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首页> 外文期刊>RSC Advances >Experimental and computational exploration of the UV-visible properties of hexaniobate and hexatantalate ions
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Experimental and computational exploration of the UV-visible properties of hexaniobate and hexatantalate ions

机译：六铌酸根和六钽酸根离子的紫外-可见性质的实验和计算探索

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The UV-visible properties of hexaniobate (HxNb6O19x-8) and hexatantalate (HxTa6O19x-8) ions were investigated experimentally and by DFT calculations. Due to the huge discrepancies among the reported values found in prior studies, the extinction coefficients of hexaniobate ions were determined from multiple samples in various media. A simple and low-cost method was then developed for the determination of the niobium content of both synthetic and industrial samples. Taking advantage of the UV spectra of the hexaniobate ions, the first protonation constant of Nb6O198- could be determined experimentally (pK(a) = 13.4 +/- 0.1 at 25 degrees C, Delta H-r = -95 kJ mol(-1) in 3 M KOH/KCl) and is in accordance with the reported values previously extrapolated from potentiometric measurements (pK(a) = 13.3 +/- 0.6). UV batch titrations performed at different ionic strengths suggest that the equilibrium between the monoprotonated and the deprotonated forms of the Lindqvist ion is accompanied by an exchange of potassium ions and that the cluster is easier to deprotonate in Na+ media. This study highlights the importance of the ion-pairing on the chemistry of these polyoxometalates. The tremendous difference between the UV spectrum of Nb6O198- and Ta6O198- was investigated by DFT computations. The UV spectra were reproduced and show a good agreement with the experimental data. Moreover this study revealed some insights into the evolution of the spectra together with the pH of the medium by studying the nature of the transitions involved in this spectral domain.
机译：实验研究了六铌酸盐（HxNb6O19x-8）和六钽酸盐（HxTa6O19x-8）离子的紫外可见性质。由于在先前研究中发现的报告值之间存在巨大差异，因此从各种介质中的多个样品中确定了己酸根离子的消光系数。然后开发了一种简单而低成本的方法来测定合成样品和工业样品中的铌含量。利用六铌酸盐离子的紫外光谱，可以通过实验确定Nb6O198-的第一质子化常数（pK（a）= 13.4 +/- 0.1在25°C，Delta Hr = -95 kJ mol（-1） 3 M KOH / KCl），并且与先前从电位测量法推断的报告值一致（pK（a）= 13.3 +/- 0.6）。在不同离子强度下进行的紫外分批滴定表明，Lindqvist离子的单质子化和去质子化形式之间的平衡伴随着钾离子的交换，并且簇更易于在Na +介质中去质子化。这项研究强调了离子对在这些多金属氧酸盐的化学反应中的重要性。通过DFT计算研究了Nb6O198-和Ta6O198-的紫外光谱之间的巨大差异。紫外光谱被复制并显示出与实验数据良好的一致性。此外，这项研究通过研究此光谱域中涉及的跃迁的性质，揭示了对光谱演化以及介质pH值的一些见解。

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