Roscioli Joseph R, Diken Eric G, Johnson Mark A, Horvath Samantha, McCoy Anne B
Sterling Chemistry Laboratories, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, USA.
J Phys Chem A. 2006 Apr 20;110(15):4943-52. doi: 10.1021/jp056022v.
A detailed picture of the structural distortions suffered by a water molecule in direct contact with small inorganic anions (e.g., X = halide) is emerging from a series of recent vibrational spectroscopy studies of the gas-phase X-.H2O binary complexes. The extended spectral coverage (600-3800 cm(-1)) presently available with tabletop laser systems, when combined with versatile argon "messenger" techniques for acquiring action spectra of cold complexes, now provides a comprehensive survey of how the interaction evolves from an ion-solvent configuration into a three-center, two-electron covalent bond as the proton affinity of the anion increases. We focus on the behavior of H2O in the X-.H2O (X = Br, Cl, F, O, and OH) complexes, which all adopt asymmetric structures where one hydrogen atom is H-bonded to the ion while the other is free. The positions and intensities of the bands clearly reveal the mechanical consequences of both (zero-point) vibrationally averaged and infrared photoinduced excess charge delocalization mediated by intracluster proton transfer (X-.H2O --> HX.OH-). The fundamentals of the shared proton stretch become quite intense, for example, and exhibit extreme red-shifts as the intracluster proton-transfer process becomes available, first in the vibrationally excited states (F-.H2O) and then finally at the zero-point level (OH-.H2O). In the latter case, the loss of the water molecule's independent character is confirmed through the disappearance of the approximately 1600 cm(-1) HOH intramolecular bending transition and the dramatic (>3000 cm(-1)) red-shift of the shared proton stretch. An unexpected manifestation of vibrationally mediated charge transfer is also observed in the low frequency region, where the 2 <-- 0 overtones of the out-of-plane frustrated rotation of the water are remarkably intense in the Cl-.H2O and Br-.H2O spectra. This effect is traced to changes in the charge distribution along the X-.O axis as the shared proton is displaced perpendicular to it, reducing the charge transfer character of the H-bonding interaction and giving rise to a large quadratic contribution to the dipole moment component that is parallel to the bond axis. Thus, all of these systems are found to exhibit distinct spectral characteristics that can be directly traced to the crucial role of vibrationally mediated charge redistribution within the complex.
一系列近期对气相X⁻·H₂O二元复合物的振动光谱研究，正呈现出与小无机阴离子（如X = 卤化物）直接接触的水分子所遭受的结构畸变的详细图景。当桌面激光系统目前可提供的扩展光谱范围（600 - 3800 cm⁻¹）与用于获取冷复合物作用光谱的通用氩“信使”技术相结合时，现在可以全面考察随着阴离子质子亲和力的增加，相互作用是如何从离子 - 溶剂构型演变为三中心、双电子共价键的。我们关注X⁻·H₂O（X = Br、Cl、F、O和OH）复合物中H₂O的行为，这些复合物都采用不对称结构，其中一个氢原子通过氢键与离子相连，而另一个是自由的。谱带的位置和强度清楚地揭示了（零点）振动平均以及由团簇内质子转移（X⁻·H₂O → HX·OH⁻）介导的红外光致过量电荷离域的力学后果。例如，共享质子拉伸的基频变得相当强烈，并随着团簇内质子转移过程的出现而呈现出极大的红移，首先在振动激发态（F⁻·H₂O）中出现，最终在零点水平（OH⁻·H₂O）出现。在后一种情况下，水分子独立特征的丧失通过约1600 cm⁻¹的HOH分子内弯曲跃迁的消失以及共享质子拉伸的显著（>3000 cm⁻¹）红移得到证实。在低频区域还观察到振动介导电荷转移的一个意外表现，其中水分子面外受挫旋转的2 ← 0泛音在Cl⁻·H₂O和Br⁻·H₂O光谱中非常强烈。这种效应可追溯到随着共享质子垂直于X⁻·O轴移动时，沿X⁻·O轴电荷分布的变化，这降低了氢键相互作用的电荷转移特征，并对平行于键轴的偶极矩分量产生很大的二次贡献。因此，发现所有这些系统都表现出独特的光谱特征，这些特征可直接追溯到复合物内振动介导电荷重新分布的关键作用。
