Hydrogen-silicon bonds, analysis

Porous silicon (por-Si) formed by electrochemical etching of (llO)-oriented p-type Si wafers is an example of novel nanostructured medium with controllable optical properties. It was found to exhibit the strong in-plane birefringence (up to 18 %) and free-carrier dichroism [1-5]. Both phenomena originate from the form anisotropy of Si nanocrystals and voids assembling the material [3-5]. Below, we report the analysis of the dichroism in por-Si on the basis of the generalized effective-medium approximation (EMA) [7] as well as prominent anisotropy of absorption by silicon-hydrogen surface bonds. 

The free iodine can readily he washed out, but CaCl2 can be removed only incompletely because all the solvents for this substance react with silicon chloride. The reaction will take place quantitatively only if the starting CaSi2 has been finely ground under inert gas or in a solvent. Grinding in air leads to partial oxidation and the reaction remains incomplete. Silicon monochloride prepared in this way is a yellow scaly product whose analysis indicates the exact 1 1 silicon to chlorine ratio. The quantity of hydrogen liberated in alkaline medium is also consistent with the three Si-Si bonds per silicon atom required for the layer structure. 

The negative pr and positive pi values obtained from the analysis suggest that the reactivity of the Si=C bond in 1-methylsilene (2a), the base compound in the analysis, is enhanced by substitution of the silicon hydrogen with resonance electron donors or... 

While complete X-ray analysis will establish the structure in the solid state, it is useful to have NMR data on the solution state that illustrate the increase of the coordination number of silicon. It would seem that NMR spectroscopy of nuclei participating directly in donor-acceptor interaction is especially important in investigating silicon compounds with an expanded coordination sphere. This requires the use of Si NMR spectroscopy since the electron shell of the silicon atom, the bond angles and lenghts are strongly affected upon complexation. Valuable information could also be obtained with by " N, N, 0, F NMR data since these elements act as donors. Chemical shifts of nuclei other than hydrogen are determined by various factors and not yet understood well anough to provide easily applied correlations of other physical properties of the molecules. 

---