Depletion of atmospheric ozone by nitrogen dioxide: a bifurcated reaction pathway

Abstract

Theoretical investigation on the reaction of NO2 with O3 in the context of ozone depletion is reported in this study. The progress of this reaction in single or concrete steps is an open and highly challenging question for both experimentalists and theoreticians. The reaction is studied through DFT methods, M06-2X, BH and HLYP and MPW1K with 6-311++G(2df,2p) basis set and also at UCCSD and CASPT2 methods with 6-31G(d,p) basis set. The single-point energies of the reactive species are calculated at CASSCF, UCCSD(T), UQCISD(T) and MRCI methods with 6-311++G(2df,2p) basis set. The reaction proceeds along an intermediate through a rate-limiting first transition state. This intermediate is followed by a second transition state, and the intrinsic reaction coordinate (IRC) calculations from this transition state show that the reaction path follows a product channel and another transition state. Two sequential transition states are calculated and the potential energy surface bifurcation is observed through a valley-ridge inflection point. The IRC calculations from third transition state lead to second product channel in which NO and O2 are formed. The energy barrier calculated for the formation of NO3 and O2 in the first product channel is 33.96 kcal/mol, and an energy barrier of 34.07 kcal/mol is calculated for the formation of NO + 2O2 product channel. The rate constant calculated at 298 K for the two product channels is 2.31 × 10−7 cm3 molecule−1 s−1. These results show that the formation of the two product channels is equally probable. The photodissociation wavelength of the reactive species involved in the reaction system is calculated using equation of motion coupled cluster (EOMCC) method. The calculated photodissociation wavelength shows that a significant amount of NO2 photofragment enters into the stratosphere and potentially involves in the depletion of ozone. The photodissociation wavelength of the reactive species calculated using EOMCC method is in good agreement with the available experimental data.