Studies on the vibrational spectra of various ammonium-centered clusters under different solvation environments have raised interest over the last thirty years. The gas-phase infrared photodissociation spectroscopy (IRPD) experiments showed that these NH₄⁺⋯X_n clusters exhibit rich spectral features from 2600 to 3400 cm⁻¹. In this work, we have simulated the vibrational spectra and analyzed couplings among vibrational quantum states in the aforementioned frequency range using ab initio anharmonic algorithms. Originating from the anharmonic couplings between NH stretching fundamentals and bending overtones, Fermi resonance (FR) is a common feature in these spectra, and its extent is determined by the magnitude of couplings and the energy matching conditions between relevant states, which are governed by the proton affinity, number, and bonding configuration of the solvation species. For weakly bound clusters consisting of rare gas atoms, FR is insignificant but not negligible; for strongly bound clusters, such as ammonium–water clusters, the hydrogen-bonded NH stretching fundamentals redshift and reach a better resonance condition, and thus light up the bending overtones as prominent FR bands. Our simulated spectra are in good agreement with previous experimental reports of these ammonium-centered clusters and provide a better understanding of the vibrational coupling behind the spectra of the NH stretching region.

中文翻译：

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溶剂化铵的振动光谱背后的非谐耦合：通过调整溶剂化环境通过费米共振激发泛音状态

在过去的三十年中，对各种以铵为中心的团簇在不同溶剂化环境下的振动光谱的研究引起了人们的兴趣。气相红外光解离光谱（IRPD）实验表明，这些NH ₄ ⁺ ⋯X _n团簇具有2600至3400 cm ^-1的丰富光谱特征。在这项工作中，我们使用了从头算方法模拟了振动光谱，并分析了上述频率范围内的振动量子态之间的耦合。非谐算法。费米共振（FR）源于NH拉伸基本原理和弯曲泛音之间的非谐耦合，是这些光谱的共同特征，其程度由耦合的大小和相关状态之间的能量匹配条件决定，该条件由质子亲和性，数量和溶剂化物质的键合构型。对于由稀有气体原子组成的弱束缚簇，FR无关紧要，但不可忽略。对于牢固结合的团簇，例如铵-水团簇，氢键结合的NH拉伸基本原理发生红移并达到更好的共振条件，从而将弯曲泛音点亮为突出的FR谱带。