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.