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.