Standard free energy of the equilibrium between the trans-monomer and the cyclic-dimer of acetic acid in the gas phase from infrared spectroscopy

• Published in: Physical chemistry chemical physics : PCCP 2015-01, Vol.17 (11), p.7477-7488
• Main Authors: Goubet, Manuel, Soulard, Pascale, Pirali, Olivier, Asselin, Pierre, Réal, Florent, Gruet, Sébastien, Huet, Thérèse R, Roy, Pascale, Georges, Robert
• Format: Article
• Language: English
• Subjects: Acetic acid; Acetic Acid - chemistry; Chemical equilibrium; Chemical Physics; Dimerization; Dissociation energy; Electronic structure; Estimates; Free energy; Gases - chemistry; Hydrogen Bonding; Mathematical analysis; Models, Molecular; Molecular Conformation; Physics; Quantum Theory; Spectra; Spectroscopy, Fourier Transform Infrared; Temperature
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp05684a

Survey jet-cooled spectra of acetic acid have been recorded in the infrared region (200-4000 cm −1 ) over a wide range of expansion conditions. From the variations of the relative intensities of the signals, vibrational transitions have been assigned unambiguously to the trans -monomer and cyclic-dimer. The IR-active fundamental frequencies have been determined at the instrumental accuracy of 0.5 cm −1 . This analysis of the jet-cooled spectra supported by electronic structure calculations permitted us to characterize the trans -monomer/cyclic-dimer equilibrium. From static cell spectra at 298 K, variations of the molar fractions ratio as a function of the total pressure were used to estimate the equilibrium constant and the Gibbs free energy of dimerization at 298 K. The very good agreement with the literature data shows that the present method is able to produce, from a single study, a free energy value as reliable as the one obtained from a large collection of data. In addition, the semi-empirical free energy value was used to estimate the accuracy of electronic structure calculations and in turn the accuracy of the derived useful information such as the dissociation energy of the complex ( i.e. the strength of the hydrogen bonds) or the relative energies within the conformational landscape. The dimerization equilibrium of acetic acid in the gas phase at 298 K has been characterized from the analysis of jet-cooled and static cell infrared spectra supported by calculations.