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.

中文翻译：

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非谐波DFT饱和和不饱和长链脂肪酸的近红外光谱模拟

基于量子力学计算的光谱模拟通常是为光谱问题提供决定性答案的终极工具，但是在近红外光谱的情况下，此类研究仍然非常罕见，尤其是对相当复杂的分子的研究。在目前的工作中，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）采用了完全非谐波光谱模拟。通过去扰动振动二阶微扰理论（DVPT2）在整个NIR区域（8000–4000 cm ^–1）中精确再现了与脂肪酸饱和度相对应的光谱特征），其中主要包含组合频段，并提供了详细的频段分配。模拟的近红外光谱反映了烷基链饱和度的影响和C═C键数的依赖性。这样就可以得出可靠的结论，即如何准确地将C═C键的存在及其在分子中的数目转化为观察到的光谱。在NIR光谱中，由于涉及长链脂肪酸环状二聚体的OH拉伸和弯曲模式的结合带而引起的基线升高被证实与短链和中链脂肪酸相似。另外，对于两个实例（亚油酸和棕榈酸），通过在致密栅格上探查单体和二聚体的相关振动势，详细研究了高度非谐OH拉伸模式。随后通过广义矩阵Numerov方法求解与时间无关的Schrodinger方程，可以改善通过DVPT2途径预测单体的2νOH模式的预测的不一致性。对于循环二聚体，对称的（还研究了a _g）和反对称（B _u）OH拉伸电势曲线。结合先前对短链和中链脂肪酸的研究，对这些观察结果进行了讨论。