DFT calculations of indirect ²⁹Si-¹H spin-spin coupling constants in organoalkoxysilanes

The performance of four basis sets (6-311+G(2d,p), IGLO-III, cc-PVTZ, and 6-31G) is evaluated in order to find a quantum mechanical technique that can be used to accurately estimate ²⁹Si-¹H spin-spin coupling constants in organoalkoxysilanes. The 6-31G basis set with the B3LYP functional is found to be an accurate, efficient, and cost-effective density functional theory method for predicting spin-spin coupling constants of organoalkoxysilanes. Knowledge of these scalar coupling constants and their dependence on structural variations is important to be able to fine-tune NMR experiments that rely on polarization transfer among nuclei, such as ²⁹Si distortionless enhancement by polarization transfer (DEPT). The effects of size and the number of unhydrolyzable alkyl groups attached to silicon and the effects of substitution of alkoxy groups with hydroxyl groups on ²⁹Si-¹H spin-spin coupling constants are investigated using this DFT method. The results show that the predicted scalar coupling between silicon and organic groups depends weakly on the degree of hydrolysis of the alkoxysilanes. The effectiveness of this method is also illustrated for the determination of spin-spin coupling constants in a species containing a siloxane bond.