NIR Spectra Simulations by Anharmonic DFT-Saturated and Unsaturated Long-Chain Fatty Acids

Spectra simulation based on quantum mechanical calculations is often an ultimate tool bringing decisive answers to spectroscopic problems, but in the case of NIR spectroscopy, such studies still remain very rare, particularly those on rather complicated molecules. In the present work we have employed fully anharmonic spectra simulation for saturated and unsaturated long-chain fatty acids (arachidic acid, palmitic acid, stearic acid, linoleic acid, linolenic acid, and oleic acid). The spectral features corresponding to the saturation of fatty acid were accurately reproduced by deperturbed vibrational second-order perturbation theory (DVPT2) throughout a wide NIR region (8000-4000 cm^-1), which contains mostly combination bands, and detailed band assignments have been provided. The effect of the saturation of the alkyl chain and the dependency of the number of C═C bonds were reflected in the simulated NIR spectra. This allowed for drawing reliable conclusions about how exactly the existence of C═C bonds and their number in a molecule are translated into the observed spectra. The baseline elevation in the NIR spectra due to the combination bands involving OH stretching and bending modes of the long-chain fatty acid cyclic dimers were confirmed to be similar to those of short- and medium-chain fatty acids. Additionally, for two examples (linoleic and palmitic acid), highly anharmonic OH stretching modes were studied in detail by probing the relevant vibrational potentials over a dense grid for monomers and dimers. Subsequent solving of the time-independent Schrodinger equation by a generalized matrix Numerov method allowed for improving the inconsistency of the prediction by the DVPT2 route of the 2νOH modes of the monomers. For the cyclic dimers, the symmetric ( A_g) and antisymmetric ( B_u) OH stretching potential curves have been investigated as well. These observations were discussed in relation to the previous investigations of short- and medium-chain fatty acids.