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.