Reaction rates of hydroxyl radical with nitric acid and with hydrogen peroxide

The rates of the reactions HO+HNO3 → H2O+NO3, (1), HO+H2O2 → H2O+HOO (2) have been studied by laser flash photolysis of reactants and resonance fluorescence of hydroxyl radicals. The recently reported high rate constants at room temperature for both reactions and the negative activation energy for Reaction (1) at low temperature have been confirmed. Results obtained here are: k1 = 1.52×10−14 exp(644/T) cm3 molecule−1 s−1 from 218–363 K and k2 = 1.81×10−12 cm3 molecule−1 s−1 at 298 K. These two reactions have been examined by transition-state theory; (1) is assigned a cyclic and (2) a chainlike transition state. Even with no potential energy barrier, the reaction coordinate of (1) involves a quantum-mechanical, temperature independent frequency; and with this model the low pre-exponential factor and negative activation energy of Reaction (1) can be explained.