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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Fifth stereoactive orbital on silicon: Relaxation of the lowest singlet excited state of octamethyltrisilane
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Fifth stereoactive orbital on silicon: Relaxation of the lowest singlet excited state of octamethyltrisilane

机译：硅上的第五个立体活性轨道：八甲基三硅烷的最低单重态激发态的弛豫

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摘要

We address relaxation pathways in the excited singlet states S _1 of saturated molecules, specifically alkylated oligosilanes. Unlike their longer peralkylated homologues, disilanes and trisilanes do not fluoresce even at low temperatures. An examination of the S _1 potential energy surface of Si _3Me _8 with density functional (TDDFT, LC-TDDFT), and ab initio (RICC2, RIADC(2)) methods with TZVP basis sets revealed only extremely shallow minima in the vicinity of funnels, accounting for the absence of fluorescence, rapid internal conversion, and photoproducts. Relaxed singlet excited state structures either contain one approximately trigonal bipyramidal Si atom or two that are halfway between tetrahedral and trigonal bipyramidal. Four of the ligands are those that the Si atom had in the ground state. Natural bond orbital analysis suggests that the fifth one is a nonbonding hybrid orbital of the lone-pair type and size intermediate between valence and Rydberg, with an only very small occupancy, yet stereochemically active. The fifth natural hybrid orbital is composed primarily of 4s, 4p, and usually to a lesser degree, also 3d atomic orbitals. The trigonal bipyramidal structure allows an optimal accommodation of the presence of both a negative and a positive charge in the Lewis structures. The excess negative charge on the distorted Si atom is shared between the nonbonding fifth hybrid orbital and σ* antibonding orbitals associated with its bonds. The positive charge resides in an adjacent σ SiSi bond orbital. A Rydberg minimum also occurs on the S 1 surface at the geometry of the radical cation.
机译：我们讨论了饱和分子（特别是烷基化低聚硅烷）的激发单重态S _1中的弛豫途径。与它们较长的全烷基化同系物不同，乙硅烷和丙硅烷即使在低温下也不会发出荧光。用TZVP基集对密度函数（TDDFT，LC-TDDFT）和从头算（RICC2，RIADC（2））方法对Si _3Me _8的S _1势能面的研究表明，在漏斗附近只有极浅的极小值，说明没有荧光，内部快速转换和光产物。弛豫的单重态激发态结构要么包含一个大约为三角形的双锥体硅原子，要么为两个位于四面体和三角形的双锥体中间的Si原子。四个配体是Si原子在基态下具有的那些。自然键轨道分析表明，第五个是价键和里德堡之间的孤对类型和大小中间的非键杂合轨道，仅具有很小的占有率，但具有立体化学活性。第五自然杂化轨道主要由4s，4p组成，通常程度也较小，还包括3d原子轨道。三角双锥体结构允许在Lewis结构中最佳地容纳负电荷和正电荷的同时存在。变形的Si原子上的过量负电荷在非键合的第五杂化轨道和与其键相关的σ*反键合轨道之间共享。正电荷位于相邻的σSiSi键轨道中。里德伯格极小值也出现在自由基阳离子的几何形状的S 1表面上。

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