Grabska Justyna, Beć Krzysztof B, Ishigaki Mika, Huck Christian W, Ozaki Yukihiro
Department of Chemistry, School of Science and Technology , Kwansei Gakuin University , Sanda , Hyogo 669-1337 , Japan.
Faculty of Chemistry , University of Wroclaw , F. Joliot-Curie 14 , 50-383 Wrocław , Poland.
J Phys Chem B. 2018 Jul 12;122(27):6931-6944. doi: 10.1021/acs.jpcb.8b04862. Epub 2018 Jun 29.
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), 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) and antisymmetric ( B) 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.
基于量子力学计算的光谱模拟通常是为光谱问题带来决定性答案的终极工具，但在近红外光谱的情况下，此类研究仍然非常罕见，尤其是针对相当复杂分子的研究。在本工作中，我们对饱和和不饱和长链脂肪酸（花生酸、棕榈酸、硬脂酸、亚油酸、亚麻酸和油酸）采用了完全非谐光谱模拟。通过去微扰振动二阶微扰理论（DVPT2）在包含大部分组合带的宽近红外区域（8000 - 4000 cm⁻¹）准确再现了与脂肪酸饱和度相对应的光谱特征，并提供了详细的谱带归属。烷基链饱和度的影响以及碳 - 碳双键数量的依赖性反映在模拟的近红外光谱中。这使得我们能够可靠地得出关于分子中碳 - 碳双键的存在及其数量如何准确转化为观测光谱的结论。确认了由于长链脂肪酸环状二聚体涉及羟基伸缩和弯曲模式的组合带导致的近红外光谱基线升高与短链和中链脂肪酸的情况相似。此外，对于两个例子（亚油酸和棕榈酸），通过在单体和二聚体的密集网格上探测相关振动势，详细研究了高度非谐的羟基伸缩模式。随后通过广义矩阵努莫罗夫方法求解与时间无关的薛定谔方程，改善了DVPT2方法对单体2νOH模式预测的不一致性。对于环状二聚体，还研究了对称（A）和反对称（B）羟基伸缩势曲线。结合之前对短链和中链脂肪酸的研究讨论了这些观察结果。
