Symmetrical cyclo-N5− hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts

Abstract

Pairing different cations (R⁺) to stabilize cyclo-N₅⁻ is the main synthesis path for non-metallic cyclo-pentazolate (cyclo-N₅⁻) salts. As novel energetic materials (EMs), crystalline packing-force of cyclo-N₅⁻ salts has been a puzzle, and whether cyclo-N₅⁻ is protonated also is a controversial issue. In this paper, four non-metallic cyclo-N₅⁻ salts, PHAC, N₂H₅N₅, NH₃OHN₅, and NH₄N₅, 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 (p-H, R–H⋯N₅⁻), is discovered to be a key stabilizing factor for cyclo-N₅⁻. Proton competition mechanism between R and cyclo-N₅⁻ in p-H HB showed that cyclo-N₅⁻ cannot be protonated into HN₅. In general, p-H HB can be adopted to estimate the stability of novel non-metallic cyclo-N₅⁻ EMs. Such findings have great significance for future design and performance prediction of novel cyclo-N₅⁻ EMs in both theoretical and experimental aspects.