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.