ABSTRCT
An extended computational approach has been utilized to explore the reactions of acids with carbonyl oxide, also known as Criegee intermediate (CI). The reactions were explored inside water cluster containing 50 water molecules. All possibilities of product formation were considered. Among the considered acids, the rate of 1,4-insertion follows the order - HCOO < HCl < HNO3. The most stable products of the reactions between the considered acids and CI have been identified.
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 The general mechanism of alkene ozonolysis is shown below8-9
These produced CIs are found to have excess internal energy to undergo unimolecular processes, generally. But with acids, these CIs undergo bimolecular reaction so efficiently.10-12 Studies 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.13
Early experimental data showed fast reactions of CI with formic acid (rate constant, kHCOOH = 1.1 X 10-11cm3 molecule-1s-1), hydrochloric acid (rate constant, kHCl= 4.0 X 10-11cm3 molecule‑1s-1) and nitric acid (kHNO3 = 5.4 X 10-10 cm3molecule-1s-1).14-18Their reaction rates are suggested with the help of their reaction rate co-efficient values.18-20 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.21-24 The obtained experimental data were also in agreement with theoretical values.13Vereecken performed quantum chemical calculation to find out the probable barrierless pathways of these CI-acid reactions in gaseous phase.13 As it has been already discovered that water plays a significant role in atmospheric and environmental processes,23-28 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. Francisco et. al. have shown that the reaction of CI with nitric acid is greatly influenced by air-water interface.29a Moreover, Lin et. al . have shown that the reaction of CI with alcohol is greatly enhanced by single water molecule.29b However, to the best of our knowledge, no such study is available where the reaction between CI and acids are considered inside the water cage. The choice of 50 water cluster is due to the fact that it provides a full cavity (volume 166.3 Å3) where the reactions of CI and acids can be studied. Although the reaction of CI with acids have been studied in gas phase13 and in air-water interface,29a the study of these reactions inside water cavity is also very essential for better understanding the fate of CI in aqueous environment.
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.30 We have used two layer ONIOM31 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.32
The rate constant of different reaction channels as well as the overall rate constant was calculated by using transition state theory33 using equation (1)
k (T ) = σr ,Γ (T ) kBT/h (qTS /qR1qR2 ) exp (-∆E0/RT) (1)
Values of the partition functions (q ) and relative energy value (∆E0 ) are calculated using CCSD/6-311++G** level of theory. The value of Γ (T ), known as the tunnelling correction factors were calculated from the unsymmetrical Eckart type potential barrier method34 and σr is the symmetry number. Other terms have usual meaning.