First Calculations of ¹⁵N−¹⁵N J Values and New Calculations of Chemical Shifts for High Nitrogen Systems: A Comment on the Long Search for HN₅ and Its Pentazole Anion

Abstract

In the potential solution observation of the long-sought-after pentazole anion (N(5)(-)), the principal experimental tool used for detection is NMR. However, in two experiments, very different conclusions were reached. To assist in the interpretation, we report predictive level coupled-cluster results for the spin-spin coupling constants and chemical shifts for all of the key species, which include NO(3)(-), N(5)(-), HN(5), N(3)(-), and MeOC(6)H(5)N(3). In the case of the shifts, an empirical estimate based on the molecule polarity enables comparison of gas-phase and observed values with expected error bars of approximately +/-10 ppm. For the scalar couplings, the evidence is that the solution effects are modest, enabling the gas-phase values (with error bars are approximately +/-5 Hz) to be accurate. The latter supports the observation of centrally (15)N labeled N(3)(-) in the cerium(IV) ammonium nitrate (CAN) solution which could only occur if the pentazole anion had been created in the experiment, yet with too short a lifetime to be observed in NMR.

Kinetic Stability and Propellant Performance of Green Energetic Materials

“…[21] As the detection of the pentazole anion in the condensed phase by NMR spectroscopy has proven difficult [22][23][24] it is still a topic of ongoing research. [25][26][27] The stability of many aryl pentazoles is well established; their decomposition barriers are typically around 20 kcal mol À1 . [25,[28][29][30] Other pentazole compounds are also known.…”

Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance

“…The detection in solution was reported in 2003 [58], but the NMR-assignment was subsequently questioned [59]. Although a recent theoretical study has provided support for the original assignment [60], it is clear that PZ at best was generated as a transient species [61]. Thus, the quest for its synthesis and isolation goes on.…”

“…The NMR spectrum, whether it is 14 N or 15 N, also consists of a single peak. The frequency of this peak is difficult to predict accurately by theoretical methods and is sensitive to the solvent [60]. As mentioned, the use of the NMR-shift for fingerprinting has already stirred up a controversy in the case of Butler's attempted synthesis of PZ.…”

“…They suggest that the reaction afforded pentazole (HN 5 ), which was held in solution as PZ complexed to Zn 2+ . The outcome of this experiment has been under scrutiny, and rather than going into the details we refer the interested reader to the relevant literature [59,60]. In any case, it seems clear that the experimental protocol will not enable isolation of PZ without significant modifications.…”

Theoretical Study on the Azido‐cyclization of 3,6‐Di(azido)‐1,2,4,5‐tetrazine

“…1 Such compounds have currently attracted significant attention from many researchers due to their novel properties of high-nitrogen compounds. [2][3][4][5][6][7][8][9] Additionally, they can be potentially utilized in numerous unique applications, such as effective precursors for carbon nanospheres 10 and carbon nitride nanomaterials, [11][12][13][14] solid fuels in micropropulsion systems, 15,16 gas generators, 17 and smoke-free pyrotechnic fuels. 18,19 Among the high-nitrogen systems, the polyazido heteroaromatic compounds were extensively studied 2,3,20,21 since they possess even higher heat of formation.…”