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.