Department of Chemistry, Faculty of Science, Benha University, P.O.Box 13518, Benha, Egypt.
J Mol Model. 2011 Dec;17(12):3299-308. doi: 10.1007/s00894-011-1017-4. Epub 2011 Mar 4.
We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg(2+) site of a defect-free surface and at Li(+) and Na(+) sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hund's rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal-support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg(2+) site of the pure surface and at Li(+) and Na(+) sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.
我们通过密度泛函理论计算和嵌入簇模型分析了 Cr、Ni、Mo 和 Pt 沉积在 MgO 晶体上的吸附和自旋态特性。我们考虑了在无缺陷表面的 Mg(2+)位和含杂质表面的 Li(+)和 Na(+)位的沉积。为了避免人为极化效应，我们将具有无边界阴离子的中等大小的簇嵌入到模拟库仑场中，这些库仑场非常接近宿主表面的Madelung 场。过渡金属原子与表面的相互作用是由吸附原子的 Hund 规则和界面处化学键形成之间的竞争决定的。我们发现，阳离子杂质显著增强了金属原子的吸附能，自旋猝灭配合物的低自旋态的吸附能总是比高自旋态的吸附能更有利。自旋极化效应倾向于保持吸附原子相对于孤立原子的自旋态。金属-载体相互作用使吸附金属的低自旋态相对于孤立金属稳定，但这种作用并不总是足以猝灭自旋。对于纯表面的 Mg(2+)位和含杂质表面的 Li(+)和 Na(+)位，Cr 和 Mo 配合物的自旋猝灭发生。自由金属的自旋态特性以及金属配合物的吸附和自旋态特性的变化与前沿轨道的能量有关。静电位能曲线提供了对所研究性质的本质的进一步理解。
