Kinetics and Degradation Mechanism of Atmospheric Isoprene (2-methyl-1, 3-butadiene (C5H8)) with Chlorine Radical and Its Derivatives – A DFT Study

Isoprene (2-methyl-1, 3-butadiene (C5H8)) is one of the most prominent and abundant non-methane hydrocarbon existing in the lower level of the troposphere. In this work, possible reaction mechanism of chlorine (Cl) radical initiated isoprene and its subsequent reactions are investigated using quantum chemical methods. The calculated thermodynamic result shows that the reaction of isoprene with the Cl radical at the terminal C=C bond position plays an important role to predict the end products. The calculated rate coefficient for the reaction between isoprene and Cl radicals (Cl addition at C1, C3, C4 and C5 positions) is found to be 4.89⨯10-11, 6.91⨯10-10, 1.63⨯10-10 and 8.12⨯10-10 cm3/molecule/sec at 298K. The branching ratio and atmospheric lifetime have been calculated from the reaction rate coefficient values of isoprene+Cl. The reaction force analysis predicts Cl radical addition at the terminal C=C bond position plays a dominant role by structural rearrangement. 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. Further, the subsequent reaction of Cl-isoprene adduct radical helps for the development of ozone layer during daytime.