Thermal decomposition and phase transitions in solid nitramines

Abstract

Processes of solid phase transition, fusion and decomposition in several nitramines (and a nitrosamine) have been investigated using differential scanning calorimetry. The following values (kcal mol–1) for the enthalpy changes, ΔH(TK) are reported: N-picryl-N-methylnitramine (tetryl), ΔHf(400.5)= 6.18±0.07, –ΔHd(445)= 94±2; 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX)ΔHf(478.5)= 8.52 ± 0.07, –ΔHd(483)= 136 ± 2; 1,3,5,7-tetranitro-1,3,5,7-tetra-azacyclooctane, (HMX), ΔHt,β→δ(ca. 460)= 2.35±0.20, –ΔHd(ca. 540)= 166±9; 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX), ΔHf(422.5)= 9.2 ±0.4, –ΔHd(ca. 500)= 124± 12; 1,5-endomethylene-3,7-dinitro-1,3,5,7-tetra-azacyclooctane (DPT), –ΔHd(ca. 471)= 35 ± 6; 1,3-dinitro-1,3-diazacyclopentane (DDCP), ΔHf(403)≃ 6.0; 1,3-dinitro-1,3-diazacyclohexane(DDCHX), ΔHt(343)= 3.7 ± 0.3, ΔHf(353)= 0.70 ±0.02; 1,3-dinitro-1,3-diazacycloheptane (DDCHP), ΔHt(370)≃ 5.6, ΔHf(376)≃ 0.95; 1,3,5-trinitroso-1,3,5-triazacyclohexane (TNOHX), ΔHt(365)= 4.25±0.15, ΔHf(376)= 0.90±0.03. The depression of the melting point of tetryl caused by pre-heating the solids is used together with literature data ((i) relating the melting point depression with the amount of gas produced during decomposition, and (ii) for the amount of gas produced on complete decomposition) to calculate first-order rate constants for decomposition in the region of the solid→liquid transition. The results indicate a “premonitory” effect at temperatures below the melting point. With the exception of RDX (apparent Ea= 45.2±0.4 kcal mol–1) the apparent activation energies for de-composition of the nitramines tend to be unrealistically high. DDCHX, DDCHP and TNOHX show plastic crystal behaviour and may be classified as “globular” molecules; the absence of this type of behaviour with DDCP and RDX is discussed in terms of factors affecting molecular rotation in the solid state.