de Smet Louis C P M, Zuilhof Han, Sudhölter Ernst J R, Lie Lars H, Houlton Andrew, Horrocks Benjamin R
Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands.
J Phys Chem B. 2005 Jun 23;109(24):12020-31. doi: 10.1021/jp044400a.
The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces: a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the beta-carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm(-1) was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the beta-carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm(-1) features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/groups.
通过实验和计算手段研究了1-烯烃与氢终止的多孔硅表面反应在硅上形成Si-C键合单层的机理。我们提出，单层形成过程与在单晶表面发生的过程相同，都是通过自由基链反应进行的：一个甲硅烷基自由基攻击1-烯烃，形成Si-C键以及β-碳原子上的自由基中心。然后，这个碳自由基可以从相邻的Si-H键中提取一个氢原子，从而使链反应得以延续。通过使用高度氘代的多孔硅和傅里叶变换红外光谱（FTIR），通过识别与所提出的链增长步骤中形成的C-D键相关的红外波段，为该机理提供了证据。在48% DF/D2O:EtOD（1:1）电解液中通过恒电流蚀刻形成的氘代多孔硅表面，其表面Si-D位点的密度比在等效H电解液中制备的氢终止多孔硅表面上的Si-H位点密度高30%。1-十一碳烯的热反应以及苯乙烯在氘代表面上的路易斯酸催化反应，都产生了烷基化表面，其C-C和C-H振动特征与在氢终止表面上相应反应中形成的相同。然而，在对氘代表面进行烷基化时，在2100 cm(-1)附近观察到一个宽带。对小分子模型进行的从头算和密度泛函理论计算表明，该波段的积分吸光度与链机理预测的C-D伸缩振动强度相当。计算还表明，β-碳原子上的氢原子与Si(111)-H表面上的氢原子之间存在大量相互作用。因此，这些在2100 cm(-1)处的宽特征被归因于表面D原子参与硅氢化反应所产生的C-D波段，而谱线展宽部分可以通过与相邻表面原子/基团的相互作用来解释。
