Probing the Structural Effects of Hydrogen Bonding in 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB): Experimental Evidence by ¹⁵N NMR Spectroscopy

Examining isotopically 15N-labeled versions of 1,3,5-triamino-2,4,6-trinitroben-zene (TATB) by NMR has experimentally demonstrated the effects of strong intra-and intermolecular hydrogen bonding on the solid-state and solution phase structure. Two isotopically labeled TATB compounds were synthesized using the wet-amination procedure: one with only the amino-nitrogen sites labeled, (15NH2)3-TATB, and the other with both the amino-and nitro-nitrogen sites labeled, (15NH2)3(15NO2)3-TATB. Isotopic and chemical purity was established by HPLC coupled to optical and mass spectroscopic detection. Solid-state NMR techniques (cross-polarization magic angle spinning (CP/MAS), 15N{1H} frequency switched, heteronuclear correlation (FSLG-HetCor)) were applied to assess the labeled nitrogen environments and revealed two distinctive nitrogen sites: one for the amino and one for the nitro. Closer examination of both sites revealed the nitro-nitrogen site represented one environment while the amino-nitrogen site represented three different environments. Equivalent solution NMR spectra show the presence of stable TATB tautomers in slow exchange with one another. A model was constructed (DFT to compute chemical shifts using the Gaussian16 revision A.03 computational code) demonstrating the inequivalence of the amino-nitrogen sites and shows essentially the equivalence of the nitro-nitrogen sites in the solid state. This provides further experimental evidence of intermolecular hydrogen bonding playing a significant role in the solid-state structure of TATB and possibly clarifies evidence of structural inequivalence suggested by some spectroscopic and crystallographic examinations. These results also may impact structure variations in future modeling studies.