Daul Claude, Niketic Svetozar, Rauzy Cédrick, Schläpfer Carl-Wilhelm
Département de Chimie, Université de Fribourg-Pérolles, 1700 Fribourg, Switzerland.
Chemistry. 2004 Feb 6;10(3):721-7. doi: 10.1002/chem.200305065.
The interconversion mechanisms between three idealized polytopal forms (a square pyramid and two trigonal bipyramids) of [M(bidentate)(2)(unidentate)] were investigated by an original combination of molecular mechanics (MM) and density functional theory (DFT) approaches. MM was used to model the mechanistic rearrangement path, and DFT to study selected points along this path. The test case was a five-coordinate [Ni(acac)(2)(py)] species. In the case of [Ni(acac)(2)(py)] it was confirmed (both by MM and by DFT) that the three polytopal forms do indeed represent shallow local minima, of which the square pyramid (SQP) is more stable than the other two. Small energy barriers that separate the three minima prevent spontaneous rearrangement among the polytopal forms in geometry-optimization simulations. The driving force for MM simulation of the polytopal rearrangements was supplied through the L-M-L angle bending terms. MM results for relative energies and geometries are fully supported by DFT. Finally, the implication of the present results to explain some racemization mechanisms of octahedral complexes (namely, the intramolecular bond rupture of tris(chelate) species, and intermolecular dissociation of bis(bidentate) species) is briefly discussed.
通过分子力学（MM）和密度泛函理论（DFT）方法的原始组合，研究了[M（双齿）(2)（单齿）]的三种理想化多面体形式（一个方锥和两个三角双锥）之间的相互转化机制。MM用于模拟机理重排路径，DFT用于研究沿此路径的选定点。测试案例是一个五配位的[Ni(acac)(2)(py)]物种。对于[Ni(acac)(2)(py)]，（通过MM和DFT）证实这三种多面体形式确实代表浅局部极小值，其中方锥（SQP）比其他两种更稳定。分隔这三个极小值的小能垒阻止了几何优化模拟中多面体形式之间的自发重排。多面体重排的MM模拟驱动力通过L-M-L角弯曲项提供。DFT完全支持MM关于相对能量和几何结构的结果。最后，简要讨论了本结果对解释八面体配合物的一些外消旋化机制（即三（螯合物）物种的分子内键断裂和双（双齿）物种的分子间解离）的意义。
