Ab initio Simulation of the Grafting of Phenylacetylene on Hydrogenated Surfaces of Crystalline Silicon Catalyzed by a Lewis Acid

Car−Parrinello simulations have been carried out to identify the grafting mechanism of phenylacetylene, a
prototypical alkyne, on the hydrogenated surfaces of crystalline silicon, catalyzed by a Lewis acid (AlCl3).
To this purpose, we have made use of a new technique, metadynamics, devised recently to deal with complex
chemical reactions in first principles simulations. The reaction mechanism, leading to a styrenyl-terminated
surface, turns out to be equivalent to the corresponding gas-phase hydrosilylation reaction by silanes that we
have identified in a previous work. The activation energies for the surface reactions (0.43, 0.42, 0.35 eV, for
H−Si(111), H−Si(100)2 × 1, and H−Si(100)1 × 1, respectively) are very close to that of the corresponding
gas-phase reaction (0.37 eV). The estimated activation free energy at room temperature is sufficiently low
for the grafting reaction to be viable at normal conditions and at low coverage on the crystalline silicon
surfaces, as already well documented to occur on the surface of porous silicon. However, the conformation
of the transition state shadows a large area of the surface, which might contribute to making the grafting
process self-limiting.

本研究通过卡-帕里内罗模拟（Car-Parrinello simulation），旨在揭示苯乙炔——一类典型炔烃——在路易斯酸（Lewis acid，AlCl₃）催化下，于氢化结晶硅表面的接枝机理。为此，我们采用了近年来提出的一种适用于第一性原理模拟中复杂化学反应研究的新技术——元动力学（metadynamics）。研究发现，生成苯乙烯基封端表面的反应机理，与我们此前工作中确定的硅烷参与的气相氢化硅烷化反应完全一致。针对不同氢化硅表面的反应活化能分别为：H−Si(111)对应0.43 eV、H−Si(100)2 × 1对应0.42 eV、H−Si(100)1 × 1对应0.35 eV，与对应气相反应的活化能（0.37 eV）极为接近。室温下估算得到的活化自由能足够低，表明该接枝反应在常规条件及结晶硅表面低覆盖度下可顺利进行，这与多孔硅表面已被广泛证实的接枝现象相符。不过，过渡态的构象会遮蔽大面积表面区域，这可能是导致接枝过程呈现自限性的重要原因。