15N NMR Chemical Shift Tensors of Substituted Hexaazaisowurtzitanes: The Intermediates in the Synthesis of CL-20

Several cage-substituted hexaazaisowurtzitanes [2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (HBIW); 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADB); and 2,6,8,12-tetraacetyl-4,10-diformyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADF)] have been synthesized as intermediate steps in the synthesis of CL-20. The 15 N NMR isotropic and chemical shift tensor principal values have been obtained for these intermediates using CP/MAS and FIREMAT methods. Solid-state 15 N NMR offers unique advantages in the study of these materials, due to solubility issues and lack of high-quality X-ray crystallographic data. In addition, the caged structures possess unique electronic properties that can be useful in further refining theoretical methods. The effects of the different functional groups on the chemical shift tensor principal values of these caged amines have been evaluated and are shown to be reflected in the total anisotropy. The position of the amine within the cage is manifested in the δ 22 values. Nitrogen-15 NMR shielding calculations using the embedded ion method (EIM) to account for long-range intermolecular interactions show a good correlation, rmsd = 4 ppm, with the observed chemical shift tensor values.