Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands.
Langmuir. 2011 Feb 1;27(3):972-80. doi: 10.1021/la104705b. Epub 2011 Jan 5.
On H-Si(111) surfaces monolayer formation with 1-alkenes results in alkyl monolayers with a Si-C-C linkage, while 1-alkynes yield alkenyl monolayers with a Si-C═C linkage. Recently, considerable structural differences between both types of monolayers were observed, including an increased thickness, improved packing, and higher surface coverage for the alkenyl monolayers. The precise origin thereof could experimentally not be clarified yet. Therefore, octadecyl and octadecenyl monolayers on Si(111) were studied in detail by molecular modeling via PCFF molecular mechanics calculations on periodically repeated slabs of modified surfaces. After energy minimization the packing energies, structural properties, close contacts, and deformations of the Si surfaces of monolayers structures with various substitution percentages and substitution patterns were analyzed. For the octadecyl monolayers all data pointed to a substitution percentage close to 50-55%, which is due the size of the CH(2) groups near the Si surface. This agrees with literature and the experimentally determined coverage of octadecyl monolayers. For the octadecenyl monolayers the minimum in packing energy per chain is calculated around 60% coverage, i.e., close to the experimentally observed value of 65% [Scheres et al. Langmuir 2010, 26, 4790], and this packing energy is less dependent on the substitution percentage than calculated for alkyl layers. Analysis of the chain conformations, close contacts, and Si surface deformation clarifies this, since even at coverages above 60% a relatively low number of close contacts and a negligible deformation of the Si was observed. In order to evaluate the thermodynamic feasibility of the monolayer structures, we estimated the binding energies of 1-alkenes and 1-alkynes to the hydrogen-terminated Si surface at a range of surface coverages by composite high-quality G3 calculations and determined the total energy of monolayer formation by adding the packing energies and the binding energies. It was shown that due to the significantly larger reaction exothermicity of the 1-alkynes, thermodynamically even a substitution percentage as high as 75% is possible for octadecenyl chains. However, because sterically (based on the van der Waals footprint) a coverage of 69% is the maximum for alkyl and alkenyl monolayers, the optimal substitution percentage of octadecenyl monolayers will be presumably close to this latter value, and the experimentally observed 65% is likely close to what is experimentally maximally obtainable with alkenyl monolayers.
在 H-Si(111)表面上，1-烯烃的单层形成导致具有 Si-C-C 键的烷基单层，而 1-炔烃则生成具有 Si-C═C 键的烯基单层。最近，观察到这两种类型的单层之间存在相当大的结构差异，包括增加的厚度、改善的堆积和更高的烯基单层的表面覆盖率。其确切的起源尚未在实验中得到澄清。因此，通过周期性重复修改后的表面的薄片的 PCFF 分子力学计算，详细研究了 Si(111)上的十八烷基和十八烯基单层。在能量最小化后，分析了不同取代百分比和取代模式的单层结构的 Si 表面的堆积能、结构性质、紧密接触和变形。对于十八烷基单层，所有数据都指向接近 50-55%的取代百分比，这是由于 Si 表面附近的 CH(2)基团的大小。这与文献和实验确定的十八烷基单层覆盖率一致。对于十八烯基单层，每条链的堆积能最小值计算为 60%左右的覆盖率，即接近实验观察到的 65%[Scheres 等人。朗缪尔 2010 年，26，4790]，并且与计算的烷基层相比，这种堆积能对取代百分比的依赖性较小。链构象、紧密接触和 Si 表面变形的分析澄清了这一点，因为即使在覆盖率超过 60%的情况下，也观察到相对较少的紧密接触和 Si 的可忽略的变形。为了评估单层结构的热力学可行性，我们通过复合高质量 G3 计算估算了一系列表面覆盖率下 1-烯烃和 1-炔烃与氢终止 Si 表面的结合能，并通过添加堆积能和结合能来确定单层形成的总能量。结果表明，由于 1-炔烃的反应放热性明显较大，即使对于十八烯基链，理论上也可以达到高达 75%的取代百分比。然而，由于空间位阻（基于范德华足迹）烷基和烯基单层的最大覆盖率为 69%，因此十八烯基单层的最佳取代百分比可能接近后一个值，并且实验观察到的 65%可能接近实验中用烯基单层获得的最大值。
