Nitrogen‐14 Nuclear Quadrupole Resonance in the Explosive Triamino‐Trinitrobenzene (TATB)

Abstract

ABSTRACT 14 N nuclear quadrupole resonance (NQR) frequencies in the explosive triamino‐trinitrobenzene (TATB) have been measured directly for the first time. The ν + lines for the three amine nitrogen sites were found at 1.584, 1.545 and 1.512 MHz, with three ν − lines also found at 1.151, 1.149 and 1.135 MHz. The lines were located by a sequential search strategy guided by density functional theory (DFT) calculations. Calculated DFT frequencies were within 4%–11% of their measured values, with at least some of the difference due to the temperature difference between calculations (0 K) and measurements (293 K). The measured ν + and ν − frequencies were paired according to the calculated frequencies and therefore tentatively assigned to the amine nitrogen sites. The 14 N spin‐lattice relaxation time T 1 was measured for each line and revealed comparatively long T 1 values in the range 15–60 s. A modified multi‐pulse inversion‐recovery method was used to measure T 1 by replacing the conventional single pulse acquisition. This method was verified against the conventional using cyclo‐tetramethylene‐tetranitramine (HMX), with known T 1 . The effective spin echo time T 2e was also measured with a multi‐pulse sequence and found to be in the range 10–30 ms (noting that T 2e can depend on the pulse separation). Knowledge of both T 1 and T 2e is important so that signal‐to‐noise ratio per unit time can be maximised for practical applications. Further work will include measuring the ν 0 lines to confirm the ( ν + , ν − ) pairs and the temperature dependence of the lines as well as extending the modelling to finite temperatures.

Abstract

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