The primary particles of silica aerogels resulting from three different mixtures of precursors - 50% tetra-methylorthosilicate (TMOS)/50% vinyltrimethoxysilane (VMTS); 50% tetraethylorthosilicate (TEOS)/50% aminopropyltriethoxysilane and 50% TEOS/50% glycidylpropoxi - as well as aerogels of pure TMOS and VTMS, have been studied by quantum mechanics density functional theory calculations (DFT), at the B3LYP/6-311 + G(d,p) level of theory). Thermo-chemical calculations have indicated that cage structures are the most favored for all materials, followed by other species (linear and cyclic) with high degrees of condensation. A vibration mode analysis based on a numerical model fitted to experimental infrared spectra, has allowed the identification of the most representative silica clusters for all the materials studied. The resulting theoretical spectra were close to their experimental counterparts. Analysis of calculated and experimental Si-29-NMR spectral data generally corroborated the derived model.