This work presents new experimental and theoretical insights on vibrational spectra of CH₃I and CD₃I in the liquid phase. For the first time, we provided the contributions from different vibrational modes to mid-infrared (MIR) and near-infrared (NIR) spectra and estimated the extent of anharmonicity in the MIR region. Direct comparison of the intensities from ATR-IR and NIR transmission spectra was possible due to normalization of ATR-IR spectra. As a reference for normalization, we applied the area of the ν_s(CH₃)/ν_s(CD₃) band recorded in transmission mode. Our results show that the corresponding vibrational modes of CH₃I and CD₃I have similar contributions to the total intensity (MIR+NIR), however, these contributions are distributed in a different way between MIR and NIR regions. As expected, most of intensity in MIR spectra originates from the fundamental transitions (>90%). The fundamental bands together with the first overtones and the binary combinations contribute to more than 99% of MIR intensity for both compounds. Therefore, reliable reconstruction of MIR spectra can be achieved by considering only these vibrational modes. On the other hand, accurate simulation of NIR spectra requires including the higher-order transitions. In the case of CD₃I, the fourth-order transitions contribute to 12.7% of NIR intensity. The contributions from NIR region are significantly smaller than those from MIR range and were estimated to be 6.7% for CH₃I and 2.3% for CD₃I. The theoretical calculations provide a reasonable estimation of the total contribution from the fundamental bands. Yet, the calculated contributions from the anharmonic transitions are different from those obtained from the experimental data. MIR spectra of CH₃I and CD₃I reveal an unexpected increase in the intensity of some overtones and combination bands indicating the presence of Fermi resonances. These resonances are responsible for differences in contributions from the first overtones and binary combinations between CH₃I and CD₃I.

这项工作提出了关于CH ₃ I和CD ₃ I在液相中的振动光谱的新的实验和理论见解。首次，我们提供了不同振动模式对中红外（MIR）和近红外（NIR）光谱的贡献，并估计了MIR区域中的非谐性程度。由于ATR-IR光谱的归一化，可以直接比较ATR-IR和NIR透射光谱的强度。作为归一化的参考，我们应用了ν的面积_小号（CH ₃）/ν_小号（CD ₃）以传送模式记录带。我们的结果表明，CH ₃ I和CD ₃的相应振动模式我对总强度（MIR + NIR）的贡献相似，但是，这些贡献在MIR和NIR区域之间的分布方式不同。不出所料，MIR光谱中的大多数强度都来自基本跃迁（> 90％）。基带，第一泛音和二元组合对这两种化合物的MIR强度贡献超过99％。因此，仅考虑这些振动模式就可以实现MIR光谱的可靠重建。另一方面，准确的NIR光谱模拟要求包括高阶跃迁。对于CD ₃I，四阶跃迁占NIR强度的12.7％。NIR区域的贡献显着小于MIR范围的贡献，对于CH ₃ I估计为6.7％，对于CD ₃ I为2.3％。理论计算提供了对基带总贡献的合理估计。但是，从非谐波跃迁计算得出的贡献与从实验数据获得的贡献不同。CH ₃ I和CD ₃ I的MIR光谱显示某些泛音和组合谱带的强度出乎意料的增加，表明存在费米共振。这些共振是造成第一泛音和CH之间二进制组合的贡献差异的原因₃我和CD ₃我。