CCB-Center for Chemistry and Biomedicine, Institute of Analytical Chemistry and Radiochemistry, Leopold-Franzens University, Innrain 80/82, 6020 Innsbruck, Austria.
School of Biological and Environmental Sciences, Kwansei Gakuin University, Sanda 669-1337, Hyogo, Japan.
Molecules. 2021 Aug 27;26(17):5212. doi: 10.3390/molecules26175212.
Anharmonic quantum chemical calculations were employed to simulate and interpret a near-infrared (NIR) spectrum of caffeine. First and second overtones, as well as binary and ternary combination bands, were obtained, accurately reproducing the lineshape of the experimental spectrum in the region of 10,000-4000 cm (1000-2500 nm). The calculations enabled performing a detailed analysis of NIR spectra of caffeine, including weak bands due to the second overtones and ternary combinations. A highly convoluted nature of NIR spectrum of caffeine was unveiled, with numerous overlapping bands found beneath the observed spectral lineshape. To properly reflect that intrinsic complexity, the band assignments were provided in the form of heat maps presenting the contributions to the NIR spectrum from various kinds of vibrational transitions. These contributions were also quantitatively assessed in terms of the integral intensities. It was found that the combination bands provide the decisively dominant contributions to the NIR spectrum of caffeine. The first overtones gain significant importance between 6500-5500 cm, while the second overtones are meaningful in the higher wavenumber regions, particularly in the 10,000-7000 cm region. The obtained detailed band assignments enabled deep interpretation of the absorption regions of caffeine identified in the literature as meaningful for analytical applications of NIR spectroscopy focused on quantitative analysis of caffeine content in drugs and natural products.
采用非谐量子化学计算模拟和解释了咖啡因的近红外（NIR）光谱。得到了一级和二级泛频以及二元和三元组合带，准确地再现了实验光谱在 10000-4000cm（1000-2500nm）区域的谱线形状。该计算能够对咖啡因的 NIR 光谱进行详细分析，包括由于二级泛频和三元组合引起的弱带。揭示了咖啡因 NIR 光谱的高度复杂性质，在观察到的光谱形状下发现了许多重叠带。为了正确反映这种内在的复杂性，以热图的形式提供了带分配，展示了各种振动跃迁对 NIR 光谱的贡献。还根据积分强度对这些贡献进行了定量评估。结果发现，组合带对咖啡因的 NIR 光谱提供了决定性的主要贡献。一级泛频在 6500-5500cm 之间具有重要意义，而二级泛频在较高波数区域具有重要意义，特别是在 10000-7000cm 区域。获得的详细带分配使我们能够深入解释文献中确定的咖啡因吸收区域，这些区域对于基于近红外光谱分析应用的定量分析药物和天然产物中咖啡因含量具有重要意义。
