Symmetrical -N hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts.

Pairing different cations (R) to stabilize -N is the main synthesis path for non-metallic cyclo-pentazolate (-N) salts. As novel energetic materials (EMs), crystalline packing-force of -N salts has been a puzzle, and whether -N is protonated also is a controversial issue. In this paper, four non-metallic -N salts, PHAC, NHN, NHOHN, and NHN, are quantitatively studied by coupling first-principle method and bond-strength analyzing technology. Different from the traditional CHON-EMs (molecular crystal) and azide-EMs (ionic crystal), the four salts are stabilized by 3D hydrogen bond (HB) networks. One new type of hydrogen bond, protonated HB (-H, R-H⋯N), is discovered to be a key stabilizing factor for -N. Proton competition mechanism between R and -N in -H HB showed that -N cannot be protonated into HN. In general, -H HB can be adopted to estimate the stability of novel non-metallic -N EMs. Such findings have great significance for future design and performance prediction of novel -N EMs in both theoretical and experimental aspects.