(596d) DFT Calculation of 29Si-1H Indirect Spin-Spin Coupling Constants In Organoalkoxysilanes

To study the reactions of organoalkoxysilanes and their hydrolyzed products, polarization transfer NMR experiments such as Distortionless Enhancement by Polarization Transfer (DEPT) are frequently employed for signal enhancement and spectral editing. However, even seemingly simple precursors such as bis(triethoxysilyl)ethane show complex non-first-order coupling patterns that cannot be interpreted without the aid of first-principles calculations. Here, we discuss the development of methodology for predicting indirect spin-spin coupling constants in organoalkoxysilanes. The performance of four basis sets: 6-31G, 6-311+G(2d,p), cc-PVTZ and IGLO-III is compared regarding the estimation of ²⁹Si-¹H spin-spin coupling constants in methyltriethoxysilane. DFT calculations with the 6-31G basis set and B3LYP functional are found to offer good agreement with the experimental coupling constants. This model chemistry is used to calculate the ²⁹Si-¹H coupling constants of MexSi(OR)y(OH)4-x-y (OR=OMe, OEt) series of silanes. It is observed that substituting alkoxy groups with hydroxyl groups has no effect on the two bond ²⁹Si coupling to ¹H in non-hydrolyzable organic group. Hence, the fully hydrolyzed version of the corresponding monomers can be used to estimate these values. This reduces the computational cost without reducing the accuracy of the results. Based on this approach, ²⁹Si-¹H scalar coupling constants in bridged alkoxysilane of interest for kinetic studes are discussed: bis(trimethoxysilyl)ethane, bis(triethoxysilyl)methane, bis(triethoxysilyl)ethane and bis(triethoxysilyl)ethene. The fully coupled ²⁹Si NMR spectral patterns of these molecules, specifically those with ethylene bridging groups, are too complex to directly measure the coupling constants based on peak splitting. We estimated these values with the aid of NMR spin system simulations and the scalar coupling constants from DFT calculations. Coupling constants of the trans conformation provide consistent predictions with the selectively decoupled ²⁹Si spectra, and are unaffected by variations of temperature and solvent.