecular structure and concentration.
• Symmetrical dimers with strong hydrogen bonds are the most stable forms.
• Differences in dimerization behavior between acetic and propionic acid can be explained by computational modeling.