Matrix infrared spectra of NUN formed by the insertion of uranium atoms into molecular nitrogen

Pulsed-laser ablated uranium atoms were codeposited with ¹⁴N₂(¹⁵N₂) and excess Ar at 12 K. The Fourier transform infrared (FTIR) spectrum revealed a single product, UN₂, which exhibited a v₃ absorption at 1051.0 cm ^-1. Ultraviolet (UV) photolysis increased the yield of UN₂ by threefold and showed that electronic excitation facilitated the insertion reaction. N₂ perturbed UN₂ bands at 1041.3 and 1031.5 cm^-1 grew sharply during matrix annealings. In ¹⁴N ¹⁵N experiments the v₁ and v₃ modes of ¹⁴NU¹⁵N were observed at 987.2 and 1040.7 cm^-1, respectively; FG matrix calculations were performed to determine F _r=8.27 mdyn/Å and F_rr=0.12 mdyn/Å and to estimate the IR-inactive v₁ modes of U¹⁴N₂ and U¹⁵N₂ at 1008.3 and 985.7 cm^-1, respectively. Energetic considerations suggest that the U+N₂ insertion reaction has little exothermicity and that the activation energy for this reaction may be provided by hypothermal uranium atoms.