1. Introduction: In atmosphere, ozone molecule occurs as open shell singlet di-radical. Ozone easily attack the double bond containing compounds and oxidized them by attacking at double bonded positions, which generates a highly reactive Criegee intermediate (CI), also known as carbonyl oxide.1-4 It is found that tropospheric alkenes are the most significant source of Criegee intermediates.5-7 Tropospheric alkenes are strongly localized and short chain alkenes are more usual to undergo ozonolysis. The general mechanism of alkene ozonolysis is shown below –
These produced CIs are found to have excess internal energy to undergo unimolecular processes, generally. But with acidic particles, these CIs undergo bimolecular reaction so efficiently.8-10Studies found that some acids like HCl, H2SO4, HNO3, HCOOH etc. are embedded on the surface of stratospheric region condensing with H2O, they are also known as polar stratospheric particles, play a major role in scavenging CIs. During these scavenging reaction with acids, atmospheric H2O budget are remarkably influenced.11
Early experimental data showed fast reactions of CI with formic acids (rate constant, kHCOOH=1*10-11cm3molecule-1s-1), hydrochloric acids (rate constant, kHCl= 4.0*10-11cm3molecule‑1s-1) and nitric acids (kHNO3 = 5.4 *10-10cm3molecule-1s-1), forming good yields.12-16 Their reaction rates are suggested with the help of their reaction rate co-efficient values.16-18 The obtained high rate co-efficient values suggested that acids play a key role in scavenging CI in the atmosphere leading to the formation of low-volatile but highly oxidized molecules.19,20 The obtained experimental data were also in agreement with theoretical values.11 Vereecken performed quantum chemical calculation to find out the probable barrier less pathways of these CI-acid reactions in gaseous phase.11 As it has been already discovered that water budget plays a significant role in atmosphere, so here in our study, all the possible pathways of the CI-acid reactions are studied in water cluster containing 50 water molecules using high level computational chemistry approach.
2. Computational details: The water cluster containing 50 molecules of water was energy minimized using TIP4P model for water by using leap module of AMBER18 package. Steepest descent method was used for energy minimization and conjugate gradient method to discard any unfavourable interactions. The energy minimized water cluster was used for all other abinitio calculations. The gradient and hessian calculations were performed using M06-2X/6-311+G* level of theory.21 We have used two layer ONIOM22 calculations by using CCSD/6-311++G** for the reactants and M06-2X/6-311+G* for the surrounding water molecules. Intermediates were characterized by all real values of the hessian matrix while transition states were characterized by one imaginary value of the hessian matrix. Open shell species were treated with unrestricted formalism. Unless otherwise noted, spin contamination values were negligibly small, < 0.5%. All these calculations were performed using Gaussian 16 suite of program.23