Disiloxanes, structural parameters

Since 1990 the ab initio calculation of NMR chemical shift parameters has been established as a reliable tool for structure elucidation of various types of compounds [1]. Based on a set of model disiloxanes, the predictive powers of various computational levels (HF, DFT, MP2) [2] for Si NMR chemical shift calculations for linear, cyclic and cage-like polysiloxanes are compared. The dependence of the calculated NMR parameters on structural changes is discussed. Finally, the performance of our theoretical approach is tested using a series of 25 siloxanes and silanes. 

The prediction of NMR chemical shifts depends crucially on the quality of the optimized structure and on the precision of the subsequent NMR calculation. Therefore various computational methods in combination with different basis sets were tested for their ability to predict the important geometrical parameters like the Si-O-Si angle and the Si-O bond length. After a full geometry optimization, four theoretical levels were used to calculate the Si NMR shift. Disiloxane, hexafluorodisiloxane, hexachlorodisiloxane and hexamethyldisiloxane were used as model compounds (Fig. 3). 

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