Kinetics of IO radicals with ethyl formate and ethyl acetate: a study using cavity ring-down spectroscopy and theoretical methods

Kinetics of IO radicals with C1, C2 aliphatic alcohols in tropospherically relevant conditions

Kinetics of the reaction of IO radicals with methanol (MeOH) and ethanol (EtOH) were experimentally studied in the gas phase using pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS). IO radicals were produced in situ at the reaction zone by photolysing a mixture of precursors (CH₃I + O₃ + N₂) at 248 nm and thereby electronically excited at 445.04 nm. The rate coefficients for the reactions of (IO + MeOH) and (IO + EtOH) were measured at a total pressure of 60 Torr/N₂ in the range of 258-360 K. At room temperature, the experimental rate coefficients of the title reactions were measured to be [Formula: see text] and [Formula: see text]. Dependencies of the kinetics with photolysis laser fluence and experimental pressures were verified. Effects of pressure over the kinetic behaviour of the studied systems were observed to be insignificant within the statistical uncertainties when studied in the range of ~ 30-150 Torr/N₂, whereas a minor and linear fluence dependency was observed within the studied limit. From the measured kinetic parameters, the atmospheric lifetimes of MeOH and EtOH were calculated in the tropospherically relevant conditions regarding their reactions with important atmospheric oxidants like Cl atom, OH and IO radicals. To complement experimental results, kinetics and thermochemistry for the title reactions were investigated theoretically via canonical variational transition state (CVT) theory in combination with small curvature tunnelling (SCT) corrections with a dual-level Interpolated Single Point Energy (ISPE) approach at the CCSD(T)/def2-QZVPP//M06-2X/def2-TZVPP level of theory/basis set in the temperatures between 200 and 400 K. Good degree of agreement was encountered between experimentally measured and theoretically calculated rate coefficients. This article also discusses the thermochemical parameters and kinetic branching ratios (BRs) of all the pathways involved in the title reactions.

Experimental and Theoretical Investigation of Reactions of Formyl (HCO) Radicals in the Gas Phase: (I) Kinetics of HCO Radicals with Ethyl Formate and Ethyl Acetate in Tropospherically Relevant Conditions

Formyl (HCO) radicals were generated in situ in the gas phase via the photolysis of glyoxal in N₂ at 248 nm using the pulsed laser photolysis-cavity ring-down spectrometry technique, and the absorption cross-section of the radical was measured to be σ_HCO = (5.3 ± 0.9) × 10^-19 cm² molecule^-1 at 298 K and 615.75 nm, which was the probing wavelength. The kinetics of the reactions of HCO radicals with ethyl formate (EF) and ethyl acetate (EA) were investigated experimentally in the temperature range of 260-360 (±2) K at a pressure of 60 Torr/N₂. The absolute rate coefficient for the reaction between HCO and EF was measured to be kHCO+EFExpt(298 K) = (1.39 ± 0.30) × 10^-14 cm³ molecule^-1 s^-1 at ambient temperature, whereas that for the reaction of HCO with EA was measured tobe kHCO+EATheory(298 K) = (2.05 ± 0.43) × 10^-14 cm³ molecule^-1 s^-1. The reaction of HCO with EA was faster than that with EF, which might be due to the greater stability of the formed radical intermediate due to hyperconjugative and inductive effects. The dependency of the measured kinetics on experimental pressures and laser fluences was examined within a certain range. To complement the experiments, kinetics of the title reactions in the temperature range of 200-400 K were deciphered theoretically via the canonical variational transition-state theory with small-curvature tunneling and interpolated single-point energy (CVT/SCT/ISPE) method using a dual-level approach at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory/basis set. A good degree of agreement was detected between the rate coefficients measured experimentally and those calculated theoretically both at room temperature and throughout the range of temperatures studied. The kinetic branching ratios and thermochemistry of the reactions were investigated to understand the thermodynamic feasibility and kinetic lability of each pathway throughout the studied temperatures. Atmospheric implications of these reactions of HCO radicals are also discussed.