Reaction rate of H atoms with N₂O in hot water

The rate constant of H^• atoms with N₂O in water has been measured by a competition method up to 300 °C. Radiolysis with 2.5 MeV electrons generated H^• atoms, and the HD product from their reaction with deuterated tetrahydrofuran (THF-d8) was measured with mass spectroscopy. The concentration of THF-d8 was changed by an order of magnitude in the presence of 25 mM N₂O to obtain the ratio of rate constants. To determine the rate constant of H^• with THF-d8, a similar competition vs. 0.2 mM OH^– ion was also measured. The reaction rate of H^• with OH^– has been accurately determined vs. temperature in previous work, allowing the two unknown rate constants to be deduced. Rate constant of H^• with THF-d8 follows the Arrhenius law ln(k/M⁻¹s⁻¹)=27.33 - (32.30 kJ/mol)/RT. Rate constant of H^• with N₂O follows the Arrhenius law ln(k/M⁻¹s⁻¹)=24.50 - (30.42 kJ/mol)/RT. In all likelihood, the N₂O reaction proceeds via cis-HNNO^• radical intermediate as in the gas phase, but with participation of a bridging water molecule in the 1,3 hydrogen shift to form N₂ and ^• OH products. - Highlights: • Rate constant for H with N₂O in water follows Arrhenius behavior to at least 350 °C. • Room temperature rate is 2×10⁵ M⁻¹ s⁻¹ and activation energy is 31 kJ/mole. • Reaction probably proceeds through cis-HNNO intermediate as in gas phase.