Cappa Christopher D, Smith Jared D, Messer Benjamin M, Cohen Ronald C, Saykally Richard J
Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
J Phys Chem A. 2007 Jun 7;111(22):4776-85. doi: 10.1021/jp070551c. Epub 2007 Apr 18.
X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately "hyper-coordinated" solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.
已在氧 K 边附近测量了 4M 和 6M 氢氧化钾水溶液的 X 射线吸收光谱。向水中加入 KOH 后，在远低于液态水前缘特征能量（535eV）处出现了一个新的光谱特征（532.5eV），这归因于 OH⁻离子。除了由于溶剂化 OH⁻离子吸收而导致的光谱变化外，计算得到的 XA 光谱表明，第一溶剂化层水分子呈现出与 bulk 液态水不同的吸收光谱。有人认为，这种光谱变化主要源于第一壳层水分子未占据分子轨道的直接电子扰动，其次才是第一水合壳层内局部氢键网络的几何畸变。实验和计算得到的 XA 光谱均表明，OH⁻离子与溶剂化水分子之间的相互作用性质与卤化盐水溶液中相应的相互作用在这种直接轨道畸变方面存在根本差异。对 Mulliken 电荷布居的分析表明，这种差异的根源在于溶剂化水分子氢原子之间电荷不对称性的差异。电荷不对称性是由阴离子存在引起的电场效应以及各离子的电荷转移共同诱导产生的。计算结果还表明，OH⁻离子以主要为“超配位”的溶剂化壳层存在，并且 OH⁻离子不易向周围水分子提供氢键。
