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,
  1. 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.
  2. 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).
  3. 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.
  4. 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
  5. 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.
  6. 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.