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

Abstract

Examining isotopically 15N-labeled versions of 1,3,5-triamino-2,4,6-trinitrobenzene (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.

Syntheses and characterization of isotopically labeled 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB)**

“…( 15 NH 2 ) 3 ‐TATB has one resonance at 120 ppm assigned to the amino‐group; ( 15 NO 2 ) 3 ‐TATB has one at 369 ppm assigned to the nitro‐group; ( 15 NH 2 ) 3 ( 15 NO 2 ) 3 ‐TATB has two assigned to both nitro‐ and amino‐groups. Previous work [12] using different NMR parameters showed the broader amino resonance reflects three inequivalent amine environments.…”

“…Figure 1 shows the isotopically labeled analogues and which labeled precursors were used to produce them. These compounds were synthesized using the wet-amination method [29], with adaptation to laboratory conditions for the synthesis as previously described for ( 15 NH 2 ) 3 -TATB [12]. Except for the 13 C-labeled material, the synthesis procedure starts with TCB.…”

“…These methods are reviewed in detail elsewhere. Briefly, spectroscopic and spectrometric characterization methods were used: infrared spectroscopy (IR) [16] samples were placed on an attenuated reflectance (ATR) cell, spectra were obtained on a Bruker INVENIO ® R R&D FTIR Spectrometer with a Platinum ATR attachment; SS-NMR [12] -spectra of 15 N-, 13 C-, and 2 H-labeled samples were recorded at 300 MHz on a Bruker NEO NMR spectrometer, 10-mg samples were loaded into the middle third of a 2 mm o.d. diameter zirconia NMR rotor; spectra were collected using a Revolution NMR HX MAS probe at a spinning rate of 15 kHz for 15 N samples and without spinning for the 2 H samples; powder x-ray diffraction (PXRD) [32] -Bruker D8 Advance diffractometer using Cu wavelength, with a K alpha 1 and K alpha 2 edge component; mass spectrometry electrospray ionization (ESIÀ ), atmospheric pressure chemical ionization (APCIÀ ), electron impact ionization (EIÀ ), and optical diode array detection (DAD) -method reported previously [33]; pyrolysis coupled with cryo-trapping EI-MS -(Py-EI-MS)-Frontier Laboratories EGA/PY 3030D multi-functional pyrolyzer; differential scanning calorimetry (DSC) -TA instruments Q2000 [34].…”

“…Although many of the assignments are commonly accepted in these studies, there still is controversy over others. 15 N substitution has discerned inequivalence in the amino sites in the solid‐state structure of TATB [12] by NMR. In earlier NMR work, 15 N substitution in the amino group was used to assign 13 C positions in TATB dissolved in strong acids [13].…”