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.