Fig 2 . Energetic for the formation of CI by the reaction of C2H4 with O3. The optimized geometries of the intermediates and transition states were also shown. Bond lengths are in Å.
CI-formation takes place via ozonolysis of C2H4. First step is 1,3-dipolar cycloaddition of ozone to the double bond of ethane leading to primary ozonide or 1st molozonide, 1 . The reaction is found to be exergonic by 75.3 kcal/mol. In 2nd step, the carbonyls (as 1,3-compounds) again undergo 1,3-cycloaddition, leading to the formation of secondary ozonide 2 (more stable than primary ozonide). The 3rd step is the breaking of C-O bonds to generate the Criegee intermediate (carbonyl oxides) and H2CO which involves a barrier of 25.1 kcal/mol. Overall, the formation of CI is exergonic suggesting the high rate of the reaction between C2H4 and O3. The studied mechanism is in tune with previously studied reactions of C2H4 with O3.22-28,35
We then turned our attention to investigate all the possible reaction channels of some acids with CI. The choice of the acids namely, hydrochloric acid, formic acid and nitric acid for the reaction with CI is stimulated by their high abundance in atmosphere.
3.3 Reaction between hydrochloric acid and CI : It is generally speculated that HCl molecule inside the water cavity will become hydrated H+ and Cl- very quickly and therefore, will not get the time to react with CI. To investigate this, we have performed Born-Oppenheimer molecular dynamics (BOMD) simulation at M06-2X/6-31+G* for 10 ns. Figure 3 shows that the HCl molecule remains intake inside the cavity without being hydrated till 10 ns. This interval of time may be sufficient enough for HCl to undergo reactions with CI before being hydrated. We, therefore, investigated its reaction channels with CI.