GIPAW (gauge including projected augmented wave) and local dynamics in C-13 and Si-29 solid state NMR: the study case of silsesquioxanes (RSiO1.5)(8)

Octameric silsesquioxanes (RSiO1.5)(8) are versatile and interesting nano building blocks, suitable for the synthesis of nanocomposites with controlled porosity. In this paper, we revisit the Si-29 and C-13 solid state NMR spectroscopy for this class of materials, by using GIPAW (gauge including projected augmented wave) first principles calculations [Pickard & Mauri, Phys. Rev. B, 2001, 63, 245101]. Full tensorial data, including the chemical shift anisotropies (CSA) and the absolute orientation of the corresponding principal axes systems (PAS), were calculated. Subsequent averaging of the calculated tensors (due to fast reorientation of the R groups around the Si-C bonds) allowed for the interpretation of the strong reduction of CSA and dipolar couplings for these derivatives. Good agreement was observed between the averaged calculated data and the experimental parameters. Interesting questions related to the interplay between X-ray crystallography and solid state NMR are raised and will be emphasized.