Formation of chloroalkenylalkoxy radical intermediates
Under the atmospheric condition, chloroalkenyl peroxy radical criegee
intermediates I2a, I2b, I2c, and I2d react with the presence of nitrogen
monoxide (NO) radical possesses chloroalkenylalkoxy radical
intermediates I3a, I3b, I3c, and I3d respectively and their optimized
structure are shown in Figure. 5. As shown in Figure.
6, the intermediates I3a, I3b, I3c, and I3d are formed with the
activation energy values of 24.786, 29.171, 30.775, and 29.493 kcal/mol
respectively. During this reaction, the condensed fukui function
analysis is used to find the active site of the chloroalkenyl peroxy
radical criegee intermediates I2a, I2b, I2c, and I2d, where the active
sites are found to be at terminal oxygen atom for each criegee
intermediates. The condensed fukui function values are found to be
-0.297, -0.187, -0.194 and -0.199 at the O16 position of each
intermediates I3a, I3b, I3c, and I3d respectively, which tends to
attract the electron-deficient electrophilic atom N17 with the condensed
Fukui function value of -0.162. The NO radical addition reaction is
observed through the transition states TS3a, TS3b, TS3c, and TS3d with
the single imaginary frequency value of -110.945, -182.017, 88.346, and
210.912 cm-1 respectively. The TSs are confirmed by
IRC calculation through PES. During the NO radical addition reaction
with the criegee intermediate I2a, the bond length of (O18—N17) and
(O16—O15) atoms are found to be 1.1 Å and 1.4Å, whereas in the
formation of intermediate I3a bond length extended around 0.01Å and 0.05
Å respectively. During the addition of NO radical with the intermediate
I2b, the bond length of O15—O16 and N17—O18 atoms are found to be
1.4Å and 1.1Å and in the formation of intermediate I3b, the bond length
elongated around 0.02Å and 0.08Å respectively. Similarly, the formation
of intermediate I3c, the bond length of (N17—O18) and (O16—O15)
atoms lengthened around 0.2Å and 0.02Å respectively. As shown in
(Tabe 3) the formation of intermediate I3a, I3b, I3c, and I3d
is endothermic with the enthalpy (ΔH) values of 23.741, 28.643, 30.495,
and 30.599 kcal/mol and that they are endoergic with the Gibbs free
energy (ΔG) value of 26.908, 30.893, 32.901, and 33.117 kcal/mol
respectively which reveals that the reactions are less feasible and
non-spontaneous. Further, the covalent bond between O15 and O16 atoms in
an intermediates I3a, I3b, I3c, and I3d gets completely cleaved to form
an intermediate I4a, I4b, I4c, and I4d respectively. As shown in
(Table 4), the intermediates I4a, I4b, I4c, and I4d are formed
in a barrier less reaction. During the formation of intermediate I4a,
I4b, I4c, and I4d, the nitrogen dioxide (NO2) eliminated
into the atmosphere due to cleavage of the covalent bond between O15 and
O16. In these steps, NO2 elimination from respective
intermediates are observed through the transition states TS4a, TS4b,
TS4c, and TS4d. The reaction enthalpy for the elimination of
NO2 from an intermediates I3a, I3b, I3c, and I3d are
found to be exothermic with their corresponding values are -43.304,
-37.914, -34.574, and -36.904 kcal/mol, and also the Gibbs free energies
are found to be exoergic with the values of -41.848, -37.133, -34.118,
and -36.941 kcal/mol respectively. By the negative values of the
thermodynamic parameters of NO2 elimination, the
reactions are more feasible and spontaneous. Thus, the eliminated
NO2 helps to increase the level of ozone under
atmospheric photo-oxidation.
As shown in Figure 7, intermediates I4a, I4b, I4c, and I4d
undergoes decomposition to form four different product P1, P2, P3, and
P4 as a SOAs through transition states TS5a, TS5b, TS5c, and TS5d which
is confirmed by the IRC calculation. The product P1, P2, P3, and P4 are
formed with the activation energy values of 7.635, 13.572, 6.231, and
13.143 kcal/mol respectively and it is shown in relative energy profileFigure 8 . Both P1 and P2 are the products obtained from the
addition of chlorine atom to C1 and C3 position respectively and they
are formed by the homolytic bond fission between respective C—C atoms.
Other end products of the isoprene reactions are P3 (methyl vinyl ketone
(MVK)) and P4 (methacrolein (MACR)) and they are the potential end
products obtained from the addition of chlorine atom to the terminal
carbon C4 and C5 atoms. The addition of Cl atom at terminal position C4
and C5 also generates the CH2Cl and it subsequently
reacts to produce formyl chloride. As given in (Table 4) , the
thermodynamic parameters for the formation of end products P1, P2, P3,
and P4 are found to be exothermic with the enthalpy values of -6.334,
-0.539, -0.129, and -1.157 kcal/mol respectively. Moreover, they are
found to be exoergic with the Gibbs free energy values of -10.353,
-3.778, -5.479, and -2.189 kcal/mol respectively. From the thermodynamic
results, the negative values reveal that the formation of product P1,
P2, P3, and P4 are more feasible and spontaneous. During the formation
of products, obtained entropy values 13.483, 10.858, 17.942, and 8.871
mol-1 K-1 are also indicated that
the reactions are spontaneous and irreversible. The potential end
products P1, P2, P3, and P4 are reactive in its own right and can
contribute to O3 production as well as provide a source
of free radicals through their photolysis.