Energy and reaction force profiles
In Figure 14 & 15, the energy and reaction force profiles along with the partition of the reaction coordinates are shown. As stated by the transition state theory (TST), an energy profile of the all reaction path are characterized in three critical points such as two minimum points, one reactant (\(\xi_{R})\) and another product (\(\xi_{p})\), and one maximum point for the transition structure (\(\xi_{\text{TS}})\). In the reaction force profile, the critical points are divided into three separate reaction regions that are reactants\(\ \xi_{R}\rightarrow\xi_{1}\),\(\xi_{1}\rightarrow\xi_{2}\) and \(\xi_{2}\rightarrow\)product\({(\ \xi}_{p})\). The first region is the preparation region associated with the reactants (\(\xi_{R}\),\(\xi_{1}\)), where the structural distortion takes place mainly by the bond stretching, angle bending, etc. In this region, when the negative (retarding) reaction force reaches its greatest strength at the point\(\xi_{1}\), the reaction force \(F\left(\xi\right)\) becomes minimum. The second region is a transition state region which is located from \(\xi_{1}\) to its maximum point at \(\xi_{2}\) and where the structural rearrangements take place by the bond breaking and bond forming. All of these extensive changes produce a positive reaction force which starts at \(\xi_{1}\)that gradually overcomes the retarding one. At the point\(\ \xi_{\text{TS}}\), the positive deriving force is dominant and continues to increase until it reaches its maximum reaction force at\(\xi_{2}\) where the system has been changed as the states of the products. Finally, the third region is the product region which is located from the F (ξ) maximum at \(\xi_{2}\) to its minimum point at\(\xi_{p}\). In this region, the system involves structural relaxation to reach its final state after the maximum point at \(\xi_{2}\) and the reaction force F (ξ) declines to zero to reach the equilibrium geometry of the products.
On the other hand, reaction force calculation has been performed to estimate the amount of work done in each elementary step of the chemical reaction. The calculated work is done for the formation of Cl isoprene adduct radical intermediates I1a, I1b, I1c, and I1d are listed in supplementary (Table S5) . From the (Table S5), the formation of Cl-isoprene adduct radical intermediates (I1a, I1b, I1c, and I1d), 70%, 77.5%, 74.1%, and 77.6% of activation energy is due to the geometrical rearrangement and the remaining 30%, 22.5%, 25.9%, and 22.4% of activation energy is due to electronic reordering respectively. In this mechanism, the potential end products P3 (MVK with CH2O) and P4 (MACR with CH2O) is formed through the addition of Cl, O2 and NO radical pathway. During the formation P3, 71% & 29% of activation energies are mainly due to the geometrical rearrangement and electronic reordering respectively. Similarly, the formation of P4, 72% & 28% of activation energies were due to geometrical rearrangement and electronic reordering respectively. On the other hand, the potential end products P6 (MVK with H2O2) and P7 (MACR with H2O2) is formed through the addition of Cl, O2, and H2O pathway. In the formation of P6 and P7, 75% & 76% of activation energies are due to geometrical rearrangement, the remaining 25% & 24% of activation energies are due to electronic reordering respectively listed in(Table S6). This analysis shows that the potential product P3 and P4 are formed with less activation energy than the potential product of P6 and P7, which confirms that the formation of MVK and MACR through Cl, O2, and NO radical addition pathway is more favorable than the formation of MVK and MACR through Cl, O2, and H2O addition pathway. The kinetic study also reveals that the formation of MVK and MACR via Cl, O2, and NO radical addition pathway is more favorable. The relative energy profile also shows that the formation of MVK through Cl, O2, and NO radical addition pathway has less activation energy than the formation of MVK through Cl, O2, and H2O addition pathway. Similarly, the formation of MACR through Cl, O2, and NO radical addition pathway has less activation energy than the formation of MACR via Cl, O2, and H2O addition pathway. Thus the result shows that the H2O molecule can affect the formation of SOAs such as MVK and MACR. All previous results are in good agreement with this reaction force analysis.