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.

Automated summary

Examining isotopically 15N-labeled versions of TATB, this study investigates the effects of intra- and intermolecular hydrogen bonding on its solid-state and solution phase structure. The results demonstrate the presence of distinctive nitrogen sites and provide further evidence of the role of hydrogen bonding in the TATB structure.