Properties of Pure Nitryl Bromide. Thermal Behavior, UV/Vis and FTIR Spectra, and Photoisomerization to trans-BrONO in an Argon Matrix

BrNO2 is produced by the heterogeneous reaction of ClNO2, highly diluted in He, with an aqueous bromide solution and subsequent trapping at −155 °C. The crude product is purified by trap to trap condensation and fractional sublimation in vacuo. At 300 K and a pressure of 1 mbar, a half-life of about 1 h is observed. The decomposition produces Br2 and NO2. The UV/vis and IR absorption cross sections of gaseous BrNO2 are determined in the ranges 185−600 nm and 2000−400 cm-1, respectively. Irradiation of matrix-isolated BrNO2 with visible light causes nearly quantitative isomerization to trans-BrONO, while ClNO2 is converted by UV light into a mixture of cis- and trans-ClONO. It is possible to assign all fundamentals of BrNO2 and trans-BrONO by comparison to the respective band position of the analogous molecules XNO2 and XONO (X = Cl, F).