P-063

Jogilė Mačytė

jogile.macyte@ff.vu.lt

Justinas Čeponkus, Valdas Šablinskas

Institute of Chemical Physics, Vilnius University, Lithuania

Structural Analysis of Caproic Acid Using Matrix Isolation FTIR Spectroscopy and Ab Initio Calculations

Caproic acid, also known as hexanoic acid (C6H12O2), is a simple saturated aliphatic carboxylic acid. Microbial fermentation and waste-to-chemical conversion are effective methods for producing it in a cleaner and more efficient way [1]. Carboxylic acids are often studied using vibrational spectroscopy since they serve as good model systems for understanding processes in more complex molecular structures [2]. Most matrix isolation studies have focused on smaller carboxylic acids, starting from formic acid and ending at propionic acid, to investigate hydrogen bond complexes. The larger acids, such as caproic acid, have been studied less since possibility to exist in form of different conformers makes the interpretation of their vibrational spectra more challenging. The main goal of this work is to identify the possible structures of caproic acid using matrix isolation infrared spectroscopy together with ab initio calculations.

Geometries and vibrational spectra of possible caproic acid conformers were calculated using B3LYP and MP2 levels of theory, with the cc-pVTZ basis set. B3LYP calculations showed that the TTTTT conformation has the lowest energy, meaning it is the most stable form. The second conformer is TG±G±TT, which is ∆𝐸 = 1.54 kJ/mol higher in energy. In contrast, MP2 calculations produced a different result: the most stable conformer is TG±G±TT, which is 1.06 kJ/mol lower in energy than TTTTT. We recorded experimental FTIR spectra of caproic acid trapped in argon and neon matrices.

When we compared the experimental spectra with the calculated spectra of the two lowest-energy conformers, we were able to match specific spectral bands to each conformer. For example, in the argon matrix, the intense band at 1093 cm-1 was assigned to CH2 twisting and OH deformation vibrations of TG±G±TT conformer. Another band, observed at 1116 cm-1 can be assigned to TTTTT conformer, this band is associated with CH2 wagging and OH deformation vibrations.

Such findings allowed us for the first time to come to conclusion that at least two conformers of caproic acid coexist in low-temperature matrices, with different abundance what is reasoned by higher stability of the TTTTT conformer.

[1] W. S. Chen et al. Environmental Science & Technology, Vol. 51, No. 12, pp. 7159–7168 (2017).

[2] J. Ceponkus et al. Lithuanian Journal of Physics, Vol. 49, No. 1, pp. 53–62 (2009).