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.