Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide–Water Binary Complex
Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide–Water Binary Complex
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碘化物-水二元复合物中模式相关隧道分裂的光谱特征
DOI：
10.1021/acs.jpca.0c00853
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Johnson, Mark A.
中科院分区：
文献类型：
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作者：
Talbot, Justin J.;Yang, Nan;Huang, Meng;Duong, Chinh H.;McCoy, Anne B.;Steele, Ryan P.;Johnson, Mark A.
1996年首次报道的碘-水簇离子（I -·H2O）的气相振动谱，是涉及离子微水化的最困难、长期存在的光谱难题之一。虽然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，其中只有一个OH基团与离子形成氢键，但I -·H2O光谱显示出具有部分分解的旋转模式的多重结构，这些结构还受到量子核自旋统计量的影响。在这项研究中，这种复杂的行为通过实验方法的组合来解开，包括在温度控制的离子阱中制备离子，以及通过应用无标签、双色IR-IR双共振光谱来简化光谱。双共振谱分析表明，基态隧穿分裂约为20 cm-1，这与构成多重结构的峰间距相同。从振动谱的全耦合六维计算中得到的结果进一步支持了这些发现。潜在的能级结构可以被理解为实验可测量的结果，振动模式相关的隧道分裂（在地面振动状态的情况下，与ka = 0和1的能级之间的旋转能量间隔相当），以及费米共振相互作用。后者包括氢键OH延伸和组合带，涉及HOH弯曲泛音和水分子相对于离子的受挫平移和旋转位移的软模式激发。这些非谐波耦合产生紧密间隔的波段，通过借用OH拉伸基本面的强度在IR中激活。
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 withKa= 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.