Effect of NH and HCOOH on the HO + HO → HO + HO reaction in the troposphere: competition between the one-step and stepwise mechanisms.

The HO + HO → HO + HO reaction is an important reservoir for both radicals of HO and HO catalyzing the destruction of O. Here, this reaction assisted by NH and HCOOH catalysts was explored using the CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling. Two possible sets of mechanisms, (i) one-step routes and (ii) stepwise processes, are possible. Our results show that in the presence of both NH and HCOOH catalysts under relevant atmospheric temperature, mechanism (i) is favored both energetically and kinetically than the corresponding mechanism (ii). At 298 K, the relative rate for mechanism (i) in the presence of NH (10, 2900 ppbv) and HCOOH (10 ppbv) is respectively 3-5 and 2-4 orders of magnitude lower than that of the water-catalyzed reaction. This is due to a comparatively lower concentration of NH and HCOOH than HO which indicates the positive water effect under atmospheric conditions. Although NH and HCOOH catalysts play a negligible role in the reservoir for both radicals of HO and HO catalyzing the destruction of O, the current study provides a comprehensive example of how acidic and basic catalysts assisted the gas-phase reactions.