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 15 N-labeled nitro groups, were photolyzed at 15 K, and various species involving paramagnetic oxides of nitrogen (NO, NO 2 ) 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 15 N-hfs tensors. Regioselectivity of photodecomposition within the two crystal lattices, monitored by the ratio of 15 N and 14 N isotopes in the NO and NO 2 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.