Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046, United States.
J Phys Chem A. 2010 Dec 9;114(48):12613-21. doi: 10.1021/jp105644p. Epub 2010 Nov 15.
Application of polarization transfer techniques such as DEPT and INEPT in (29)Si NMR investigation of bridged silane polymerization requires knowledge of indirect (29)Si-(1)H scalar coupling constants in the silane system. However, the fully coupled (29)Si NMR spectra of these molecules, specifically those containing ethylene bridging groups, are too complicated to measure the coupling constants directly by visual inspection. This is because unlike hydrocarbon systems where one-bond proton-carbon coupling constants exceed other coupling constants by an order of magnitude, in silanes the closest proton-silicon pairs are separated by two bonds and all coupling coefficients (both homonuclear and heteronuclear) are of similar magnitude. In these systems, theoretical tools are required to interpret the spectra of even simple molecules. Here, we determine density functional theory estimates of (29)Si-(1)H scalar coupling constants and use these along with homonuclear coupling constant estimates to resolve the nontrivial nature of these spectra. We also report a Karplus equation consistent with the dihedral angle dependence of the three-bond homo- and heteronuclear coupling in the ethylene bridge. By thermal averaging of DFT coupling constants, a good initial guess of the coupled (29)Si spectral pattern is made, which is easily refined by curve fitting to determine estimates of all coupling constants in the system.
在（29）Si NMR 研究桥联硅烷聚合中，应用极化转移技术（如 DEPT 和 INEPT）需要了解硅烷体系中间接（29）Si-（1）H 标量耦合常数。然而，这些分子的完全耦合（29）Si NMR 谱，特别是那些含有乙烯桥接基团的分子，非常复杂，无法通过目视检查直接测量耦合常数。这是因为与烃系统不同，在烃系统中，一键质子-碳耦合常数比其他耦合常数大一个数量级，而在硅烷中，最近的质子-硅对相隔两个键，并且所有耦合系数（同核和异核）都具有相似的大小。在这些系统中，即使是简单的分子，也需要理论工具来解释其光谱。在这里，我们确定了（29）Si-（1）H 标量耦合常数的密度泛函理论估计值，并使用这些值以及同核耦合常数估计值来解析这些光谱的复杂性质。我们还报告了一个与乙烯桥三键同核和异核耦合的二面角依赖性一致的 Karplus 方程。通过对 DFT 耦合常数进行热平均，可以得到耦合（29）Si 光谱模式的良好初始猜测，然后通过曲线拟合很容易对其进行细化，以确定系统中所有耦合常数的估计值。
