Symmetrical cyclo -N 5 - hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts.

Pairing different cations (R + ) to stabilize cyclo -N 5 - is the main synthesis path for non-metallic cyclo-pentazolate ( cyclo -N 5 - ) salts. As novel energetic materials (EMs), crystalline packing-force of cyclo -N 5 - salts has been a puzzle, and whether cyclo -N 5 - is protonated also is a controversial issue. In this paper, four non-metallic cyclo -N 5 - salts, PHAC, N 2 H 5 N 5 , NH 3 OHN 5 , and NH 4 N 5 , 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⋯N 5 - ), is discovered to be a key stabilizing factor for cyclo -N 5 - . Proton competition mechanism between R and cyclo -N 5 - in p -H HB showed that cyclo -N 5 - cannot be protonated into HN 5 . In general, p -H HB can be adopted to estimate the stability of novel non-metallic cyclo -N 5 - EMs. Such findings have great significance for future design and performance prediction of novel cyclo -N 5 - EMs in both theoretical and experimental aspects.