A deuterium labeling, FTIR, and ab initio investigation of the solution-phase thermal reactions of alcohols and alkenes with hydrogen-terminated silicon surfaces

The reactions of alcohols and alkenes with hydrogen-terminated silicon surfaces have been investigated using infrared spectroscopy and deuterium labeling of the reagents and the surface termination. Transmission FTIR spectra were obtained on samples of electrochemically grown porous silicon or mechanically abraded silicon wafers to obtain a sufficient signal-to-noise ratio. The spectral assignments are supported by ab initio calculations on small molecule models at the MP2/6-311++G(d,p) level of theory. A convenient method for the preparation of fully deuterated (D-terminated) silicon wafers is reported; however, fully deuterated porous silicon could not be prepared this way. The spectrum of partially deuterated porous silicon could be assigned on the basis of the computed harmonic vibration frequencies for Q(3)Si-SiH2-SiQ(3) and Q(3)Si-SiHD-SiQ(3) where Q is a pseudo-hydrogen atom with the atomic mass of Si. The reaction of O-deuterated alcohols and water on porous silicon produced Si-D stretching and Si-HD scissor modes in the infrared spectrum. The kinetics were consistent with either a dissociative adsorption or an electrochemical corrosion mechanism for this reaction. However, in all cases a net decrease of Si-H/D species on the surface was observed. The magnitude of this decrease is consistent with hydrogen evolution from a hydridic reflectivity of the surface termination analogous to the formation of SiO2 via hydrolysis of molecular hydrosilanes. The Si-H/D, OxSi-H, and Si-O vibrations could be assigned using small molecule models of the form QOSiH(2)SiQ(3), QOSiH(2)OQ, and (QO)(3)SiH. Significant amounts of silicon alkoxide species are formed even in the presence of water, but the major process in wet solvents is hydrogen evolution and oxide formation. The currently accepted mechanism for the hydrosilylation of alkenes by hydrogen-terminated silicon surfaces involves the attack of a silyl radical on the double bond to produce a Si-C bond and a carbon-centered radical. In principle, this carbon radical may abstract a hydrogen atom from the surface and propagate a chain; however, using deuterated silicon wafers no C-D stretching vibrations could be detected. This indicates that under the conditions employed (1 M alkene solutions in refluxing toluene) the carbon radical abstracts a hydrogen atom from the solvent or another alkene molecule. Ab initio calculations on small molecule models were used to investigate theoretically the shift to low frequency in the Si-H vibrations on the formation of Si-C bonded species at the surface and this effect is attributed to the replacement of Si-H-2 with C-Si-H functionality at the surface.