This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c9cp01312a Formation of Criegee intermediates and peroxy acids: a computational study of gas-phase 1,3-cycloaddition of ozone with catechol† Debojit Kumar Deb and Biplab Sarkar * A detailed theoretical investigation of gas-phase 1,3-cycloaddition of ozone with catechol is presented to explore the discrepancies in previous theoretical and experimental rate constants. DFT based PBE, TPSS, B3LYP, B3PW91, M06-2X, wB97XD, MN15 and high-level CCSD(T) methods are used for the calculation. Canonical transition state theory has been used to calculate the rate coefficients of individual steps. The calculated rate coefficients are compared with the experimental and previously calculated rate constant. The possible pathways for primary ozonide (POZ) formation and subsequent reactions to yield the Criegee Intermediates (CI) and peroxy acids (POA) are investigated. The endo-POZ may undergo conversion to exo-POZ or form the Creigee Intermediates. This work shows a novel pathway by which the exo-POZ can form more stable and chemically different species, peroxy acids, by abstracting an H atom from the OH group. 1 Introduction Biogenic and anthropogenic emissions of volatile organic compounds (VOCs) from biomass burning, agro-industrial settings and gasoline combustion produce precursors for secondary organic aerosol (SOA) formation. 1 Aromatic hydrocarbons are an important class of VOCs that are present in the troposphere. 2–4 Dihydroxy- benzenes such as catechol are the most common gas phase organic constituents (B50 ppbv) resulting from biomass burning, pyrolysis and combustion. 5–7 The major aromatic compounds found in the troposphere, like benzene and toluene, react mainly with hydroxyl radicals to produce phenol and cresol isomers and even further oxidation leads to the formation of catechols (1,2-benzenediols) in high yields of about 80%. 8–10 In addition, from anthropogenic sources such as wood combustion 11 and atmospheric oxidation of gasoline, catechol can be directly emitted into the atmosphere. These VOCs once generated are chemically transformed by reac- tions with either ozone, hydroxyl and nitrate radicals, and chlorine atoms or by photo dissociations affecting the Earths radiative budget. 8,10,12–14 Moreover, the net ozone production cycle in the troposphere, which is quantitatively expressed by the ozone creation potential (OCP), 12,15–19 is substantially influenced by these VOCs. Ozone at the same time is beneficiary and harmful. Absorp- tion of the harmful UV radiation in the stratosphere by ozone is vital for all life on Earth. On the contrary, being a very powerful oxidizer and a greenhouse gas when present in the troposphere it is quite harmful. Tropospheric ozone can be brought down by reducing the emission of VOCs. Gas phase oxidation initiated by reaction with ozone is an important pathway for the degradation and transformation of unsaturated organic compounds in the atmosphere. The kinetic parameters of ozone reactions in the gas phase for numerous classes of hydrocarbons are found in the literature. 2–4 Though the ozonolysis of aromatic compounds is unfavorable due to the loss of resonance in the aromatic ring, from experiments relatively fast ozonolysis of catechol has been observed. 20 Different experimental techniques based on spectroscopy as well as quantum chemical calculations showed that 1,3 cycloaddition of ozone Department of Chemistry, North-Eastern Hill University, Shillong 793022, India. E-mail: biplabs@nehu.ac.in; Tel: +91 364 2722603 † Electronic supplementary information (ESI) available: Coordinates for optimized geometries of reactants, pre-reactive complexes, transition states and products for the 1,3 cycloaddition of ozone with catechol. IRC paths for the steps involved in 1,3 cycloaddition of ozone with catechol (Fig. S1–S4). Scan for TS search (Fig. S5). The forward and backward energy barriers for the formation of primary ozonides from endo and exo addition of ozone to catechol (Table S1). Absolute energies, zero point energies (ZPE), thermal correction to energy (DE), thermal correction to enthalpy (DH), thermal correction to Gibbs free energy (DG) in atomic units, total partition function (Q) of stable conformers, transition states and pre-reactive complexes and imaginary frequencies of transition states (Im o in cm À1 ) obtained from B3LYP/ 6-311+G(2df,2p) and MN15/6-311+G(2df,2p) methods (Tables S2 and S3). Energies (in a.u.) of conformers, transition states and pre-reactive complexes calculated at CCSD(T)/cc-pVXZ(X = 2, 3) and the corresponding T1 diagnostics values and the CCSD(T)/CBS limit for the structure obtained from the B3LYP/6-311+G(2df,2p) method (Table S4). Zero-point corrected relative energies (kJ mol À1 ) of the reactants, pre-reactive complexes, transition states, intermediates, and products calculated by the B3LYP/6-311+G(2df,2p) and CCSD(T)/CBS//B3LYP/6-311+G(2df,2p) methods (Table S5). Forward energy barriers (E F ), reverse energy barriers (E R ) in kJ mol À1 for the individual reactions involved in the 1,3 cycloaddition of ozone with catechol (Table S6). See DOI: 10.1039/c9cp01312a Received 7th March 2019, Accepted 16th May 2019 DOI: 10.1039/c9cp01312a rsc.li/pccp PCCP PAPER Published on 16 May 2019. Downloaded by North Eastern Hill University on 5/30/2019 9:50:01 AM. View Article Online View Journal