Characterization of the equilibrium between acetic acid and its dimer
The Gibbs free energy of dimerization of acetic acid at room temperature has been determined with a method based exclusively on infrared spectroscopy, thus ending the controversy on the ability of the technique to produce high-quality data for thermodynamics. This work has been accomplished with the Jet-AILES apparatus, a supersonic jet coupled to the high-resolution infrared spectrometer of the AILES beamline, and developed jointly by teams from the PhLAM (Lille1), MONARIS (Paris VI) and IPR (Rennes 1) laboratories and the AILES beamline at SOLEIL. The spectrum of acetic acid vapor cooled by adiabatic expansion was recorded in a wide range of the infrared domain. The flexibility of the experiment, in terms of expansion conditions, allowed for a definitive attribution of observed signals at the instrumental accuracy. The excellent agreement between the present results and the numerous data obtained from a large variety of different techniques demonstrates that our methodology allows the accurate determination of the dimerization equilibrium from a single analysis.
Figure 1: Structures of the trans-acetic acid (a) and its cyclic dimer (b).
Carboxylic acids are organic compounds widely used in industry, and are among the most abundant species of the interstellar medium and planetary atmospheres. In the gas phase, their dominant structure is a cyclic dimer (Figure 1): acetic acid vapor at room temperature is composed of about 85% dimer against only 15% monomer. Acetic acid – one of the smallest member of the family- is a model system for the characterization of physico-chemical properties of carboxylic acids, explaining why it has been the subject of many studies. Concerning infrared spectroscopy, both the attribution of some absorption bands and the vapor composition used to remain ambiguous, despite several studies performed in different experimental conditions: in static cell at room temperature, in supersonic jet, in cryogenic matrix, each one being compared with many theoretical calculations.
Concerning the thermochemistry of the monomer-dimer equilibrium, the dimerization free energy values spread over more than 5 kJ/mol (i.e. a 30% relative uncertainty) showing that this point has always been subject to controversy, whether it is experimental, analysis or calculation problems.
Jet-AILES and electronic structure calculations as diagnostic tools.
Figure 2: Infrared spectra (550-2000 cm-1, resolution 0,5 cm-1) of acetic acid vapor recorded in several adiabatic expansion conditions. From front to rear, monomer signals are clearly increasing while the dimer signals tend to decrease.
During this study, acetic acid vapor has been observed in various adiabatic expansion conditions thanks to Jet-AILES apparatus. Several sizes of slit nozzles, an accurate flowmetry of the sample and its carrier gas and a thermal regulation of the whole injection system allowed controlling precisely the production and destruction of the dimer in the vapor probed by the infrared beam. Relative variations of the absorption signals intensity with flow conditions led to a clear attribution of the observed infrared bands (Figure 2). In addition, the jet-cooled conditions led to a strong reduction of the hot bands sequences, often responsible for shifts of absorption bands at room temperature up to several tens of cm-1. The frequency of each vibrational band was thus pointed out with unprecedented accuracy.
This preliminary analysis showed that the probed vapor is composed of only the trans-monomer and its cyclic dimer (Figure 1). It paves the way for a two-step thermodynamic characterization of the monomer-dimer equilibrium. First, quantum calculations on which the study holds were calibrated using the experimental data. Then, isolated and unperturbed infrared bands were selected using these calculations, in order to determine the monomer/dimer molar fractions ratio as a function of the total pressure of the gas mixture.
The chosen infrared bands were recorded at room temperature over a range of pressures from 4 mTorr to 1.5 Torr thanks to experimental set-ups implemented on the AILES beamline and at MONARIS. The difficulty to quantify a dimerization reaction at room temperature lies in the fact that both the monomer and dimer are present at all times into the probed sample. In this study, the use of the calculated bands intensities allowed to determine the molar fractions as a function of the total pressure. From this, an equilibrium constant (proportional to the slope of the curve, see Figure 3) and the dimerization free energy at 298 K were derived. The very good agreement between this value and the most recent literature value, obtained from a large number of different analyses, shows that this infrared spectroscopic study-based method allows characterizing accurately a dimerization equilibrium from a single analysis.
Figure 3: Weighted ratio of the molar fractions of the dimer (xD) and the monomer (xM) as a function of total pressure. Black diamonds = average, red circles = maximum, blue squares = minimum. The equilibrium constant is equal to the slope time the standard pressure (750 Torr).
This simpler and faster new method will soon be tested on other carboxylic acids such as formic and oxalic acids. The first one, simpler without methyl groups, will serve as another model system, when the latter, more complex with its two carboxyl groups, will be used for testing this method toward larger members of this family.