Reaction of the hydrogen atom with nitrous oxide in aqueous solution – pulse radiolysis and theoretical study

Abstract

UB3LYP/cc-pVTZ computations using C-PCM, IEF-PCM, and SMD water-solvent models have been performed for the reaction of the H˙ atom with nitrous oxide (N₂O) producing N₂ and ˙OH in aqueous solution. The H˙ atom attacks the oxygen atom in the N₂O molecule resulting in the formation of the [H–ONN]^‡ transition state and its decomposition into ˙OH and N₂. This direct path requires 54.2 kJ mol⁻¹ (PCM) or 54.6 kJ mol⁻¹ (SMD) compared to 53.0 kJ mol⁻¹ in a vacuum. The H˙ atom addition to the nitrogen end leads to the [H–NNO]^‡ transition state decaying to a cis-HNNO intermediate that after transformation to [NNOH]^‡ finally produces ˙OH and N₂. The total energy expense associated with the indirect mechanism, 67.6 kJ mol⁻¹ (PCM) or 65.5 kJ mol⁻¹ (SMD), is slightly smaller compared to 67.7 kJ mol⁻¹ computed for the reaction in vacuum. The temperature dependence of the reaction rate constant obtained based on the pulse radiolysis measurements in N₂O-saturated 0.1 M HCl solution over the temperature range of 296–346 K shows the activation energy (62.6 ± 2.1) or (59.9 ± 2.1) kJ mol⁻¹ depending on a form of the pre-exponential factor in the Arrhenius equation, A or A′ × T, respectively. The activation energy, almost three times higher than observed in gases at temperatures below 500 K, indicates predominance of the direct reaction path via [H–ONN]^‡. The indirect mechanism may also contribute, but in contrast to the gas phase reaction neither tunnelling from [H–NNO]^‡ to [NNOH]^‡ nor collisional stabilization of [H–NNO]^‡ occurs in solution.