The local physical structure of amorphous hydrogenated boron carbide: Insights from magic angle spinning solid-state NMR spectroscopy

摘要

Magic angle spinning solid-state nuclear magnetic resonance spectroscopy techniques are applied to the elucidation of the local physical structure of an intermediate product in the plasma-enhanced chemical vapour deposition of thin-film amorphous hydrogenated boron carbide (B_xC:H_y) from an orthocarborane precursor. Experimental chemical shifts are compared with theoretical shift predictions from ab initio calculations of model molecular compounds to assign atomic chemical environments, while LeeGoldburg cross-polarization and heteronuclear recoupling experiments are used to confirm atomic connectivities. A model for the B_xC:H_y intermediate is proposed wherein the solid is dominated by predominantly hydrogenated carborane icosahedra that are lightly cross-linked via nonhydrogenated intraicosahedral B atoms, either directly through BB bonds or through extraicosahedral hydrocarbon chains. While there is no clear evidence for extraicosahedral B aside from boron oxides, 40% of the C is found to exist as extraicosahedral hydrocarbon species that are intimately bound within the icosahedral network rather than in segregated phases.