The appearance of multiple bands in the N–H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend–stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (${\mathrm{\nu }}_{\text{sym}}^{\text{D}}$), asymmetric stretchin (${\mathrm{\nu }}_{\text{asym}}^{\text{D}}$), and bending overtone ($2{\mathrm{\nu }}_{\text{bend}}^{\text{D}}$) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (${\mathrm{\nu }}_{\text{bend}}^{\text{D}}$ + ${\mathrm{\nu }}_{\text{bend}}^{\text{A}}$) and with the combination of the intermolecular translational mode over the donor bending overtone (${\mathrm{\nu }}_{\text{trans}}$ + $2{\mathrm{\nu }}_{\text{bend}}^{\text{D}}$). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm⁻¹ for the N–H bend–stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend–stretch Fermi resonance coupling.

中文翻译：

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氢键的振动光谱特征引起胺中的NH伸缩弯曲费米共振：甲胺簇和其他N–H⋯N氢键复合物

在中性甲胺二聚体和三聚体的红外光谱的NH拉伸区域中出现多个谱带，是NH弯曲-拉伸非谐耦合的信号。从头开始进行非谐计算，以揭示在甲胺二聚体光谱中观察到的各个谱带的起源。利用离散变量表示方法构建了一个七维势能面，涉及氢键供体和受体沿分子间平移模式的对称和不对称拉伸和弯曲振动。所得的二聚体光谱显示出五个带，可以归因于对称拉伸（${\mathrm{\nu }}_{\text{象征}}^{\text{d}}$），不对称拉伸素（${\mathrm{\nu }}_{\text{不对称}}^{\text{d}}$）和弯曲泛音（$2{\mathrm{\nu }}_{\text{弯曲}}^{\text{d}}$）供体部分。这些伴随着由施主和受主的弯曲振动引起的结合带出现（${\mathrm{\nu }}_{\text{弯曲}}^{\text{d}}$ + ${\mathrm{\nu }}_{\text{弯曲}}^{\text{一种}}$）以及分子间平移模式结合施主弯曲泛音（${\mathrm{\nu }}_{\text{反式}}$ + $2{\mathrm{\nu }}_{\text{弯曲}}^{\text{d}}$）。三聚体的光谱基本上由在二聚体中看到的所有特征组成，其中带位置有少量变化。基于二态去扰动模型和从头算非谐计算的实验光谱分析，得出了N–H弯曲-拉伸费米共振耦合的约40 cm ^-1的矩阵元素。通常，氢键结合的氨基的红外光谱描述了由于弯曲拉伸费米共振耦合而产生的三组谱带。