Determination of ²⁹Si-¹H spin-spin coupling constants in organoalkoxysilanes with nontrivial scalar coupling patterns

Application of polarization transfer techniques such as DEPT and INEPT in ²⁹Si NMR investigation of bridged silane polymerization requires knowledge of indirect ²⁹Si-¹H scalar coupling constants in the silane system. However, the fully coupled ²⁹Si 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 ²⁹Si-¹H 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 ²⁹Si spectral pattern is made, which is easily refined by curve fitting to determine estimates of all coupling constants in the system.