The 15N chemical shifts in mixed NB2Si and NBSi2 environments of Si3B3N7-A theoretical investigation

Nuclear magnetic resonance (NMR) chemical shifts in solids may be calculated by ab initio methods approximating the solid state by molecular clusters. We employed this technique to obtain estimates of 15N chemical shifts in NB2Si and NBSi2 environments in the solid state. Such nitrogen environments are found in amorphous (Si/B/N-)ceramics which exhibit very interesting features such as high thermal and mechanical stability. We based our calculations on cutouts of hypothetical Si3B3N7 crystals suggested by Kroll and Hoffmann [Silicon boron nitrides: hypothetical polymorphs of Si3B3N7, Angew. Chem. Int. Ed. 37 (1998) 2527]. Taking the systematic errors of our calculations into account we expect the chemical shifts in NBSi2 environments around -293±5ppm. Chemical shifts in NB2Si environments are expected at -272±6ppm. The range of the calculated chemical shifts in NBSi2 environments coincides with experimental chemical shifts in molecular compounds. Experimental chemical shifts of NB2Si nitrogen in molecules appear at lower field than our calculated chemical shifts in the solid state.