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首页> 外文期刊>The journal of physical chemistry, B. Condensed matter, materials, surfaces, interfaces & biophysical >Raman Spectroscopic Measurements and ab Initio Molecular Orbital Studies of Cadmium(II) Hydration in Aqueous Solution
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Raman Spectroscopic Measurements and ab Initio Molecular Orbital Studies of Cadmium(II) Hydration in Aqueous Solution

机译：水溶液中镉（II）水合的拉曼光谱测量和从头算分子轨道研究

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摘要

The weak, polarized Raman band assigned to the ν↓(l) CdO↓( 6)mode of the hexaaquo Cd (II) ion ( for the GdO↓( 6) unit, Th with H-atoms) has been studied over the temperature range from 25 to152℃. The isotropic scattering geometry in R format was employed in order to measure the true vibrational comtribution of the band and account for the Boltzmann temperature factor B and frequency factor. The band profile as a function of temperature has been examined analytically to extract the parameters: position of btu full width at half-height (fwhh), integral intensity of the band, and relative molar scattering coefficient,S↓(h), over the temperature range measured. The dependence on concentration has also been measured,The 358 cm↑(-1) stretching mode of the hexaaquo Cd(II) shifts only 3 cm↑(-1) to lower frequencies but broad cm↑(-1) for a 127 ℃ temperature increase. Two depolarized modes at 235 and 185 cm↑(-1) could b u↓(2)(e↓(g)) and u↓(5)(f↓(2g)), respectively. The Raman spectroscopic data suggest that the hexaaquo Cd(II) ion is thermodynamically stable in perchlorate solution over the temperature and concentration range measured These findings are in contrast to CdSO↓(4) solutions, recently measured by one of us, where sulfa water molecule of the first hydration sphere. Ab initio geometry optimizations of [Cd(OH↓(2))↓(6)↑(2+)] out at the Hartree-Fock and second-order Mφ11er-Plesset levels of theory, using various basis sets up to 6-31+ G↑(*). The global minimum structure of the Cd(II) hexaaquo ion corresponds with symmetry T↓(h).The vibrational frequencies of the [Cd(OH↓(2))↓(6)↑(2+)]cation were also calculated. The unscaled frequencies of the CdO↓(6) unit are lower than the experimental frequencies (ca. 16%), but scaling the calculated Hartree vibrational frequencies (HF/6-31G↑(*), HF/6-31+G↑(*), and MP2/6-31G↑(*) levels) reproduces ti frequencies. The theoretical binding energy for the hexaaquo Cd(II) ion was calculated and acc ounts for ca.66% of the experimental hydration enthalpy of Cd(II). Frequency calculations are also reported for a Cd(OH↓(2))↓(18)↑(2+) cluster with 6 water molecules in the inner sphere and 12 water molecules in the outer sphere. The global minimum of this cluster corresponds with symmetry T. The vi CdO6 mode (unscaled) occurs at 335 cm↑(-1), quite close to the experimental value. The water molecules of the first sphere form stong H-bonds with the water molecules in the second hydration shell because of the polarizing effect of the cadmium ion. The importance of the second hydration sphere is discussed.
机译：在整个温度范围内，研究了分配给六水合Cd（II）离子的ν↓（l）CdO↓（6）模式的弱极化拉曼带（对于GdO↓（6）单元，Th带有H原子）摄氏25至152度。采用R格式的各向同性散射几何形状是为了测量频带的真实振动贡献，并考虑玻尔兹曼温度因子B和频率因子。解析分析了谱带随温度变化的函数，以提取以下参数：btu半高全宽的位置（fwhh），谱带的积分强度以及相对摩尔散射系数S↓（h）。测量的温度范围。还测量了对浓度的依赖性，六水合镉（358）的358 cm↑（-1）拉伸模式仅将3 cm↑（-1）移至较低频率，但在127℃时移至宽cm↑（-1）。温度升高。在235和185 cm↑（-1）处的两种去极化模式分别可以是b↓（2）（e↓（g））和u↓（5）（f↓（2g））。拉曼光谱数据表明六水合Cd（II）离子在高氯酸盐溶液中在测量的温度和浓度范围内是热力学稳定的。这些发现与我们一个人最近测量的CdSO↓（4）溶液相反，在该溶液中，磺胺水分子第一个水化球在Hartree-Fock和二阶Mφ11er-Plesset理论水平上，[Cd（OH↓（2））↓（6）↑（2+）]的从头算几何优化，使用多达6-31的各种基础集+ G↑（*）。 Cd（II）六水合离子的整体最小结构与对称性T↓（h）相对应。还计算了[Cd（OH↓（2））↓（6）↑（2+）]阳离子的振动频率。 CdO↓（6）单位的未缩放频率低于实验频率（约16％），但缩放计算的Hartree振动频率（HF / 6-31G↑（*），HF / 6-31 + G↑ （*）和MP2 / 6-31G↑（*）级别）再现ti频率。计算了六水合Cd（II）离子的理论结合能，得出了Cd（II）实验水合焓的约66％。还报道了Cd（OH↓（2））↓（18）↑（2+）团簇的频率计算，该团簇的内球面有6个水分子，外球面有12个水分子。该簇的整体最小值与对称性T相对应。vi CdO6模式（无标度）出现在335 cm↑（-1）处，非常接近实验值。由于镉离子的极化作用，第一球的水分子与第二水合壳中的水分子形成牢固的氢键。讨论了第二水合作用的重要性。

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