Lee Han Myoung, Diefenbach Martin, Suh Seung Bum, Tarakeshwar P, Kim Kwang S
National Creative Research Initiative Center for Superfunctional Materials, Department of Chemistry, Division of Molecular and Life Sciences, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea.
J Chem Phys. 2005 Aug 15;123(7):074328. doi: 10.1063/1.2000256.
Using molecular-orbital analysis, we have elucidated the quantum-chemical origin of the intriguing phenomena in sequential hydration energies of the gold cation, which is known to be the most conspicuous among all transition metals. The hydration energy of Au+ with the second water molecule is found to be much larger than that with the first water molecule. Owing to the large relativistic effect of gold (i.e., significant lowering of the 6s orbital energy and significant raising of the 5d orbital energy), the highest occupied molecular orbital of the hydrated gold cation has a large portion of the 6s orbital. As the electron density of the 6s orbital populates in a large outer spherical shell far off the gold nucleus, the p orbitals (or sp hybridized lone-pair orbitals) of the water molecules are able to overlap with the outer part of the 6s orbital in the dihydrated gold cation, resulting in the unusual skewed overlap of p-6s-p orbitals (not the atom-to-atom bond overlap). No previous molecular-orbital analysis has reported this peculiar skewed orbitals overlap. Since this skewed orbitals overlap is saturated with two water molecules, this property is responsible for the low coordination number of the gold ion.
通过分子轨道分析，我们阐明了金阳离子连续水合能中有趣现象的量子化学起源，金阳离子在所有过渡金属中最为显著。发现Au⁺与第二个水分子的水合能远大于与第一个水分子的水合能。由于金的相对论效应较大（即6s轨道能量显著降低，5d轨道能量显著升高），水合金阳离子的最高占据分子轨道有很大一部分是6s轨道。由于6s轨道的电子密度分布在远离金原子核的大外球壳中，水分子的p轨道（或sp杂化孤对轨道）能够与二水合金阳离子中6s轨道的外部重叠，导致p - 6s - p轨道出现异常的倾斜重叠（而非原子间键的重叠）。此前没有分子轨道分析报道过这种特殊的倾斜轨道重叠。由于这种倾斜轨道重叠在两个水分子时就达到饱和，这种性质导致了金离子的低配位数。
