Rate measurements of reactions of OH by resonance absorption. Part 3.—Reactions of OH with H2, D2 and hydrogen and deuterium halides

Abstract

The rates of the reactions of OH with H₂, D₂, HCl and DCl have been determined between 210 and 460 K using time-resolved resonance absorption to monitor removal of OH following creation of the radicals by flash photolysis of mixtures containing H₂O or N₂O + H₂. Arrhenius rate laws are obeyed by all four of these reactions throughout this range of temperature: k_{OH + H2}= 1.8 × 10^–11 exp[–19.4 kJ mol^–1/RT] cm³ molecule^–1 s^–1, k_{OH + D2}= 1.2₅× 10^–11 exp[–21.5 kJ mol^–1/RT] cm³ molecule^–1 s^–1, k_{OH + HCl}= 4.1 × 10^–12 exp[–4.4 kJ mol^–1/RT] cm³ molecule^–1 s^–1, k_{OH + DCl}= 4.7 × 10^–12 exp[–6.5 kJ mol^–1/RT] cm³ molecule^–1 s^–1.

The observed kinetic isotope effects are compared with predictions based on 3- and 4-atom models of the transition states using semi-empirical potential functions to estimate the transition state properties. Preliminary measurements on the reactions of OH with HBr and HI are also reported. The rate constants at 298 K are respectively 4.5 × 10^–12 and 9 × 10^–12 cm³ molecule^–1 s^–1.