Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan; University of Wroclaw, F. Joliot-Curie 14, 50-383 Wrocław, Poland.
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan.
Spectrochim Acta A Mol Biomol Spectrosc. 2017 Oct 5;185:35-44. doi: 10.1016/j.saa.2017.05.024. Epub 2017 May 13.
Quantum chemical reproduction of entire NIR spectra is a new trend, enabled by contemporary advances in the anharmonic approaches. At the same time, recent increase of the importance of NIR spectroscopy of biological samples raises high demand for gaining deeper understanding of NIR spectra of biomolecules, i.e. fatty acids. In this work we investigate saturated and unsaturated medium-chain fatty acids, hexanoic acid and sorbic acid, in the near-infrared region. By employing fully anharmonic density functional theory (DFT) calculations we reproduce the experimental NIR spectra of these systems, including the highly specific spectral features corresponding to the dimerization of fatty acids. Broad range of concentration levels from 5·10M in CCl to pure samples are investigated. The major role of cyclic dimers can be evidenced for the vast majority of these samples. A highly specific NIR feature of fatty acids, the elevation of spectral baseline around 6500-4000cm, is being explained by the contributions of combination bands resulting from the vibrations of hydrogen-bonded OH groups in the cyclic dimers. Based on the high agreement between the calculated and experimental NIR spectra, a detailed NIR band assignments are proposed for hexanoic acid and sorbic acid. Subsequently, the correlations between the structure and NIR spectra are elucidated, emphasizing the regions in which clear and universal traces of specific bands corresponding to saturated and unsaturated alkyl chains can be established, thus demonstrating the wavenumber regions highly valuable for structural identifications.
量子化学全近红外光谱再现是当前非谐方法进步带来的新趋势。与此同时，生物样品近红外光谱的重要性不断增加，这对深入了解生物分子（如脂肪酸）的近红外光谱提出了更高的要求。在这项工作中，我们研究了饱和和不饱和中链脂肪酸，己酸和山梨酸在近红外区域的情况。通过采用完全非谐密度泛函理论（DFT）计算，我们再现了这些体系的实验近红外光谱，包括对应于脂肪酸二聚化的高度特异的光谱特征。我们研究了从 CCl 中 5·10M 到纯样品的广泛浓度范围。对于绝大多数这些样品，可以证明环状二聚体的主要作用。脂肪酸的一个高度特异的近红外特征是光谱基线在 6500-4000cm 左右的升高，这可以通过氢键 OH 基团振动产生的组合带的贡献来解释，这些组合带存在于环状二聚体中。基于计算和实验近红外光谱之间的高度一致性，我们为己酸和山梨酸提出了详细的近红外带分配。随后，阐明了结构与近红外光谱之间的相关性，强调了可以建立对应于饱和和不饱和烷基链的特定带的清晰和通用痕迹的区域，从而证明了对于结构鉴定非常有价值的波数区域。
