Spin trapping of nitrogen dioxide radical from photolytic decomposition of nitramines

The photochemical (lambda < 400 nm) decomposition of some monocyclic and polycyclic nitramines produces .NO2, which can be detected in the respective nitramine crystals at 77 K by EPR (electron paramagnetic resonance). In solutions of perdeutero-dimethylsulfoxide (DMSO-d6) the .NO2 produced by photolytic decomposition of dissolved nitramines can be spintrapped by the solvent to give a radical having the structure CD3-(SO2)-(NO.)-CD3. In this article, we examine this reaction for two nitramines: cyclotrimethylenetrinitramine (RDX) and hexanitrohexaazaisowurzitane (HNIW), which are energetic materials. The decay of the spin-adduct radical (I) follows first-order kinetics for both nitramines studied, having a rate constant (k) of congruent to 7.1 x 10(-4) s-1. The net growth in spin concentration of (1) measured from EPR spectra is fitted by a first-order rate equation taking into account the simultaneous competitive decay rate of spin adduct (I). Using the rate data and EPR spin concentration data, the ratio of free .NO2 produced per parent nitramine molecule is estimated as 1:1 for RDX and 4:1 for HNIW. Biological implications of trapping of .NO2 by dimethyl sulfoxide are discussed.