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

UB3LYP/cc-pVTZ computations using C-PCM, IEF-PCM, and SMD water-solvent models have been performed for the reaction of the H-center dot atom with nitrous oxide (N2O) producing N-2 and (OH)-O-center dot in aqueous solution. The H-center dot atom attacks the oxygen atom in the N2O molecule resulting in the formation of the [H-ONN](double dagger) transition state and its decomposition into (OH)-O-center dot and N-2. 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-center dot atom addition to the nitrogen end leads to the [H-NNO](double dagger) transition state decaying to a cis-HNNO intermediate that after transformation to [NNOH](double dagger) finally produces (OH)-O-center dot and N-2. 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' x 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](double dagger). The indirect mechanism may also contribute, but in contrast to the gas phase reaction neither tunnelling from [H-NNO](double dagger) to [NNOH](double dagger) nor collisional stabilization of [H-NNO](double dagger) occurs in solution.