Fulton John L, Chen Yongsheng, Heald Steve M, Balasubramanian Mahalingam
Fundamental Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Chem Phys. 2006 Sep 7;125(9):094507. doi: 10.1063/1.2346548.
X-ray absorption fine structure (XAFS) spectroscopy was used to measure the first-shell structure about Ca2+ in high-temperature aqueous solution. XAFS spectra were acquired at the Ca K edge at temperatures up to 400 degrees C and pressures up to 350 bars. For the system at 400 degrees C, both Ca (4038.5 eV) and Cl (2822.4 eV) K-edge data were acquired and a global model was used to fit the two independent sets of XAFS data. Measurements were made at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. Above 250 degrees C, a significant number of Ca2+-Cl- direct contact ion pairs form in agreement with existing thermodynamic data for this system. For a 1 m CaCl2 solution at 400 degrees C, the mean coordination structure about Ca2+ contains 3.2+/-0.6 water molecules at an average Ca-O distance of 2.356+/-0.026 A and 1.8+/-0.7 Cl- at a Ca-Cl distance of 2.677+/-0.007 A. An evaluation of the Ca and Cl preedge and near-edge (x-ray absorption structure) spectra provided further confirmation of the change in the Ca2+ first-shell structure and symmetry. Overall these measurements provide a structural basis for understanding solvation of Ca2+ in hydrothermal systems. These results also provide important new insights into the structural aspects of Ca2+ ion pairing that are the basis of many biological processes under ambient conditions.
X射线吸收精细结构（XAFS）光谱用于测量高温水溶液中Ca2+的第一配位层结构。在高达400摄氏度的温度和高达350巴的压力下，在Ca K边采集XAFS光谱。对于400摄氏度的体系，采集了Ca（4038.5电子伏特）和Cl（2822.4电子伏特）K边数据，并使用全局模型拟合两组独立的XAFS数据。测量是在阿贡国家实验室先进光子源的弯转磁铁束线（20号扇区）进行的。高于250摄氏度时，大量Ca2+-Cl-直接接触离子对形成，这与该体系现有的热力学数据一致。对于400摄氏度下的1 m CaCl2溶液，Ca2+周围的平均配位结构包含3.2±0.6个水分子，平均Ca-O距离为2.356±0.026 Å，以及1.8±0.7个Cl-，Ca-Cl距离为2.677±0.007 Å。对Ca和Cl的预边和近边（X射线吸收结构）光谱的评估进一步证实了Ca2+第一配位层结构和对称性的变化。总体而言，这些测量为理解热液系统中Ca2+的溶剂化提供了结构基础。这些结果还为Ca2+离子配对的结构方面提供了重要新见解，而离子配对是许多环境条件下生物过程的基础。
