Formation of Cl-isoprene adduct radicals
Isoprene can react with Cl radical at the C=C double bond positions such as C1, C3, C4, and C5 which lead to the formation of Cl isoprene adduct radical (C5H8Cl) intermediates (I1a, I1b, I1c, and I1d) and their optimized structures are shown inFigure 1 The intermediates I1a, I1b, I1c, and I1d are formed along with the activation energy values of 7.63, 13.57, 12.14 and 13.14 kcal/mol respectively and their corresponding relative energy profile is shown in Figure 2 . In isoprene, the condensed Fukui function value of C3, C1, C4, and C5 atoms are found to be 0.051, 0.038, -0.016, and -0.025 respectively. In the Cl radical, the condensed fukui function value is found to be 0.043. During the formation of Cl isoprene adduct radical intermediates (I1a, I1b, I1c and I1d), the obtained condensed Fukui function value shows that the nucleophilic carbon C3, C1, C4 and C5 atoms tend to interact with the electron deficient electrophilic Cl radical. During this Cl radical addition, the π bond is cleaved homolyticly in between the (C=C) atoms and it leads to form the covalent bond between (C—Cl) atoms. In the case of bond formation, the Cl radical able to accommodate the electron as a lone pair which can stabilize the structures by resonance delocalization. As shown inFigure 1 , the intermediates I1a, I1b, I1c, and I1d are formed through the transition states TS1a, TS1b, TS1c, and TS1d, where the obtained single imaginary frequency values are -318.896 cm-1, -241.151 cm-1, -829.152 cm-1, and -241.115 cm-1respectively, confirms the transition state on PES by IRC calculations. During the addition of Cl radical at C1 position, the bond length of (C1—C2), (C1—C3) and (C1=C4) atoms are found to be 1.502 Å, 1.461 Å and 1.343 Å respectively. Whereas in the formation of intermediate I1a, the bond length of (C1—C2), (C1—C3) and (C1=C4) atoms elongated around 0.01 Å, 0.04 Å and 0.1 Å respectively. After that addition of Cl radical at C3 position, the bond length of (C3=C5), and (C3—C1) atoms are found to be 1.338 Å and 1.461 Å respectively. The previously reported bond length of C=C and C—C atoms are 1.33Å and 1.54Å respectively and these results are in good agreement with the obtained results [40]. During the formation of intermediate I1b, the bond length gets elongated around 0.128 Å and 0.042 Å. During Cl radical addition at C4 position, the bond length between C4 and C1 atoms are found to be 1.339 Å, while in the formation of intermediate I1c, the bond length is extended around 0.14 Å. In the formation of intermediate I1d, the bond length between C5 and C3 atom elongated around 0.15Å, The Cl—C bond distances in intermediates I1c and I1d are 1.83Å and 1.83Å respectively where the bond length of Cl—C atoms is very similar in both intermediates. Another two adduct radical intermediates I1a and I1b with Cl radical addition to the internal positions C1 and C3 atoms contain well localized double bonds and the Cl—C bond distances in intermediates I1a and I1b are 1.90Å and 1.88Å respectively, which is a little higher than those of the intermediates I1c and I1d with the terminal Cl radical addition. In the present work, the obtained bond length of C—Cl atoms is well correlated with the previously available experimental investigation[41]. As given in (Table 1 ), the formation of Cl isoprene adduct radical intermediates I1a, I1b, I1c, and I1d are found to be exothermic with the enthalpy (ΔH) values of -13.230, -13.852, -26.326, and -26.111 kcal/mol and these intermediates are also found to be exoergic with the Gibbs free energy (ΔG) values of -11.647, -13.100, -26.421, -25.231 kcal/mol respectively. The calculated thermodynamical values are well correlated with previous results[42] and the results are compared in (Table 1) . The ΔH<0 and ΔG<0 reveals that the reactions are spontaneous and more feasible. The thermodynamic results reveal that the terminal addition of Cl radical with the isoprene is thermodynamically more favorable than the Cl addition of the internal positions (C1 and C3). Wenfang Lei and Renyi Zhang also reported that the Cl radical addition at terminal positions (C4 and C5) has lower enthalpy (ΔH) values than the Cl radical addition at the internal positions (C1 and C3) [43]. Hence, the Cl radical addition reaction reveals that the addition of Cl radical at the terminal position is thermodynamically more favourable in the marine boundary.