Formation of chloroalkenyl peroxy radical criegee intermediates
The Cl isoprene adduct radical intermediates I1a, I1b, I1c, and I1d lead
to form a chloroalkenyl peroxy radical criegee intermediates I2a, I2b,
I2c, and I2d by the addition of O2 molecule and their
optimized structures are shown in (Figure 3) . The criegee
intermediates I2a and I2d are formed with the activation energy values
of 2.245, 2.594 kcal/mol respectively, I2b and I2c formed in a
barrierless reaction and their relative energy profile are shown inFigure 4. These criegee intermediates are formed through the
transition states TS2a, TS2b, TS2c, and TS2d with the single imaginary
frequency values of -278.725, -108.025, -178.434, and -341.418
cm-1 respectively. In Cl isoprene adduct radical
intermediates (I1a, I1b, I1c, & I1d), the active sites are found to be
at carbon (C4, C5, C1, and C3) positions and their obtained
corresponding condensed fukui function values are -0.439, -0 45, -0.267,
and -0.265. These active sites tend to interact with the electron-rich
nucleophilic oxygen O15 atom with the condensed fukui function value of
-0.046. During the O2 addition at the C4 position, the
bond length of (C4—O15), (C4—Cl) and (O15—O16) atoms are found to
be 2.897 Å, 1.472 Å, and 1.189 Å respectively, While in the formation of
intermediate I2a, the bond length is elongated around 0.11Å, 1.46Å, and
0.06Å respectively. In O2 addition at the C5 position,
the bond length between C5—C3 atom is found to be 1.467 Å, while in
the formation of intermediate I2b the bond length elongated around
0.05Å. During the addition of O2 molecule at chiral
carbon atom C1, the bond length of (C1—C3), (C1—C4), and (C1—C2)
atoms are found to be 1.402Å, 1.479Å, and 1.496Å whereas in the
formation of intermediate I2c the bond length lengthened around 0.1Å,
0.06Å, and 0.01Å respectively due to an increased σ nature of (C—C)
bond. Simultaneously, the π bond is being transferred to the newly
formed C—O2 bond. At the time of O2addition at the C3 position, the bond length of (C3—C1), (C3—C5),
and (C3—H9) atoms are found to be 1.399Å, 1.476Å, and 1.085Å
respectively and in the formation of criegee intermediate I2d, the bond
length become elongated around 0.1Å, 0.04Å, and 0.01Å respectively.
During the formation of criegee intermediates, the bond angle decreases
with increasing bond length due to the high electronegativity of Cl
isoprene adduct radical intermediates I1a, I1b, I1c, and I1d. Thus, the
formation of chloroalkenyl peroxy radical criegee intermediate I2a, I2b,
I2c, and I2d are found to be exothermic with the enthalpy values of
-30.523, -30.067, -17.498 and -17.430 kcal/mol and also found to be
exoergic with the Gibbs free energy values of -26.308, -24.315, -12.544,
and -13.9 kcal/mol respectively. The thermodynamically calculated
enthalpy (ΔH) and Gibbs free energy (ΔG) values are compared with the
previously available literature [44], listed in
(Table 2) . The negative values of enthalpy reveal that the
reactions are spontaneous and more feasible. The spontaneity of the
crieege intermediates in the atmosphere further leads to a subsequent
reaction with NO or OH radical.
After that, the intermediates I2a, I2b, I2c, and I2d, the reactions
occur with two different steps. As shown in Scheme. 1, one step
is the formation of intermediates I3a, I3b, I3c, and I3d by the addition
of NO radical and another step is the formation of intermediates I6,
I6c, and I6d by decomposition.