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Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide–Water Binary Complex
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2020-04-02 , DOI: 10.1021/acs.jpca.0c00853
Justin J. Talbot 1 , Nan Yang 2 , Meng Huang 3 , Chinh H. Duong 2 , Anne B. McCoy 3 , Ryan P. Steele 1 , Mark A. Johnson 2
Affiliation  

The gas-phase vibrational spectrum of the isolated iodide–water cluster ion (I·H2O), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion microhydration. Although the spectra of the smaller halides are well described in the context of an asymmetrical ground-state structure in which only one OH group is hydrogen-bonded to the ion, the I·H2O spectrum displays multiplet structures with partially resolved rotational patterns that are additionally influenced by quantum nuclear spin statistics. In this study, this complex behavior is unraveled with a combination of experimental methods, including ion preparation in a temperature-controlled ion trap and spectral simplification through applications of tag-free, two-color IR–IR double-resonance spectroscopy. Analysis of the double-resonance spectra reveals a vibrational ground-state tunneling splitting of about 20 cm–1, which is on the same order as the spacing between the peaks that comprise the multiplet structure. These findings are further supported by the results obtained from a fully coupled, six-dimensional calculation of the vibrational spectrum. The underlying level structure can then be understood as a consequence of experimentally measurable, vibrational mode-dependent tunneling splittings (which, in the case of the ground vibrational state, is comparable to the rotational energy spacing between levels with Ka = 0 and 1), as well as Fermi resonance interactions. The latter include the hydrogen-bonded OH stretches and combination bands that involve the HOH bend overtones and soft-mode excitations of frustrated translation and rotation displacements of the water molecule relative to the ion. These anharmonic couplings yield closely spaced bands that are activated in the IR by borrowing intensity from the OH stretch fundamentals.

中文翻译:

碘-水二元络合物中依赖模式的隧道分裂的光谱学特征

所述分离的碘化物的水簇离子的气相振动光谱(I - ·H 2 O),在1996年首次报道,礼物最困难的,长期的分光涉及离子microhydration难题之一。尽管较小的卤化物的光谱在不对称基态结构的背景下得到了很好的描述,在基态结构中,只有一个羟基与氢键合,I ·H 2O光谱显示具有部分解析的旋转模式的多重结构,该旋转模式另外受量子核自旋统计的影响。在这项研究中,通过多种实验方法的组合来阐明这种复杂的行为,包括在温度控制的离子阱中进行离子制备以及通过应用无标签双色IR-IR双共振光谱仪简化光谱。对双共振频谱的分析显示,振动基态隧穿裂隙约为20 cm –1,其顺序与组成多重结构的峰之间的间隔相同。从振动光谱的全耦合六维计算获得的结果进一步支持了这些发现。然后,可以将底层的层级结构理解为根据实验可测量的,依赖于振动模式的隧穿裂隙的结果(在地面振动状态下,该裂隙与具有K a的层级之间的旋转能间隔相当)= 0和1),以及费米共振相互作用。后者包括氢键键合的OH延伸和结合带,这些结合带涉及HOH弯曲泛音以及水分子相对于离子的受阻平移和旋转位移的软模式激发。这些非谐耦合产生紧密间隔的条带,这些条带通过从OH拉伸基本原理中借用强度而在IR中被激活。
更新日期:2020-04-03
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