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 (29)Si-(1)H 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 (29)Si 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 (29)Si-(1)H 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.

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In this quantum chemical investigation, NH(3) physisorption onto a model of copper sulfate impregnated silica is compared with pure silica and copper sulfate adsorbents. The physisorption process is modeled as direct binding of the NH(3) molecule to the adsorption site of the dry adsorbents and as displacement of a H(2)O molecule by NH(3) in the hydrated complexes. The surface of silica is represented by a hydroxyl group attached to a silsesquioxane cage, H(7)Si(8)O(12)(OH) and silica impregnated with CuSO(4) by the most stable configuration of the cluster containing a CuSO(4) ion pair placed adjacent to the silica cage. H(2)O is systematically added to the dehydrated adsorbents to investigate the role of water in NH(3) adsorption. Modeling hydrated environments of each type of adsorbent is focused on H(2)O molecules that directly coordinate with the active sites. The results indicate that the binding energy of adsorbing NH(3) onto the mixed adsorbent is greater than in pure silica. This enhanced binding in the mixed adsorbent is consistent with improved Brønsted acidity of the silanol in the presence of CuSO(4).

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.