Introduction
The reactions of non-methane hydrocarbons (NMHCs) with atmospheric oxidizing species (such as hydroxyl (OH) radical, nitrate (NO3) radical, ozone (O3) molecule and chlorine radical (Cl˙)) are most important in the earth’s troposphere and these contribute to the formation of secondary organic aerosols (SOAs)[1–4]. Among such non-methane hydrocarbons, 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 which is released into the atmosphere through various biogenic and anthropogenic processes [5,6]. Like methane, the excretion of isoprene in the world is around 500-600 Tgc year-1 due to the unfinished combustion of diesel fuels, gasoline, automobile exhausts, petroleum refining, plastic, or rubber industries and other biogenic volatile organic compounds (BVOCs)[7–9]. Due to this excessive emission, isoprene plays a central role in the tropospheric chemistry related to air quality and climate change. The abundant isoprene in the atmosphere undergoes several chemical processes, which is leading to their potential removal from the atmosphere and transformation processes in the atmosphere. Especially, oxidation processes can reduce the atmospheric lifetime of biogenic isoprene in the daytime and it contributes to the formation of secondary organic aerosols (SOAs)[10].
For the last few decades, there are several experimental and theoretical studies have been reported on the kinetics of reactions of isoprene with atmospheric oxidants. M.L. Ragains and B.J. Finloyson Pitts have experimentally studied the kinetics and mechanism of the reaction of Cl atom with isoprene at 760 Torr and 298K using relative rate techniques[11]. Another important experimental study on isoprene was done byYuri Bedjanian et al., where they studied the kinetics of the reaction of isoprene+Cl and obtained the overall rate coefficient over the temperature range of 233-320 K at 1 Torr[12]. Inseon Suh and Renyi Zhang have investigated the degradation mechanism of Cl initiated isoprene and measured the rate constant between Cl atom and isoprene is (4.0±0.3) ⨯10-10 cm3molecule-1 S-1 at 5 to 10 Torr and 298K [13]. Subsequently, Inseon Suh et al. both experimental and theoretically study the reaction of isoprene+NO3 and they measured the rate coefficient in the pressure range of 5-7 Torr at 298±2K using chemical ionization mass spectrometry (CIMS) detection [14]. Recently, Victoria P. Barber et al reported the formation of four carbon criegee intermediate from ozonolysis of isoprene [15]. Earlier this year, a theoretical investigation on isoprene was done by Xirui Guo et al., where they studied the reaction of isoprene with Cl radical and the reaction rate constants for tight and loose transition states were calculated by RRKM theory and canonical variational transition state theory (CVT) respectively [16]. From the earlier literature, the OH radicals and O atoms are well known most important oxidants in the troposphere, which are responsible for the removal of VOCs in the atmosphere. Meanwhile, Cl radicals are also highly reactive in the coastal and marine boundaries. This highly reactive Cl radical is produced in the marine boundary as a consequence of both direct emission and chemical processes. Generally, particle bound chlorine released into the gas phase through heterogeneous processes on particulate surfaces or sea salt aerosols. Recently, Luo et al reported that the emission of Cl radicals from seawater is estimated to be 1792.6 Tg Cl yr-1 [17] and it is comparably higher than that of other sources (such as dust 15 Tg Cl yr-1 and volcanic eruptions 2 Tg Cl yr-1). Therefore, in the present study aimed to investigate the degradation of isoprene with Cl radical along with their intermediates, transition states and product radicals in the presence of O2, NO and H2O molecules. The earlier investigation mainly focused on primary reaction of isoprene with Cl radical which hinders the understanding of secondary reactions from Cl radical initiated isoprene and formation of SOAs from Cl radical initiated isoprene. A detailed theoretical study on the kinetics of Cl initiated isoprene and the secondary reaction is yet to be known. Therefore, in this work, a quantum chemical study is used to understand the initial and subsequent reaction of isoprene+Cl. The canonical variational transition (CVT) state theory [18]including small curvature tunneling (SCT)[19] is used to calculate the relative rate coefficient for the reaction of isoprene+Cl. The obtained rate coefficients are used to determine the lifetime of isoprene for the removal of Cl radicals. In addition to that, the reaction force analysis is also calculated to find out the structural fate of the reaction and their results are correlated with the relative energy profile in the present work.