Vibrational spectroscopy of periodic mesoporous organosilicas (PMOs) and their precursors:: A closer look

The vibrational (Raman and IR) spectra of four different periodic mesoporous organosilica (PMO) materials and the corresponding bis-silylated precursors were investigated. An assignment of the most prominent bands was possible with the help of supporting and complimentary theoretical DFT calculations. The hitherto widespread belief that vibrational spectroscopy is an appropriate tool to examine the status of the silicon-carbon bond in organosilicon/organosilica compounds is revisited and finally discarded. A thorough analysis of the spectra revealed that (i) v(Si-C) stretching vibrations are neither eigenmodes of the precursors nor of the PMO materials, (ii) any other vibration involving the Si-C fragment is not necessarily a characteristic group frequency, (iii) there is no simple analogy between different PMO materials with different organic bridges (each material is a single case), and (iv) that, as a consequence, vibrational spectroscopy is not a reliable method for monitoring the integrity of the Si-C bonds in PMOs and other related organosilica materials (xerogels and aerogels). Carrying out solid-state NMR spectroscopy is an absolute necessity in order to give valid statements regarding the extent of Si-C bond cleavage, which occurred during the synthesis of organosilica materials.