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Structure Determination of Phosphoric Acid and Phosphate Ions in Aqueous Solution Using EXAFS Spectroscopy and Large Angle X-ray Scattering
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2018-08-29 00:00:00 , DOI: 10.1021/acs.jpca.8b05641 Ingmar Persson 1 , Mylène Trublet 2 , Wantana Klysubun 3
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2018-08-29 00:00:00 , DOI: 10.1021/acs.jpca.8b05641 Ingmar Persson 1 , Mylène Trublet 2 , Wantana Klysubun 3
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The structures of hydrated phosphoric acid and phosphate ions (H2PO4–, HPO42–, and PO43–) in aqueous solution have been determined by P K-edge EXAFS and large angle X-ray scattering (LAXS). The P–O bond distance in all phosphate species studied is close to 1.53 Å. The P–(O)···Oaq distances have been refined to ca. 3.6 Å from the LAXS data giving a P–O···Oaq bond angle close to tetrahedral, suggesting that each oxygen or OH group of phosphoric acid and dihydrogen phosphate, on average, hydrogen bind three water molecules. The (P−)O(−H)···Oaq and (P−)O···(H−)Oaq hydrogen bonds in hydrated phosphoric acid and the H2PO4– ion are shorter than the hydrogen bonds in neat water. This supports previous infrared spectroscopic studies claiming that the hydrogen bonds in hydrated phosphoric acid and phosphate ions are stronger than the hydrogen bonds in neat water. Phosphoric acid and phosphate ions can therefore be regarded as structure making solutes. This is the first study applying transmission mode X-ray absorption spectroscopy (XAS) data collection on the P K-edge. It shows that XAS spectra collected in transmission mode have a much better S/N ratio than data collected in fluorescence mode, allowing accurate determination of P–O bond distances. Furthermore, P K-edge EXAFS data collected in fluorescence mode display a higher amplitude at high k than expected due to increasing radiated volume of the sample with increasing energy as the total absorption decreases sharply with increasing energy of the X-rays. As a result, the fluorescence signal becomes nonproportional to the intensity of the X-ray beam over the EXAFS spectrum. This results in an increasing amplitude of the EXAFS function with increasing energy of the X-ray beam resulting in too small Debye–Waller coefficients.
中文翻译：
EXAFS光谱和大角度X射线散射测定水溶液中的磷酸和磷酸根离子的结构
水合磷酸和磷酸根离子的（H结构2 PO 4 -，HPO 4 2-和PO 4 3-）的水溶液已被P K边缘EXAFS和大角度X-射线散射（LAXS）来测定。在所研究的所有磷酸盐种类中，P–O键的距离均接近1.53Å。P–（O）··· aq距离已精炼为大约。来自LAXS数据的3.6Å给出了一个接近四面体的P–O··O aq键角，表明磷酸和磷酸二氢的每个氧或OH基团平均而言都与三个水分子结合。（P-）O（-H）··o aq和（P-）O···（H-）O· aq水合磷酸中的氢键和H 2 PO 4 –离子比纯净水中的氢键短。这支持了以前的红外光谱研究，声称水合磷酸和磷酸根离子中的氢键比纯净水中的氢键强。因此，磷酸和磷酸根离子可以被认为是构成结构的溶质。这是首次在P K边缘上应用透射模式X射线吸收光谱（XAS）数据收集的研究。结果表明，透射模式下收集的XAS光谱的信噪比比荧光模式下收集的数据好得多，从而可以精确确定P–O键的距离。此外，以荧光模式收集的P K-edge EXAFS数据在高k下显示出更高的幅度由于总吸收随X射线能量的增加而急剧下降，因此随着能量的增加，样品的辐射体积也随之增加。结果，荧光信号变得与EXAFS光谱上的X射线束强度不成比例。这会导致EXAFS函数的振幅增加，而X射线束的能量也会增加，从而导致Debye-Waller系数太小。
更新日期：2018-08-29
中文翻译：
EXAFS光谱和大角度X射线散射测定水溶液中的磷酸和磷酸根离子的结构
水合磷酸和磷酸根离子的（H结构2 PO 4 -，HPO 4 2-和PO 4 3-）的水溶液已被P K边缘EXAFS和大角度X-射线散射（LAXS）来测定。在所研究的所有磷酸盐种类中，P–O键的距离均接近1.53Å。P–（O）··· aq距离已精炼为大约。来自LAXS数据的3.6Å给出了一个接近四面体的P–O··O aq键角，表明磷酸和磷酸二氢的每个氧或OH基团平均而言都与三个水分子结合。（P-）O（-H）··o aq和（P-）O···（H-）O· aq水合磷酸中的氢键和H 2 PO 4 –离子比纯净水中的氢键短。这支持了以前的红外光谱研究，声称水合磷酸和磷酸根离子中的氢键比纯净水中的氢键强。因此，磷酸和磷酸根离子可以被认为是构成结构的溶质。这是首次在P K边缘上应用透射模式X射线吸收光谱（XAS）数据收集的研究。结果表明，透射模式下收集的XAS光谱的信噪比比荧光模式下收集的数据好得多，从而可以精确确定P–O键的距离。此外，以荧光模式收集的P K-edge EXAFS数据在高k下显示出更高的幅度由于总吸收随X射线能量的增加而急剧下降，因此随着能量的增加，样品的辐射体积也随之增加。结果，荧光信号变得与EXAFS光谱上的X射线束强度不成比例。这会导致EXAFS函数的振幅增加，而X射线束的能量也会增加，从而导致Debye-Waller系数太小。
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