Müller David, Dopfer Otto
Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.
Phys Chem Chem Phys. 2022 May 11;24(18):11222-11233. doi: 10.1039/d2cp01192a.
Due to the weak interactions of He atoms with neutral molecules and ions, the preparation of size-selected clusters for the spectroscopic characterization of their structures, energies, and large amplitude motions is a challenging task. Herein, we generate HOHe ( ≤ 9) and HOHe ( ≤ 5) clusters by stepwise addition of He atoms to mass-selected ions stored in a cryogenic 22-pole ion trap held at 5 K. The population of the clusters as a function of provides insight into the structure of the first He solvation shell around these ions given by the anisotropy of the cation-He interaction potential. To rationalize the observed cluster size distributions, the structural, energetic, and vibrational properties of the clusters are characterized by calculations up to the CCSD(T)/aug-cc-pVTZ level. The cluster growth around both the open-shell HO and closed-shell HO ions begins by forming nearly linear and equivalent OH⋯He hydrogen bonds (H-bonds) leading to symmetric structures. The strength of these H-bonds decreases slightly with due to noncooperative three-body induction forces and is weaker for HO than for HO due to both enhanced charge delocalization and reduced acidity of the OH protons. After filling all available H-bonded sites, addition of further He ligands around HO ( = 3-4) occurs at the electrophilic singly occupied 2p orbital of O leading to O⋯He p-bonds stabilized by induction and small charge transfer from HO to He. As this orbital is filled for HO, He atoms occupy in the = 4-6 clusters positions between the H-bonded He atoms, leading to a slightly distorted regular hexagon ring for = 6. Comparison between HOHe and CHOHHe illustrates that CH substitution substantially reduces the acidity of the OH protons, so that only clusters up to = 2 can be observed. The structure of the solvation sub-shells is visible in both the binding energies and the predicted vibrational OH stretch and bend frequencies.
由于氦原子与中性分子和离子之间的相互作用较弱，制备用于光谱表征其结构、能量和大幅度运动的尺寸选择团簇是一项具有挑战性的任务。在此，我们通过向存储在5 K低温22极离子阱中的质量选择离子逐步添加氦原子，生成了HOHe（≤9）和HOHe（≤5）团簇。团簇的丰度作为 的函数，通过阳离子 - 氦相互作用势的各向异性，深入了解了这些离子周围第一个氦溶剂化壳层的结构。为了合理化观察到的团簇尺寸分布，通过高达CCSD(T)/aug-cc-pVTZ水平的计算对团簇的结构、能量和振动性质进行了表征。开壳层HO和闭壳层HO离子周围的团簇生长首先通过形成近乎线性且等效的OH⋯He氢键（H键），从而形成对称结构。由于非协同三体诱导力，这些H键的强度随 略有降低，并且由于电荷离域增强和OH质子酸度降低，HO的H键比HO的弱。在填充所有可用的氢键位点后，HO（ = 3 - 4）周围进一步添加氦配体发生在O的亲电单占据2p轨道上，导致O⋯He p键通过诱导和从HO到He的小电荷转移而稳定。当该轨道被HO填满时，氦原子占据 = 4 - 6团簇中氢键连接的氦原子之间的位置，导致 = 6时形成略微扭曲的正六边形环。HOHe和CHOHHe之间的比较表明，CH取代大大降低了OH质子的酸度，因此只能观察到高达 = 2的团簇。溶剂化子壳层的结构在结合能以及预测的振动OH伸缩和弯曲频率中都可见。
