Heat of Formation of OBrO: Experimental Photoionization Study†

The appearance energy of BrO+ from the dissociative ionization of OBrO was determined from the photoionization efficiency spectrum of BrO+ over the wavelength range λ = 90−122 nm by using a discharge flow−photoionization mass spectrometer apparatus coupled to a vacuum- ultraviolet synchrotron radiation source. Bromine dioxide was generated in a flow tube reactor by first forming BrO via the reaction O(3P) + Br2 and then allowing the BrO to react on the cold flow tube wall. Species present in addition to OBrO were BrO, HOBr, Br2O, and residual Br2. The results are perturbed by a signal from BrO+ formed by direct photoionization of BrO and by the dissociative ionization of Br2O. It was possible to correct for these perturbations to obtain a clean appearance energy plot that yielded a threshold at λ = 98.65 ± 0.23 nm. This gives AE298(BrO+,OBrO) = 12.568 ± 0.029 eV. Taking known thermodynamic quantities, this result yields (OBrO) = 173.4 ± 4.3 kJ mol-1 (and (OBrO) = 163.9 ± 4.4 kJ mol-1). This is the first e...