Huang Qian-Rui, Endo Tomoya, Mishra Saurabh, Zhang Bingbing, Chen Li-Wei, Fujii Asuka, Jiang Ling, Patwari G Naresh, Matsuda Yoshiyuki, Kuo Jer-Lai
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan.
Phys Chem Chem Phys. 2021 Feb 19;23(6):3739-3747. doi: 10.1039/d0cp05745b.
Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured in three experimental set-ups to capture their complex spectral features as a result of bend/umbrella-stretch Fermi resonance (FR). Multiple bands were observed between 2800 and 3000 cm-1 corresponding to the methyl groups for MMA and DMA. On the other hand, the corresponding spectrum of TMA is relatively simple, exhibiting only four prominent bands in the same frequency window, even though TMA has a larger number of methyl groups. The discrete variable representation (DVR) based ab initio anharmonic algorithm with potential energy surface (PES) at CCSD/aug-cc-pVDZ quality is able to capture all the experimentally observed spectral features across all three amines, and the constructed vibrational Hamiltonian was used to analyze the couplings that give rise to the observed FR patterns. It was observed that the vibrational coupling among CH stretch modes on different methyl groups is weak (less than 2 cm-1) and stronger vibrational coupling is found to localize within a methyl group. In MMA and DMA, the complex feature between 2850 and 2950 cm-1 is a consequence of closely packed overtone states that gain intensities by mixing with the stretching modes. The simplification of the spectral pattern of TMA can be understood by the red-shift of the symmetric CH3 stretching modes by about 80 cm-1 relative to MMA, which causes the symmetric CH3 stretch to shift outside the FR window.
在三种实验装置中测量了甲胺（MMA）、二甲胺（DMA）和三甲胺（TMA）单体及其团簇中甲基的振动光谱，以捕捉由于弯曲/伞状伸缩费米共振（FR）而产生的复杂光谱特征。在2800至3000 cm-1之间观察到多个与MMA和DMA的甲基相对应的谱带。另一方面，TMA的相应光谱相对简单，即使TMA有更多的甲基，在相同的频率窗口中也只显示出四个突出的谱带。基于离散变量表示（DVR）的从头算非谐算法以及CCSD/aug-cc-pVDZ质量的势能面（PES）能够捕捉所有三种胺类中所有实验观察到的光谱特征，并使用构建的振动哈密顿量来分析产生观察到的FR模式的耦合。观察到不同甲基上的CH伸缩模式之间的振动耦合较弱（小于2 cm-1），并且发现较强的振动耦合局限在一个甲基内。在MMA和DMA中，2850至2950 cm-1之间的复杂特征是紧密堆积的泛音状态的结果，这些泛音状态通过与伸缩模式混合而获得强度。TMA光谱模式的简化可以通过相对于MMA对称CH3伸缩模式红移约80 cm-1来理解，这导致对称CH3伸缩移至FR窗口之外。
