Investigation of FOX‐7 Polymorphs: new polymorphs – ϵ and ζ

Abstract

Polymorphism of 1,1‐diamino‐2,2‐dinitroethene (FOX‐7, DADNE) has investigated by X‐ray powder diffraction (XRD), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), IR‐spectroscopy. Two previously unknown polymorph modifications of FOX‐7 ‐ ϵ and ζ were discovered. ϵ‐FOX‐7 has prepared by hydrolysis of 2‐(dinitromethylene)‐5,5‐dinitropyrimidine‐4,6(1H,3H,5H)‐dione in a media with pH up 7–8 to Ho≈−3 and 2‐(dinitromethylene)imidazolidine‐4,5‐dione in aqueous ammonia. ϵ‐FOX‐7 is a stable and exists in temperature range 273–378 K. ζ‐polymorph of FOX‐7 can be obtained at ϵ‐FOX‐7 heating to 378–380 K. ζ→ϵ polymorph transition is a reversible and occurs at 361.7±3.1 K. At 433–453 K ζ‐FOX‐7 transforms to γ‐polymorph. ζ→γ phase transition temperature range was determined using IR‐spectroscopy because it is invisible on DSC curves. IR‐spectra of ϵ and ζ polymorphs are significantly distinct from α‐, β‐ and γ‐FOX‐7. ϵ‐FOX‐7, in contrast to monoclinic α‐polymorph, has orthorhombic type of crystal system (space group P212121). Based on received data, new scheme of FOX‐7 polymorph transition is proposed.

Exploring the Photochemistry of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7) Spanning Simple Bond Ruptures, Nitro-to-Nitrite Isomerization, and Nonadiabatic Dynamics

“…Several phases of FOX-7 denoted as α to ζ have been characterized, among them α-FOX-7, which is stable at temperatures below 389 K. ,− Single crystal X-ray diffraction studies reveal a monoclinic structure with four formula units per unit cell. As a typical push–pull ethylene (Scheme ), the carbon–carbon bond in α-FOX-7 is 145 pm long, which is shorter than a carbon–carbon single bond in ethane (154 pm), but longer than a carbon–carbon double bond in ethylene (134 pm) .…”

“…A comparison of these features with the infrared spectra of distinct phases of FOX-7 reveals that FOX-7 exists in the monoclinic α phase, which is stable at temperatures below 389 K (Table ). ,− Infrared spectra of the freshly prepared FOX-7 and results of differential thermal analysis are presented in Figures S6 and S7.…”

Ultraviolet-Initiated Decomposition of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7)

“…Thermal decomposition of solid FOX-7 above 500 K produces carbon monoxide (CO), carbon dioxide (CO 2 ), ammonia (NH 3 ), hydrogen cyanide (HCN), cyanic acid (HOCN), isocyanic acid (HNCO), formic acid (HCOOH), water (H 2 O), nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrous acid (HONO), nitrous oxide (N 2 O), and dinitrogen trioxide (N 2 O 3 ). , In such cases, hydroxyl radicals (OH) and NO 2 were found to abstract hydrogen to generate H 2 O and HONO . At elevated temperatures, α-FOX-7 transitions to a β-phase above 385 K and eventually to γ- and δ-FOX-7 before thermally decomposing. ,,− ,− Recently, ε- and ζ-phases have also been synthesized that convert to a γ-phase upon heating . Although subtle changes were observed in the infrared spectrum upon cooling to cryogenic temperatures under vacuum, a distinct low-temperature phase has not been observed .…”

“…17,18,28−31,33−36 Recently, εand ζ-phases have also been synthesized that convert to a γ-phase upon heating. 37 Although subtle changes were observed in the infrared spectrum upon cooling to cryogenic temperatures under vacuum, a distinct low-temperature phase has not been observed. 17 While different reaction characteristics are observed among the various phases, changes due to deuteration of FOX-7 were found to affect only the α-phase but not the γ-phase.…”