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.