Reconciling the Debate on the Existence of Pentazole HN₅ in the Pentazolate Salt of (N₅)₆(H₃O)₃(NH₄)₄Cl

The successful synthesis of the pentazolate salt (N₅)₆(H₃O)₃(NH₄)₄Cl has received considerable attention, as it ends the long search for a method for the bulk preparation of cyclo-N₅^-, a molecular ring with high energy density (Zhang, C.; et al. Science 2017, 355, 374.). A debate has recently arisen on the possible existence of a neutral HN₅ species in the pentazolate salt (Huang, R.-Y.; et al. Science 2018, 359, eaao3672.; Jiang, C.; et al. Science 2018, 359, eaas8953.). Herein, we show that the debate can be reconciled by the temperature effect on the proton transfer. At a low temperature (123 K), the proton transfer from H₃O⁺ to cyclo-N₅^- is energetically unfavorable; therefore, few neutral HN₅ species exist in the pentazolate salt, which is consistent with the single-crystal X-ray diffraction measurements (Zhang, C.; et al. Science 2017, 355, 374.). As the temperature increases toward room temperature, endothermic proton transfer becomes increasingly feasible, promoting the formation of H₂O···HN₅ via H₂O-H-N₅ as an intermediate species. In addition, the confusion over the apparent absence of a peak in the measured infrared spectrum corresponding to the out-of-plane bending of H₃O⁺ can be resolved by the computationally established ultrafast interconversion among the neutral and anionic species under ambient conditions.