Toward elimination of discrepancies between theory and experiment: The gas-phase reaction of N2O5 with H2O
New reaction mechanisms are presented and the corresponding reaction rate constants are calculated for the homogeneous gas-phase reaction N2O5 + nH2O ↔ 2HNO3 + (n − 1)H2O with n = 1,2,3 using ab initio methods and canonical variational transition state theory including tunneling corrections. The reaction barriers for the new mechanisms are 21.1 kcal mol−1 for n = 1, 18.9 kcal mol−1 for n = 2 and for the two mechanisms with three water molecules 14.2 and 19.2 kcal mol−1. Using the new reaction mechanism the rate constant for N2O5 hydrolysis with n = 1 is k1 = 5.2 × 10−25 cm3 molecule−1 s−1 at 298 K, which is in much better agreement with the experimental value being only two orders of magnitude smaller, compared to the old mechanism which is ten orders of magnitude smaller than the experimental value. Also the rate constant for the third order process—second order with respect to [H2O]—is in better agreement with experiment compared with the old mechanism (seven compared to approximately twelve orders of magnitude). For possible future confirmation of the new reaction mechanisms we determined kinetic isotope effects for the reactions and obtained KIEs of 1.55 and 1.09 for n = 1 and n = 2 water molecules, respectively, compared to 1.11 and 1.44 for the old mechanisms.