Victor G. Khamaganov

Determination of rate constants for reactions of some hydrohaloalkanes with OH radicals and their atmospheric lifetimes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with partly halogenated alkanes using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units 10−13 cm3 molecule−1 s−1): (8.1−1.2+1.5) exp{−(1516±53)/T} for CHF2Cl (HCFC-22); (10.3−1.5+1.8) exp{−(1588±52)/T} for CH2FCF3 (HFC-134a); (11.3−1.6+2.1) exp{−(918±52)/T} for CHCl2CF2Cl (HCFC-122); (9.2−2.0+2.5) exp{−(1281±85)/T} for CHFClCF2Cl (HCFC-123a).The atmospheric lifetimes for the substances have been estimated to be 12.6, 12.9, 1.05, and 4.8 years, respectively, and the accuracy of the estimates is discussed.

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols.

Rate constants for the gas phase reactions of OH(•) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(•) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.

Rate constants for reactions of OH radicals with some Br-containing haloalkanes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with two halons and three of their proposed substitutes and also with CHClBr-CF3 using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units are 10−13 cm3 molecule−1 s−1): (9.3−0.9+1.0) exp{−(1326±33)/T} for CHF2Br; (7.2−0.6+0.7) exp{−(1111±32)/T} for CHFBrCF3; (8.5−0.8+0.9) exp{−(1113±35)/T} for CH2BrCF3; (12.8−1.2+1.5) exp{−(995±38)/T} for CHClBrCF3. The rate constants at 298 K have been estimated to be <2×10−17 cm3 molecule−1 s−1 for CF3Br and CF2Br—CF2Br. The atmospheric lifetimes due to hydroxyl attack have been estimated to be 5.5, 3.3, 2.8, and 1.2 years for CHF2Br, CHFBr—CF3, CH2Br—CF3 and CHClBr—CF3, respectively.

Measurements of rate constants for the OH reactions with bromoform (CHBr3), CHBr2Cl, CHBrCl2, and epichlorohydrin (C3H5ClO).

Measurements of the rate constants for the gas phase reactions of OH radicals with bromoform (CHBr3) and epichlorohydrin (C3H5ClO) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 230-370 K. The temperature dependences of the rate constants can be represented by the following expressions: kBF(230-370 K) = (9.94 ± 0.76) × 10(-13) exp[-(387 ± 22)/T] cm(3) molecule(-1) s(-1) and kECH(230-370 K) = 1.05 × 10(-14)(T/298)(5.16) exp(+1082/T) cm(3) molecule(-1) s(-1). Rate constants for the reactions of OH with CHCl2Br and CHClBr2 were measured between 230 and 330 K. They can be represented by the following expressions: kDCBM(230-330 K) = (9.4 ± 1.3) × 10(-13) exp[-(513 ± 37)/T] cm(3) molecule(-1) s(-1) and kCDBM(230-330 K) = (9.0 ± 1.9) × 10(-13) exp[-(423 ± 61)/T] cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 57, 39, 72, and 96 days, respectively. The total atmospheric lifetimes of the Br-containing methanes due to both reaction with OH and photolysis were calculated to be 22, 50, and 67 days for CHBr3, CHClBr2, and CHCl2Br, respectively.

Absolute Rate Coefficient of the Gas-Phase Reaction between Hydroxyl Radical (OH) and Hydroxyacetone: Investigating the Effects of Temperature and Pressure

The rate coefficient (k1) of the reaction between hydroxyl radical and hydroxyacetone, which remained so far controversial, was determined over the temperature range 290-500 K using pulsed-laser photolysis coupled to pulsed-laser induced fluorescence (PLP-PLIF). Hydroxyl radical was generated by pulsed photolysis of H2O2 at 248 nm. The results show that at a pressure of 50 Torr He, the rate coefficient obeys a negative temperature dependence k1(T) = (1.77 ± 0.19) × 10(-12) exp((353 ± 36)/T) cm(3) molecule(-1) s(-1) for temperatures between 290 and 380 K, in good agreement with the results of Dillon et al. (Phys. Chem. Chem. Phys. 2006, 8, 236) at 60 Torr He. However, always at 50 Torr He but for the higher temperature range 410-500 K, a positive temperature dependence was found: k1(T) = (1.14 ± 0.25) × 10(-11) exp(-(378 ± 102)/T) cm(3) molecule(-1) s(-1), close to the expression obtained by Baasandorj et al. (J. Phys. Chem. A 2009, 113, 10495) for pressures of 2 and 5 Torr He but at lower temperatures, 280-360 K, where their k1(T) values are well below these of Dillon et al. and of this work. Moreover, the rate coefficient k1(301 K) determined as a function of pressure, from 10 to 70 Torr He, shows a pronounced decrease once the pressure is below ∼40 Torr He, thus explaining the disparity between the higher-pressure data of Dillon et al. and the lower-pressure results of Baasandorj et al. The pressure dependence of k1 and of its temperature-dependence below ∼400 K is rationalized by the reaction proceeding via a hydrogen-bonded prereactive complex (PRC) and a submerged transition state, such that at high pressures collisionally thermalized PRCs contribute additional reactive flux over and through the submerged barrier. The high-pressure rate coefficient data both of Dillon et al. and of this work over the combined range 230-500 K can be represented by the theory-based expression k1(T) = 5.3 × 10(-20) × T(2.6) exp(1100/T) cm(3) molecule(-1) s(-1).

High Accuracy Measurements of OH Reaction Rate Constants and IR Absorption Spectra: Substituted 2-Propanols

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 × 10(-11) exp{-883/T} + 1.30 × 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 × 10(-12) exp{-1207/T} + 7.85 × 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 × 10(-12) exp{-1718/T} + 7.32 × 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 × 10(-20) × (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated.

Photochemical properties of some Cl-containing halogenated alkanes.

Rate constants for the gas-phase reactions of OH radicals with three partially halogenated alkanes, CH3Cl (kMC), CHFClCFCl2 (k122a), and CH2FCFCl2 (k132c), were measured using a discharge flow-electron paramagnetic resonance technique over the temperature range from 298 to 460 K. The temperature dependences of the rate constants can be represented by the expressions kMC(298-460 K) = (3.09 ± 0.94) × 10(-12) exp[-(1411 ± 85)/T] cm(3) molecule(-1) s(-1), k122a(298-460 K) = (1.26 ± 0.24) × 10(-12) exp[-(1298 ± 66)/T] cm(3) molecule(-1) s(-1), and k132c(298-370 K) = (8.1 ± 2.2) × 10(-13) exp[-(1247 ± 89)/T] cm(3) molecule(-1) s(-1). The atmospheric lifetimes of CH3Cl, CHFClCFCl2, and CH2FCFCl2 due to their reaction with OH were estimated to be 1.6, 3.5, and 4.5 years, respectively. The UV absorption cross sections of halogenated ethanes, CHFClCFCl2, and CH2FCFCl2, were measured at T = 295 K between 190 and 240 nm, as were those for CHCl2CF2Cl (HCFC-122), CHCl2CF3 (HCFC-123), CHFClCF2Cl (HCFC-123a), and CH3CFCl2 (HCFC-141b). The atmospheric lifetimes due to stratospheric photolysis were also estimated.

Photochemical properties of hydrofluoroethers CH3OCHF2, CH3OCF3, and CHF2OCH2CF3: reactivity toward OH, IR absorption cross sections, atmospheric lifetimes, and global warming potentials.

Rate constants for the gas phase reactions of OH radicals with three partially fluorinated ethers, CH3OCF3 (kHFE-143a), CH3OCHF2 (kHFE-152a), and CHF2OCH2CF3 (kHFE-245fa2), were measured using a discharge flow-electron paramagnetic resonance technique over the temperature range 298-460 K. The temperature dependences of the rate constants can be represented by the following expressions: kHFE-143a(T) = (1.10 ± 0.20) × 10(-12) × exp{-(1324 ± 61)/T} cm(3) molecule(-1) s(-1); kHFE-152a(T) = (11.6 ± 4.2) × 10(-12) × exp{-(1728 ± 133)/T} cm(3) molecule(-1) s(-1); and kHFE-245fa2(T) = (3.04 ± 0.57) × 10(-12) × exp{-(1665 ± 66)/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 5.2, 1.9, and 5.6 years, respectively. The IR absorption cross sections of these fluorinated ethers were measured between 400 and 2000 cm(-1), and their global warming potentials were estimated.