Surface [4+2] Cycloaddition Reaction of Thymine on Si(111)7x7 Observed by Scanning Tunneling Microscopy
Surface [4+2] Cycloaddition Reaction of Thymine on Si(111)7x7 Observed by Scanning Tunneling Microscopy
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DOI：
10.1021/jp404576z
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发表时间：
2013-07-18
影响因子：
3.7
通讯作者：
Leung, K. T.
中科院分区：
文献类型：
--
作者：
Chatterjee, A.;Zhang, L.;Leung, K. T.
采用扫描隧道显微镜（STM）和基于密度泛函理论（DFT）的计算方法研究了胸腺嘧啶（Thy）在Si（111）7x7表面吸附的早期阶段。在空态和填充态 STM 图像中均观察到与吸附的胸腺嘧啶分子相对应的明亮突起。空状态图像中的这些明亮突起表现出三种不同程度的强度[低（L）、中（M）和高（H）]。 L 和 M 突出物存在于 7 x 7 晶胞中的吸附原子-重原子对上，表明 Thy 吸附的双齿构型。游离的 Thy 分子被发现通过两步过程与 Si 表面相互作用：Thy 首先经历酮-烯醇互变异构化，形成更稳定的二烯醇互变异构体，然后通过 [4 + 2] 环化反应与 Si 吸附原子-重原子对结合，产生两种不同的副产物。我们的 DFT/B3LYP/6-31++G(d,p) 计算显示了三种可能的环加成产物，其中 1,4-环己二烯副产物比 3,6- 和 2,5-环己二烯更稳定。我们的计算还表明，与表面上已经存在的双齿 Thy 副产物形成氢键合的 Thy 分子是可行的。对 Thy 在 7 x 7 表面上的三种不同曝光的统计分析表明，L 突出物具有最高的相对表面浓度 (80%)，M (16%) 和 H 特征 (496) 明显不太受欢迎。这些结果使我们将 L 和 M 突出分别归因于 1,4- 和 3,6- 环加成产物，其中最不受欢迎的 H 突出分配给与二齿 Thy 氢键连接的 Thy 分子，全部连接到吸附原子-重原子对。 Si表面上[4+2]环加成产物的观察证实了前体二烯醇Thy互变异构体的形成。 Thy 的这种表面介导的两步反应途径对于表面来说是独特的，与主要通过酸或碱催化途径在溶液相中观察到的酮-烯醇互变异构化相反。我们的 STM 研究与我们单独的 XPS 工作相结合，表明这种类型的互变异构化也可以在 Si 表面上观察到，并且这可以引发 Thy 分子与表面的后续环加成反应。
The early stage of thymine (Thy) adsorption on Si(111)7x7 surface is studied with scanning tunneling microscopy (STM) and density functional theory (DFT)-based computational method. Bright protrusions corresponding to the adsorbed thymine molecules are observed in both empty-state and filled-state STM images. These bright protrusions in the empty-state images exhibit three different degrees [lower (L), medium (M), and higher (H)] of intensities. The L and M protrusions are found on the adatom-restatom pairs in the 7 x 7 unit cell, indicating bidentate configurations for Thy adsorption. A free Thy molecule has been found to interact with the Si surface by a two-step process: Thy first undergoes keto-enol tautomerization to form the more stable dienol tautomer, which then binds to the Si adatom-restatom pair via the [4 + 2] cycloadclition reaction, leading to two different adproducts. Our DFT/B3LYP/6-31++G(d,p) calculations show three plausible cycloaddition products, with the 1,4-cydohexadiene adproduct being more stable than 3,6- and 2,5-cydohexadienes. Our calculations also suggest the viability of formation of a Thy molecule hydrogen-bonded with the bidentate Thy adproducts already on the surface. Statistical analysis for three different exposures of Thy on the 7 x 7 surface reveals that the L protrusion has the highest relative surface concentration (80%), with the M (16%) and H features (496) being significantly less popular. These results lead us to attribute the L and M protrusions to the 1,4- and the 3,6-cycloaddition products, respectively, with the least popular H protrusion assigned to a Thy molecule hydrogen-bonded to the bidentate Thy, all attached to an adatom-restatom pair. The observation of [4 + 2] cycloaddition products on the Si surface confirms the formation of precursor dienol Thy tautomer. This surface-mediated two-step reaction pathway for Thy is unique for surfaces, in contrast to keto-enol tautomerization that is mainly observed in the solution phase either by acid- or base-catalyzed pathways. Our STM study coupled with our separate XPS work have demonstrated that this type of tautomerization can also be observed on Si surfaces, and this can initiate the subsequent cycloaddition reactions of Thy molecule with the surface.