Institut de Química Computacional and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Spain.
Chemistry. 2009 Dec 7;15(47):13111-23. doi: 10.1002/chem.200901224.
Recently, it was shown that genuine Ng-Ng chemical bonds are present in the endohedral fullerenes Ng(2)@C(60) in the case of Ng=Xe, while it is more debatable whether a chemical bond exist for Ng=Ar and Kr. The lighter homologues with helium and neon are weakly bonded van der Waals complexes. The presence of a noble gas dimer inside the cage is expected to modify the exohedral reactivity of the C(60) cage with respect to that of free C(60). To investigate the impact of encapsulated diatomic noble gas molecules on the chemical reactivity of C(60), we analyzed the thermodynamics and the kinetics of [4+2] Diels-Alder cycloaddition of 1,3-cis-butadiene at all nonequivalent bonds in free C(60), Ng@C(60), and Ng(2)@C(60) (Ng=He, Ne, Ar, Kr, and Xe). Our BP86/TZP calculations reveal that introduction of single noble gas atoms in Ng@C(60) and noble gas dimers He(2) and Ne(2) in Ng(2)@C(60) has almost no effect on the exohedral reactivity compared to free C(60), in agreement with experimental results. In all these cases cycloaddition is clearly favored at the [6,6] bonds in the fullerene cage. For the endohedral compounds He(2)@C(60) and Ne(2)@C(60) a slight preference (by less than 2 kcal mol(-1)) for bonds closer to the C(5) symmetry axis is found. This picture changes dramatically for the endohedral compounds with heavier noble gas dimers. Encapsulation of these noble gas dimers clearly enhances the reaction, both under thermodynamic and kinetic control. Moreover, in the case of Xe(2)@C(60), addition to [6,6] and [5,6] bonds becomes equally viable. These reactivity changes in endohedral fullerenes are attributed to stabilization of the LUMO, increased fullerene strain energy, and greater compression of the encapsulated Ng(2) unit along the He to Xe series.
最近的研究表明，在 Ng(2)@C(60) 富勒烯中存在真正的 Ng-Ng 化学键，其中 Ng=Xe，而 Ng=Ar 和 Kr 是否存在化学键则存在争议。与氦和氖同族的较轻同系物是弱键范德华配合物。预计笼内惰性气体二聚体的存在会改变 C(60)笼相对于游离 C(60)的外生性反应性。为了研究封装的双原子惰性气体分子对 C(60)化学活性的影响，我们分析了 1,3-顺丁二烯在游离 C(60)、Ng@C(60)和 Ng(2)@C(60)(Ng=He、Ne、Ar、Kr 和 Xe)中所有非等价键上的[4+2]Diels-Alder 环加成的热力学和动力学。我们的 BP86/TZP 计算表明，与游离 C(60)相比，在 Ng@C(60)中引入单个惰性气体原子以及在 Ng(2)@C(60)中引入 He(2)和 Ne(2)惰性气体二聚体对其外生性反应性几乎没有影响，这与实验结果一致。在所有这些情况下，环加成明显有利于富勒烯笼中的[6,6]键。对于内包化合物 He(2)@C(60)和 Ne(2)@C(60)，发现靠近 C(5)对称轴的键略有偏好(小于 2 kcal mol(-1))。对于较重惰性气体二聚体的内包化合物，这种情况发生了巨大变化。这些惰性气体二聚体的封装明显增强了反应，无论是在热力学还是动力学控制下。此外，在 Xe(2)@C(60)的情况下，除了[6,6]和[5,6]键之外，加成也变得同样可行。这些内包富勒烯的反应性变化归因于 LUMO 的稳定化、 fullerene 应变能的增加以及沿着 He 到 Xe 系列的 Ng(2)单元的压缩增大。
