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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.

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The protonation of a number of 4,6-dihydroxypyrimidine derivatives is studied, and the features of the electronic spectra of free bases and protonated forms are considered. It is shown that the alkyl substituents in position 2 increase the basicity of the compound, and the nitro group in position 5 leads to its decrease. In an acid medium (0.1−99.5% H 2 SO 4 ), 4,6dihydroxypyrimidine, 6-hydroxy-2-methylpyrimidine-4(3H)-one, and 6-hydroxy-2-ethylpyrimidine-4(3H)-one have two protonation stages, barbituric acid is protonated in three stages, and 6-hydroxy-2-methyl-5-nitropyrimidine-4(3H)-one and 6-hydroxy-2-ethyl-5-nitropyrimidine-4(3H)-one form a monocation.