Silica and hybrid silica xerogels with synthesis of vapochromic complexes are used as fiber-optic sensors (FOS) to analyze volatile organic compounds (VOCs). Most of the FOS are based on a ligth intensity change produced by the analyte. The response corresponds to a global change of the absortion spectra and it is explained as a variation in the refraction index that modifies the reflected optical signal in a wawguide i.e. This requires optimizing the sensor geometry and the physico-chemical properties of the xerogel film, with materials designed specifically for each analyte. Silica and hybrid-silica xerogels allows to analyze vapors.The aim of this proposal is to measure the vibrational spectra on Lagrange of different hybrid xerogels using different silica precursors in order to obtain information of the:1)Structure of the ordered domains promoted by the incorporation of the ethyl groups.2)Molecular interaction of volatile organic compounds (VOCs) inside Silica and hybrid silica xerogels.These results will be compared with ab-initio molecular dynamics simulations that will provide time-averaged molecular conformations of the different structures of the ordered domains.