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

利用密度泛函理论计算的方法，通过红外光解光谱研究了金属离子的溶剂化结构。所选结果的例子如下：1.对于Al / NH_3体系，插入[H-Al-NH_2]^+结构的能量比加合[Al-NH_3]^+结构的能量高。然而，增量溶剂化稳定的插入结构比加合物结构更有效，因为在[H-Al-NH 2]+的Al原子上的更大的有效电荷。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）_<n-1>]^+离子比[Al-（NH_3）_n]^+离子占优势。在Ag^+（H_2O）_3中，第三个H_2O以三配位的形式直接与Ag+结合，而在Cu^+（H_2O）_3中，第三个H_2O占据双配位的Cu+的第二个壳层上的氢键位置。三配位的偏好归因于Ag+中的低效的5s-4d杂化，而Cu+中的广泛的4s-3d杂化保留了双配位。第四个H_2O占据三配位Ag+和双配位Cu^+的第二层。Cu+（H2O）n（n = 5-7）的红外光谱随分子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，3个NH_3分子以三配位的形式直接与Cu+键合。Cu^+（NH_3）_n（n = 4和5）的红外光谱表明，Cu ^+（NH_3）_n是由多个配位数不同（2，3，4）的异构体共存。然而，随着n的增加，二配位异构体的重要性越来越大，直到n = 8时才占主导地位。这些结果表明，Cu^+具有与Cu^+（H_2O）_n一样的双重线性配位的强烈倾向。

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.