CONCLUSION
In this present work, the atmospheric reaction of isoprene initiated by
chlorine atom at various channels and subsequent reactions by the
addition of O2 molecule, NO radical, and
H2O were investigated by using quantum chemical methods.
The kinetics of the primary and subsequent reactions of isoprene were
carried out by CVT/SCT. The electronic structure of reactant, transition
state, intermediate complexes, intermediates, and products were
optimized with M06-2X/6-311+G* level of theory and their corresponding
single point energy was carried out by using CCSD (T)/6-311+G* level of
theory. All reactants, intermediates, and products possess zero
imaginary frequency and transition states possess a single imaginary
frequency. All Transition States were confirmed through Intrinsic
Reaction Coordinates (IRC) to identify the preferred reactants,
intermediates and products. The conclusions of the brief explorations
are summarized below,
- The initial reaction of isoprene was studied by Cl radical addition at
the various position with the C—C bond breaking which leads to the
formation of secondary reaction and its bond length and bond angles
were calculated from the optimized structure.
- All the reaction paths are found to be exothermic and spontaneous with
a maximum energy barrier. Whereas the potential end product (MVK,
MACR) through Cl, O2, and NO radical addition pathway
is found to be more spontaneous and feasible compared to the formation
of the end product (MVK, MACR) through Cl, O2, and
H2O addition pathway. The calculated ΔH = -0.129
kcal/mol (MVK through Cl, O2, and NO) >
ΔH= 6.219 kcal/mol (MVK through Cl, O2, and
H2O) and ΔG= -5.479 kcal/mol (MACR through Cl,
O2, and NO) > ΔG= 2.369 kcal/mol (MACR
through Cl, O2, and H2O).
- The most favorable active site of isoprene is found to be at terminal
C4 and C5 position with its corresponding value is 0.289 and 0.099 by
using Condensed Fukui Function analysis and also it elucidates the
concept of bond breaking and bond forming.
- The calculated rate constants for the reaction between isoprene and Cl
radical is found to be 4.89⨯10-11,
6.91⨯10-10, 1.63⨯10-10 and
8.12⨯10-10 cm3/molecule/sec
respectively at 278K and it is compared with the experimental rate
coefficient of 4.6⨯10-10 at 298K. The lifetime of
Cl-isoprene adduct radical is estimated to be 6.49 hours at the
temperature of 298K using the average atmospheric concentration of Cl
atom
- The reaction force analysis reveals that the geometrical rearrangement
of each structure plays a major part than electronic reordering. 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.
- Conclusively, the kinetic and thermodynamic results reveal that the
electrophilic addition of Cl radical to the terminal carbon atom plays
the dominant role in the marine boundary and H2O
molecule can affect the formation of SOAs such as MVK and MACR. The
calculated lifetime in this work reveals that the isoprene degrade
quickly in the atmosphere.