Elucidating Solvation Structures of Metal Ions by Infrared Spectroscopy and Quantum Chemical Calculations of Gas-phase Clusters

结合密度泛函理论计算，用红外光解离谱研究了金属离子的溶剂化结构。计算结果表明：1.对于Al/NH_3体系，插入的[H-Al-NH_2]结构比加合的[Al-NH_3]结构的能量要高。然而，由于[H-Al-NH2]中Al原子上的有效电荷较大，增量溶剂化比加合结构更有效地稳定了插入结构。实际上，红外光谱表明，当n[大于或等于]4.2时，[(H-Al-NH_2)(NH_3)_&lt；n-1&gt；]^比[Al-(NH_3)_n]^占优势。在Ag^(H_2O)_3中，第三个H_2O以三配位形式直接与Ag结合，而在Cu^(H_2O)_3中，它占据了二配位铜的第二层中的氢键位置。与在铜中保持二配位的广泛的4s-3D杂化相反，第三H_2O在Ag中以低效的5s-4d杂化方式与Ag相结合。第四个H_2O占据了三配位的银和二配位的铜的第二壳层。当n=5-7时，Cu(H20)n的红外光谱随尺寸的变化表明，双配位结构是进一步溶剂化过程的核心。在Cu^(NH_3)_3中，三个NH_3分子以三配位的形式直接与铜键合。当n=4和5时，Cu^(NH3)n的红外光谱表明存在多个不同配位数(2，3和4)的异构体。但是，随着n的增加，双配位异构体的重要性逐渐增大，直到n=8时才占优势。这些结果表明，与Cu^(H2O)n一样，Cu^有很强的采用双线性配位的倾向。

Solvation structures of metal ions are explored by infrared (IR) photodissociation spectroscopy with the aid of density functional theory calculations. Selected examples of the results are as follows :1. For Al / NH_3 system, the inserted [H-Al-NH_2]^+ structure is calculated to be higher in energy than the adduct [Al-NH_3]^+ structure. However, incremental solvation stabilizes the inserted structure more efficiently than the adduct structure, because of a larger effective charge on the Al atom in [H-Al-NH2]+. Actually, the IR spectra show that the [(H-Al-NH_2)(NH_3)_<n-1>]^+ ions are predominant over [Al-(NH_3)_n]^+ for n【greater than or equal】4.2. The third H_2O attaches directly to Ag+ in a tri-coordinated form in Ag^+(H_2O)_3,while it occupies a hydrogen-bonding site in the second shell of the di-coordinated Cu+ in Cu^+(H_2O)_3. The preference of the tri-coordination is attributable to the inefficient 5s-4d hybridization in Ag+, in contrast to the extensive 4s-3d hybridization in Cu+ which retains the di-coordination. The fourth H_2O occupies the second shells of the tri-coordinated Ag+ and the di-coordinated Cu^+. Size dependent variations in the IR spectra of Cu+(H20)n, for n = 5-7 suggest that the di-coordinated structure acts as the core of further solvation processes.3. In Cu^+(NH_3)_3, three NH_3 molecules are bonded directly to Cu+ in a tri-coordinated form. The IR spectra of Cu^+(NH_3)_n with n = 4 and 5 suggest the coexistence of multiple isomers, which have different coordination numbers (2, 3, and 4). With increasing n, however, the di-coordinated isomer is of growing importance until becoming predominant at n = 8. These results signify a strong tendency of Cu^+ to adopt the twofold linear coordination, as in the case of Cu^+(H_2O)_n.