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.

A Kinetic Mechanism for CF3I Inhibition of Methane-Air Flames

The influence of CF3I on the burning velocity of methane-air flame is experimentally and numerically studied. Experimental results demonstrate that the inhibition effectiveness of CF3I is very close to that of CF3Br. A detailed kinetic model of flame inhibition by CF3I is presented, based on an updated version of a previous model. The kinetic model contains 1072 reactions with 115 species including 10 iodine-containing species. Modeling results demonstrate good agreement with experimental data, and both experiments and calculations show that CF3I is only slightly less effective at reducing the burning velocity than CF3Br. The flame structure predicted from numerical simulations is analyzed and shows that main reactions of the inhibition cycle of CF3I are: H+HI=H2+I; H+I+M=HI+M; I+I+M=I2+M; H+I2=HI+I; I+CH3+M=CH3I+M; H+CH3I=CH3+HI; I+HCO=HI+CO; HI+OH=H2O+I and O+HI=I+OH.

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

The gas-phase kinetics of the reactions of IO radicals with ethyl formate (EF) and ethyl acetate (EA) were investigated experimentally using cavity ring-down spectroscopy (CRDS). IO radicals were generated in situ in the CRD reaction zone by photolyzing a mixture of (CH₃I + O₃ + N₂) at 248 nm and thereby probed at 445.04 nm. The rate coefficients for the reactions (IO + EF) and (IO + EA) were measured at a total pressure of 65 Torr of N₂ in the temperature range of 258-358 and 260-360 K, respectively. The rate coefficients for the reactions (IO + EF) and (IO + EA) were measured experimentally at room temperature to be kExpt,298KIO+EF = (3.38 ± 0.67) × 10^-14 and kExpt,298KIO+EA = (1.56 ± 0.30) × 10^-13 cm³ molecule^-1 s^-1, respectively. The effects of pressure and photolysis laser fluence on the kinetics of test reactions were found to be negligible within the experimental uncertainties for the studied range. To complement our experimental findings, the kinetics of the title reactions were investigated theoretically using canonical variational transition state theory (CVT) with small curvature tunnelling (SCT) at the CCSD(T)//M06-2X/def2-SV(P) level of theory in temperatures between 200 and 400 K. Very good agreement was observed between the experimentally measured and theoretically calculated rate coefficients for both the reactions at 298 K. The thermochemical parameters as well as the branching ratios for the title reactions are also discussed in this study.

Threshold Photoelectron Spectroscopy of IO and HOI

Iodine oxides appear as reactive intermediates in atmospheric chemistry. Here, we investigate IO and HOI by mass-selective threshold photoelectron spectroscopy (ms-TPES), using synchrotron radiation. IO and HOI are generated by photolyzing iodine in the presence of ozone. For both molecules, accurate ionization energies are determined, 9.71±0.02 eV for IO and 9.79±0.02 eV for HOI. The strong spin-spin interaction in the ³ Σ^- ground state of IO⁺ leads to an energy splitting into the Ω=0 and Ω=±1 sublevels. Upon ionization, the I-O bond shortens significantly in both molecules; thus, a vibrational progression, assigned to the I-O stretch, is apparent in both spectra.

Kinetic and Mechanistic Studies of the I2/O3 Photochemistry

The atmospherically relevant chemistry generated by photolysis of I2/O3 mixtures has been studied at 298 K in the pressure range from 10 to 400 hPa by using a laboratory flash photolysis setup combining atomic resonance and molecular absorption spectroscopy. The temporal behaviors of I, I(2), IO, and OIO have been retrieved. Conventional kinetic methods and numerical modeling have been applied to investigate the IO self-reaction and the secondary chemistry. A pressure independent value of k(IO + IO) = (7.6 +/- 1.1) x 10(-11) cm(3) molecule-1 s(-1) has been determined. The pressure dependence of the branching ratios for the I + OIO and IOIO product channels in the IO + IO reaction have been determined and have values of 0.45 +/- 0.10 and 0.44 +/- 0.13 at 400 hPa, respectively. The branching ratios for the 2I + O(2) and I(2) + O(2) product channels are pressure independent with values of 0.09 +/- 0.06 and 0.05 +/- 0.03, respectively. The sensitivity analysis indicates that the isomer IOIO is more thermally stable than predicted by theoretical calculations. A reaction scheme comprising OIO polymerization steps has been shown to be consistent with the temporal behaviors recorded in this study. For simplicity, the rate coefficient has been assumed to be the same for each reaction (OIO)(n) + IO --> (OIO)(n+1), n = 1, 2, 3, 4. The lower limit obtained for this rate coefficient is (1.2 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) at 400 hPa. Evidence for the participation of IO in the polymerization mechanism also has been found. The rate coefficient for IO attachment to OIO and to small polymers has been determined to be larger than (5 +/- 2) x 10(-11) cm(3) molecule(-1) s(-1) at 400 hPa. These results provide supporting evidence for atmospheric particle formation induced by polymerization of iodine oxides.

Kinetic Study of IO Radical with RO2 (R = CH3, C2H5, and CF3) Using Cavity Ring-Down Spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).