Correlations between Structure and Near-Infrared Spectra of Saturated and Unsaturated Carboxylic Acids. Insight from Anharmonic Density Functional Theory Calculations

By near-infrared (NIR) spectroscopy and anharnionic density functional theory (DFT) calculations, we investigate five kinds of saturated and unsaturated carboxylic acids belonging to the group of short chairt fatty acids: propionic acid, butyric acid, acrylic acid, crotonic acid, and vinylacetic acid. The experimental NIR spectra of these five kinds,of carboxylic,acids are reproduced by quantum chemical calculations in a broad spectral region of 7500-4000 cm(-1)and for a wide range of concentrations. By employing anhannonic GVPT2 calculations on DFT level, a detailed interpretation of experimental spectra is achieved, elucidating structure-spectra correlations of thesemolecules in the NIR region. We emphasize the spectral features due to saturated and unsaturated alkyl chains, the location of a C=C bond within the alkyl chain, and the dimerization of carboxylic acids. In particular, the existence of a terminal C=C bond leads to the appearance of highly specific NIR bands. These pronounced bands are located at wavenurribers where no overlapping with other stincturespecific bands occurs, thus making them good structural markers. Most of the spectral differences between these two groups of molecules remain subtle, and would be difficult to reliably ascribe without quantum chemically calculated NIR spectra. Moreover, anharnronic DFTcalculations provide insightson The manifestation of hydrOgen bonding throughidistinctiye-spectral features corresponding to cyclic dimers. The regiltingspectral baseline elevation is common for all five investigated carboxylic acids, and remains-consistent with previous results on acetic acid.