Low-Temperature Reactions in Single Crystals of Two Polymorphs of the Polycyclic Nitramine 15N-HNIW

Low-temperature photolysis of HNIW single crystals was investigated by electron paramagnetic resonance spectroscopy. Crystals of the α and β polymorphs, with either fully or selectively 15N-labeled nitro groups, were photolyzed at 15 K, and various species involving paramagnetic oxides of nitrogen (NO, NO2) were observed after annealing between 15 and 300 K. The orientation of NO trapped in the lattice of β-HNIW was inferred from the orientation of its g- and 15N-hfs tensors. Regioselectivity of photodecomposition within the two crystal lattices, monitored by the ratio of 15N and 14N isotopes in the NO and NO2 radicals, showed that no single reaction pathway produced these products and that concerted three-bond fragmention was not a dominant pathway. Analysis of packing similarities and reaction differences between the α and β polymorphs of HNIW confirmed that the cavities that accommodate water in the α hydrate play an important role in its solid-state reactivity.