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

The successful synthesis of the pentazolate salt (N-5)(6)(H3O)(3)(NH4)(4)Cl has received considerable attention, as it ends the long search for a method for the bulk preparation of cyclo-N-5(-), 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-N-5(-) 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 center dot center dot center dot HN5 via H2O-H-N-5 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.