Raman and Infrared Spectroscopic Investigations on Aqueous Alkali Metal Phosphate Solutions and Density Functional Theory Calculations of Phosphate—Water Clusters
Raman and Infrared Spectroscopic Investigations on Aqueous Alkali Metal Phosphate Solutions and Density Functional Theory Calculations of Phosphate—Water Clusters
复制标题
碱金属磷酸盐水溶液的拉曼和红外光谱研究以及磷酸盐-水团簇的密度泛函理论计算
DOI：
10.1366/000370207783292037
复制
发表时间：
2007
影响因子：
3.5
通讯作者：
G. Irmer
中科院分区：
文献类型：
--
作者：
W. Rudolph;G. Irmer
用拉曼光谱和红外光谱研究了水和重水中的磷酸盐(PO43-−)溶液，包括23℃的水合物熔体在内的宽浓度范围(0.0091-5.280摩尔/L)。在低波数范围内，建立了R格式的光谱，并简要讨论了R归一化过程。归属了PO43−(AQ)四面体(TD对称性)的振动模，并与未水化PO43−(TD)和磷酸盐-水团簇PO43−·H2O(C2v)、PO43−·2H2O(D2D)、PO43−·4H2O(D2D)、PO43−·6H2O(TD)和PO43−·12H2O(T)的振动模进行了比较。还计算了一个由12个水分子和6个水分子组成的第二水化球团簇PO43−·18H2O(T)。在PO43−·12H2O团簇和PO43−·18H2O团簇的情况下，振动模的测量值和计算值的一致性最好，而对于水分子数小于12的未水合PO43−团簇或磷酸盐-水团簇的振动模的一致性要差得多。测量了ν1(A1)PO43−在水溶液中的不对称宽带形状，并解释了其不对称和宽带形状。然而，与正常水中的模式相比，在重水中相同的模式在半高时只有一半的全宽。PO43−在水溶液中具有很强的水合性。通过比较水的形变模式ν2(H2O)和水在K3PO4溶液中的伸缩模式ν1OH和ν3OH随浓度的变化以及与纯水中相同模式的比较，证实了这一点。在∼2 40 cm−1(各向同性R谱)处检测到一个模式，并将其归属于磷酸盐与水之间形成的强氢键P-O···HOH的限制平移模式.在高浓度的K3PO4溶液(C0≥3.70mol/L)和水合物熔体中，可以检测到接触离子对(CIP)的形成。CIPS中的磷酸盐表现出Td对称性降低到C3v的对称性。在浓度较低的溶液中，存在PO43−(AQ)溶剂分离离子对和双溶剂分离离子对，而在极稀溶液中存在完全水合离子(C0≤0.005摩尔/L)。在很宽的浓度范围内，对PO43-−(AQ)的水解过程进行了拉曼定量测量。根据水解数据，已确定H3PO4在23℃时的pK3值为12.45。
Phosphate (PO43−) solutions in water and heavy water have been studied by Raman and infrared spectroscopy over a broad concentration range (0.0091–5.280 mol/L) including a hydrate melt at 23 °C. In the low wavenumber range, spectra in R-format have been constructed and the R normalization procedure has been briefly discussed. The vibrational modes of the tetrahedral PO43−(aq) (Td symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO43− (Td) and phosphate–water clusters: PO43− · H2O (C2v), PO43− · 2H2O (D2d), PO43− · 4H2O (D2d), PO43− · 6H2O (Td), and PO43− · 12H2O (T), a cluster with a complete first hydration sphere of water molecules. A cluster with a second hydration sphere of 12 water molecules and 6 in the first sphere, PO43− · 18H2O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO43− · 12H2O cluster and the PO43− · 18H2O cluster but far less so in the case of the unhydrated PO43− or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of ν1(a1) PO43− in aqueous solutions has been measured as a function of concentration and the asymmetric and broad band shape was explained. However, the same mode in heavy water has only half the full width at half-height compared to the mode in normal water. The PO43− is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing ν2(H2O), the deformation mode of water, and the stretching modes, the ν1OH and ν3OH of water, in K3PO4 solutions as a function of concentration and comparison with the same modes in pure water. A mode at ∼240 cm−1 (isotropic R spectrum) has been detected and assigned to the restricted translational mode of the strong hydrogen bonds formed between phosphate and water, P–O ··· HOH. In very concentrated K3PO4 solutions (C0 ≥ 3.70 mol/L) and in the hydrate melt, formation of contact ion pairs (CIPs) could be detected. The phosphate in the CIPs shows a symmetry lowering of the Td symmetry to C3v. In the less concentrated solutions, PO43−(aq) solvent separated ion pairs and doubly solvent separated ion pairs exist, while in very dilute solutions fully hydrated ions are present (C0 ≤ 0.005 mol/L). Quantitative Raman measurements have been carried out to follow the hydrolysis of PO43−(aq) over a very broad concentration range. From the hydrolysis data, the pK3 value for H3PO4 has been determined to be 12.45 at 23 °C.