Experimental determination of Δ H f (HOBr) and ionization potentials (HOBr): Implications for corresponding properties of HOI

The photoion yield curves of HOBr+ and Br+ from HOBr are presented. The adiabatic I.P. of HOBr is found to be 10.638±0.003 eV. Autoionizing structure in HOBr+ is tentatively assigned and leads to an adiabatic I.P. of the first excited (A L2 A′) state of ∼11.46 eV. The 0 K threshold of Br+ from HOBr (13.915±0.018 eV) implies D0(HO–Br)≤2.101±0.018 eV≡48.45±0.42 kcal/mol. Together with auxiliary thermochemistry, this value yields ΔHf2980(HOBr) ≥ −13.43 ± 0.42 kcal/mol, in excellent agreement with a recent ab initio value −14.2±1.6 kcal/mol. The resulting proton affinity of BrO is 163.8 kcal/mol. Trends observed in the properties of OX and HOX (X=F, Cl, Br) are utilized to infer new values for ΔH0f(OI) and ΔH0f (HOI). Predictions are made for I.P.(HOI). Proton affinities of OX are seen to increase with heavier X, while the X 2A″–A 2A′ splitting decreases.