Kinetics of the reactions of chlorinated methyl radicals (CH2Cl, CHCl2, and CCl3) with NO2 in the temperature range 220-360 K

The kinetics of the reactions of chlorinated methyl radicals (CH2Cl, CHCl2, and CCl3) with NO2 have been studied in direct measurements at temperatures between 220 and 360 K using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated at 193 or 248 nm by pulsed laser photolysis of appropriate precursors. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of NO2 being in large excess over radical concentrations. The bimolecular rate coefficients of all three reactions are independent of the bath gas (He or N2) and pressure within the experimental range (1-6 Torr) and are found to depend on temperature as follows: k(CH2Cl + NO2) = (2.16 +/- 0.08) x 10(-11) (T/300 K)(-1.12+/-0.24) cm3 molecule(-1) s(-1) (220-363 K), k(CHCl2 + NO2) = (8.90 +/- 0.16) x 10(-12) (T/300 K)(-1.48+/-0.13) cm3 molecule(-1) s(-1) (220-363 K), and k(CCl3 + NO2) = (3.35 +/- 0.10) x 10(-12) (T/300 K)(-2.2+/-0.4) cm3 molecule(-1) s(-1) (298-363 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about +/-25%. In the reactions CH2Cl + NO2, CHCl2 + NO2, and CCl3 + NO2, the products observed are formaldehyde, CHClO, and phosgene (CCl2O), respectively. In addition, a weak signal for the HCl formation has been detected for the CHCl2 + NO2 reaction.     

Kinetics of the reactions of CH2I, CH2Br, and CHBrCl radicals with NO2 in the temperature range 220-360 K

The kinetics of the CH2I + NO2, CH2Br + NO2, and CHBrCl + NO2 reactions have been studied at temperatures between 220 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time-resolved measurements to obtain reaction rate coefficients under pseudo-first-order conditions. The bimolecular rate coefficients of all three reactions are independent of the bath gas (He or N2) and pressure within the experimental range (2-6 Torr) and are found to depend on temperature as follows: k(CH2I + NO2) = (2.18 +/- 0.07) x 10(-11) (T / 300 K)(-1.45) (+/- 0.22) cm3 molecule(-1) s(-1) (220-363 K), k(CH2Br + NO2) = (1.76 +/- 0.03) x 10(-11) (T/300 K)(-0.86) (+/- 0.09) cm3 molecule(-1) s(-1) (221-363 K), and k(CHBrCl + NO2) = (8.81 +/- 0.28) x 10(-12) (T/300 K)(-1.55) (+/- 0.34) cm3 molecule(-1) s(-1) (267-363 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about +/-25%. In the CH2I + NO2 and CH2Br + NO2 reactions, the observed product is formaldehyde. For the CHBrCl + NO2 reaction, the product observed is CHClO. In addition, I atom and iodonitromethane (CH2INO2) or iodomethyl nitrite (CH2IONO) formations have been detected for the CH2I + NO2 reaction.     

Temperature dependence and kinetic isotope effects for the OH + HBr reaction and H/D isotopic variants at low temperatures (53-135 K) measured using a pulsed supersonic Laval nozzle flow reactor

The reactions of OH + HBr and all isotopic variants have been measured in a pulsed supersonic Laval nozzle flow reactor between 53 and 135 K, using a pulsed DC discharge to create the radical species and laser induced fluorescence on the A 2sigma <-- X 2pi (v' = 1 <-- v' = 0) transition. All reactions are found to possess an inverse temperature dependence, in accord with previous work, and are fit to the form k = A(T/298)(-n), with k1 (OH + HBr) = (10.84 +/- 0.31) x 10(-12) (T/298)(-0.67+/-0.02) cm3/s, k2 (OD + HBr) = (6.43 +/- 2.60) x 10(-12) (T/298)(-1.19+/-0.26) cm3/s, k3 (OH + DBr) = (5.89 +/- 1.93) x 10(-12) (T/298)(-0.76+/-0.22) cm3/s, and k4 (OD + DBr) = (4.71 +/- 1.56) x 10(-12) (T/298)(-1.09+/-0.21) cm3/s. A global fit of k vs T over the temperature range 23-360 K, including the new OH + HBr data, yields kT = (1.06 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.11) cm3/s, and (0.96 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.03) exp((-2.88+/-1.82 K)/T) cm3/s, in accord with previous fits. In addition, the primary and secondary kinetic isotope effects are found to be independent of temperature within experimental error over the range investigated and take on the value of (kH/kD)(AVG) = 1.64 for the primary effect and (kH/kD)(AVG) = 0.87 for the secondary effect. These results are discussed within the context of current experimental and theoretical work.     

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

Kinetic studies of the ClO + ClO association reaction as a function of temperature and pressure

The kinetics of the association reaction of ClO radicals: ClO + ClO + M --> Cl2O2+ M (1), have been investigated as a function of temperature T between 206.0-298.0 K and pressure p between 25-760 Torr using flash photolysis with time-resolved UV absorption spectroscopy. ClO radicals were generated following the photolysis of Br2/Cl2O mixtures in nitrogen diluent gas. Charge coupled device (CCD) detection of time resolved absorptions was used to monitor ClO radicals over a broad wavelength window covering the ClO (A 2Pi<-- X 2Pi) vibronic absorption bands. The high pass filtered ClO absorption cross sections were calibrated as a function of temperature between T = 206.0-320 K, and exhibit a negative temperature dependence. The ClO association kinetics were found to be more rapid than those reported in previous studies, with limiting low and high pressure rate coefficients, in nitrogen bath gas, k0 = (2.78 +/- 0.82) x 10(-32) x (T/300)(-3.99 +/- 0.94) molecule(-2) cm6 s(-1) and k(infinity) = (3.37 +/- 1.67) x 10(-12) x (T/300)(-1.49 +/- 1.81) molecule(-1) cm3 s(-1), respectively, (obtained with the broadening factor F(c) fixed at 0.6). Errors are 2sigma. The pressure dependent ClO association rate coefficients (falloff curves) exhibited some discrepancies at low pressures, with higher than expected rate coefficients on the basis of extrapolation from high pressures (p > 100 Torr). Reanalysis of data excluding kinetic data recorded below p = 100 Torr gave k0 = (2.79 +/- 0.85) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1) and k(infinity) = (3.44 +/- 1.83)x 10(-12) x (T/300)(-1.73 +/- 1.91) molecule(-1) cm3 s(-1). Potential sources of the low pressure discrepancies are discussed. The expression for k(0) in air bath gas is k0 = (2.62 +/- 0.80) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1). These results support upward revision of the ClO association rate coefficient recommended for use in stratospheric models, and the stratospheric implications of the results reported here are briefly discussed.     

A kinetic study of the reactions of Fe+ with N2O, N2, O2, CO2 and H2O, and the ligand-switching reactions Fe+.X + Y --> Fe+.Y + X (X = N2, O2, CO2; Y = O2, H2O)

A series of reactions involving Fe(+) ions were studied by the pulsed laser ablation of an iron target, with detection of ions by quadrupole mass spectrometry at the downstream end of a fast flow tube. The reactions of Fe(+) with N(2)O, N(2) and O(2) were studied in order to benchmark this new technique. Extending measurements of the rate coefficient for Fe(+) + N(2)O from 773 K to 185 K shows that the reaction exhibits marked non-Arrhenius behaviour, which appears to be explained by excitation of the N(2)O bending vibrational modes. The recombination of Fe(+) with CO(2) and H(2)O in He was then studied over a range of pressure and temperature. The data were fitted by RRKM theory combined with ab initio quantum calculations on Fe(+).CO(2) and Fe(+).H(2)O, yielding the following results (120-400 K and 0-10(3) Torr). For Fe(+) + CO(2): k(rec,0) = 1.0 x 10(-29) (T/300 K)(-2.31) cm(6) molecule(-2) s(-1); k(rec,infinity) = 8.1 x 10(-10) cm(3) molecule(-1) s(-1). For Fe(+) + H(2)O: k(rec,0) = 5.3 x 10(-29) (T/300 K)(-2.02) cm(6) molecule(-2) s(-1); k(rec,infinity) = 2.1 x 10(-9) (T/300 K)(-0.41) cm(3) molecule(-1) s(-1). The uncertainty in these rate coefficients is determined using a Monte Carlo procedure. A series of exothermic ligand-switching reactions were also studied at 294 K: k(Fe(+).N(2) + O(2)) = (3.17 +/- 0.41) x 10(-10), k(Fe(+).CO(2) + O(2)) = (2.16 +/- 0.35) x 10(-10), k(Fe(+).N(2) + H(2)O) = (1.25 +/- 0.14) x 10(-9) and k(Fe(+).O(2) + H(2)O) = (8.79 +/- 1.30) x 10(-10) cm(3) molecule(-1) s(-1), which are all between 36 and 52% of their theoretical upper limits calculated from long-range capture theory. Finally, the role of these reactions in the chemistry of meteor-ablated iron in the upper atmosphere is discussed. The removal rates of Fe(+) by N(2), O(2), CO(2) and H(2)O at 90 km altitude are approximately 0.1, 0.07, 3 x 10(-4) and 1 x 10(-6) s(-1), respectively. The initially formed Fe(+).N(2) and Fe(+).O(2) are converted into the H(2)O complex at approximately 0.05 s(-1). Fe(+).H(2)O should therefore be the most abundant single-ligand Fe(+) complex in the mesosphere below 90 km.     

CF3CF=CH2 and (Z)-CF3CF=CHF: temperature dependent OH rate coefficients and global warming potentials

Rate coefficients over the temperature range 206-380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF(3)CF=CH(2)), k(1)(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF(3)CF=CHF), k(2)(T), which are major components in proposed substitutes for HFC-134a (CF(3)CFH(2)) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N(2)). For CF(3)CF=CH(2), the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k(1)(T)=(1.26+/-0.11) x 10(-12) exp[(-35+/-10)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K)=(1.12+/-0.09) x 10(-12) cm(3) molecule(-1) s(-1). For (Z)-CF(3)CF=CHF, the rate coefficients are given by the non-Arrhenius expression k(2)(T)=(1.6+/-0.2) x 10(-18)T(2) exp[(655+/-50)/T] cm(3) molecule(-1) s(-1) where k(2)(296 K)=(1.29+/-0.06) x 10(-12) cm(3) molecule(-1) s(-1). Over the temperature range most relevant to the atmosphere, 200-300 K, the Arrhenius expression k(2)(T)=(7.30+/-0.7) x 10(-13) exp[(165+/-20)/T] cm(3) molecule(-1) s(-1) reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2sigma (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF(3)CF=CH(2) and (Z)-CF(3)CF=CHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss.     

Wide temperature range (T = 295 K and 770-1305 K) study of the kinetics of the reactions HCO + NO and HCO + NO2 using frequency modulation spectroscopy

The rate constants for , HCO + NO --> HNO + CO, and , HCO + NO(2)--> products, have been measured at temperatures between 770 K < T < 1305 K behind reflected shock waves and, for the purpose of a consistency check, in a slow flow reactor at room temperature. HCO radicals were generated by 193 nm excimer laser photolysis of diluted gas mixtures containing glyoxal, (CHO)(2), and NO or NO(2) in argon and were monitored using frequency modulation (FM) absorption spectroscopy. Kinetic simulations based on a comprehensive reaction mechanism showed that the rate constants for the title reactions could be sensitively extracted from the measured HCO profiles. The determined high temperature rate constants are k(1)(769-1307 K) = (7.1 +/- 2.7) x 10(12) cm(3) mol(-1) s(-1) and k(2)(804-1186 K) = (3.3 +/- 1.8) x 10(13) cm(3) mol(-1) s(-1). The room temperature values were found to be in very good agreement with existing literature data and show that both reactions are essentially temperature independent. The weak temperature dependence of can be explained by the interplay of a dominating direct abstraction pathway and a complex-forming mechanism. Both pathways yield the products HNO + CO. In contrast to , no evidence for a significant contribution of a direct high temperature abstraction channel was found for . Here, the observed temperature independent overall rate constant can be described by a complex-forming mechanism with several product channels. Detailed information on the strongly temperature dependent channel branching ratios is provided. Moreover, the high temperature rate constant of , OH + (CHO)(2), has been determined to be k(7) approximately 1.1 x 10(13) cm(3) mol(-1) s(-1).     

Shock tube/laser absorption measurements of the reaction rates of OH with ethylene and propene

Reaction rates of hydroxyl (OH) radicals with ethylene (C?H?) and propene (C?H?) were studied behind reflected shock waves. OH + ethylene → products (rxn 1) rate measurements were conducted in the temperature range 973-1438 K, for pressures from 2 to 10 atm, and for initial concentrations of ethylene of 500, 751, and 1000 ppm. OH + propene → products (rxn 2) rate measurements spanned temperatures of 890-1366 K, pressures near 2.3 atm, and initial propene concentrations near 300 ppm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH?)?-CO-OH, and monitored by laser absorption near 306.7 nm. Rate constants for the reactions of OH with ethylene and propene were extracted by matching modeled and measured OH concentration time-histories in the reflected shock region. Current data are in excellent agreement with previous studies and extend the temperature range of OH + propene data. Transition state theory calculations using recent ab initio results give excellent agreement with our measurements and other data outside our temperature range. Fits (in units of cm3/mol/s) to the abstraction channels of OH + ethylene and OH + propene are k? = 2.23 × 10? (T)(2.745) exp(-1115 K/T) for 600-2000 K and k? = 1.94 × 10? (T)(2.229) exp(-540 K/T) for 700-1500 K, respectively. A rate constant determination for the reaction TBHP → products (rxn 3) was also obtained in the range 745-1014 K using OH data from behind both incident and reflected shock waves. These high-temperature measurements were fit with previous low-temperature data, and the following rate expression (0.6-2.6 atm), applicable over the temperature range 400-1050 K, was obtained: k? (1/s) = 8.13 × 10?12 (T)(7.83) exp(-14598 K/T).     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.     

Kinetic study of the reactions of Cl atoms with CF3CH2CH2OH, CF3CF2CH2OH, CHF2CF2CH2OH, and CF3CHFCF2CH2OH

The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF(3)CH(2)CH(2)OH (1), CF(3)CF(2)CH(2)OH (2), CHF(2)CF(2)CH(2)OH (3), and CF(3)CHFCF(2)CH(2)OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm(3) molecule(-1) s(-1)): k(1) = (4.42 +/- 0.48) x 10(-11) exp(-255 +/- 20/T); k(1)(303) = (1.90 +/- 0.17) x 10(-11), k(2) = (2.23 +/- 0.31) x 10(-11) exp(-1065 +/- 106/ T); k(2)(303) = (6.78 +/- 0.63) x 10(-13), k(3) = (8.51 +/- 0.62) x 10(-12) exp(-681 +/- 72/T); k(3)(303) = (9.00 +/- 0.82) x 10(-13) and k(4) = (6.18 +/- 0.84) x 10(-12) exp(-736 +/- 42/T); k(4)(303) = (5.36 +/- 0.51) x 10(-13). The quoted 2sigma uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (R(F)-CHO; R(F) = -CH(2)CF(3), -CF(2)CF(3), -CF(2)CHF(2), and -CF(2)CHFCF(3)), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the alpha-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm(3) molecule(-1) s(-1)) = (5.8 +/- 1.4) - (1.56 +/- 0.13) x (ionization potential (in eV)).     

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.     

Reaction kinetics of the addition of atomic sulfur to nitric oxide

The reaction of S((3)P(J)) with NO ((2)Pi) in an Ar bath gas has been studied by the laser photolysis-resonance fluorescence technique over 300-810 K at pressures from 60 to 800 mbar. The observed second-order rate constants are close to the low-pressure limit. Fitting of Troe's formalism to experiment, with an estimated F(cent) = 0.78 exp(-T/7445) and k(infinity) given subsequently, yields k(0) = (6.2+/-0.6) x 10(-33) exp(+ (940+/-40)/T) cm(6) molecule(-2) s(-1). Error limits are +/-25%. A theoretical analysis of this value suggests that the average energy transferred during collisions between Ar and the excited intermediate is DeltaE = -360(-160) (+90) cm(-1). Over 300-800 K, the high-pressure limit is predicted to be k(infinity) = 2.2 x 10(-10) (T/300)(0.24) cm(3) molecule(-1) s(-1). Doublet and quartet adducts between S and NO were characterized via CBS-QB3 theory. The kinetic data can be rationalized with SNO ((2)A(')) as the major product, and an ab initio estimate of Delta(f)H(298) for SNO is 176+/-8 kJ mol(-1).     

Ab initio study of the ClO + NH2 reaction: prediction of the total rate constant and product branching ratios

The mechanism for ClO + NH2 has been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies have been refined by single-point calculations with the modified Gaussian-2 method, G2M(CC2). Ten stable isomers have been located and a detailed potential energy diagram is provided. The rate constants and branching ratios for the low-lying energy channel products including HCl + HNO, Cl + NH2O, and HOCl + 3NH (X(3)Sigma(-)) are calculated. The result shows that formation of HCl + HNO is dominant below 1000 K; over 1000 K, Cl + NH2O products become dominant. However, the formation of HOCl + 3NH (X(3)Sigma(-)) is unimportant below 1500 K. The pressure-independent individual and total rate constants can be expressed as k1(HCl + HNO) = 4.7 x 10(-8)(T(-1.08)) exp(-129/T), k(2)(Cl + NH2O) = 1.7 x 10(-9)(T(-0.62)) exp(-24/T), k3(HOCl + NH) = 4.8 x 10(-29)(T5.11) exp(-1035/T), and k(total) = 5.0 x 10(-9)(T(-0.67)) exp(-1.2/T), respectively, with units of cm(3) molecule(-1) s(-1), in the temperature range of 200-2500 K.     

Kinetics of the NCN + NO reaction over a broad temperature and pressure range

Rate coefficients for the reaction (3)NCN + NO → products (R3) were measured in the temperature range 251-487 K at pressures from 10 mbar up to 50 bar with helium as the bath gas. The experiments were carried out in slow-flow reactors by using pulsed excimer laser photolysis of NCN(3) at 193 or 248 nm for the production of NCN. Pseudo-first-order conditions ([NCN](0) ? NO) were applied, and NCN was detected time-resolved by resonant laser-induced fluorescence excited near 329 nm. The measurements at the highest pressures yielded values of k(3) ～ 8 × 10(-12) cm(3) s(-1) virtually independent of temperature and pressure, which indicates a substantially smaller high-pressure limiting value of k(3) than predicted in earlier works. Our experiments at pressures below 1 bar confirm the negative temperature and positive pressure dependence of the rate coefficient k(3) found in previous investigations. The falloff behavior of k(3) was rationalized by a master equation analysis based on a barrierless association step (3)NCN + NO ? NCNNO((2)A″) followed by a fast internal conversion NCNNO((2)A″) ? NCNNO((2)A'). From 251-487 K and above 30 mbar, the rate coefficient k(3) is well represented by a Troe parametrization for a recombination/dissociation reaction, k(3)(T,P) = k(4)(∞)k(4)(0)[M]F(k(4)(0)[M] + k(4)(∞))(-1), where k(4) represents the rate coefficient for the recombination reaction (3)NCN + NO. The following parameters were determined (30% estimated error of the absolute value of k(3)): k(4)(0)[M=He] = 1.91 × 10(-30)(T/300 K)(-3.3) cm(6) s(-1)[He], k(4)(∞) = 1.12 × 10(-11) exp(-23 K/T) cm(3) s(-1), and F(C) = 0.28 exp(173 K/T).     

Shock-tube study of the thermal decomposition of CH3CHO and CH3CHO + H reaction

The thermal decomposition of acetaldehyde, CH3CHO + M --> CH3 + HCO + M (eq 1), and the reaction CH3CHO + H --> products (eq 6) have been studied behind reflected shock waves with argon as the bath gas and using H-atom resonance absorption spectrometry as the detection technique. To suppress consecutive bimolecular reactions, the initial concentrations were kept low (approximately 10(13) cm(-3)). Reaction was investigated at temperatures ranging from 1250 to 1650 K at pressures between 1 and 5 bar. The rate coefficients were determined from the initial slope of the hydrogen profile via k1 = [CH3CHO]0(-1) x d[H]/dt, and the temperature dependences observed can be expressed by the following Arrhenius equations: k1(T, 1.4 bar) = 2.9 x 10(14) exp(-38 120 K/T) s(-1), k1(T, 2.9 bar) = 2.8 x 10(14) exp(-37 170 K/T) s(-1), and k1(T, 4.5 bar) = 1.1 x 10(14) exp(-35 150 K/T) s(-1). Reaction was studied with C2H5I as the H-atom precursor under pseudo-first-order conditions with respect to CH3CHO in the temperature range 1040-1240 K at a pressure of 1.4 bar. For the temperature dependence of the rate coefficient the following Arrhenius equation was obtained: k6(T) = 2.6 x 10(-10) exp(-3470 K/T) cm(3) s(-1). Combining our results with low-temperature data published by other authors, we recommend the following expression for the temperature range 300-2000 K: k6(T) = 6.6 x 10(-18) (T/K) (2.15) exp(-800 K/T) cm(3) s(-1). The uncertainties of the rate coefficients k1 and k6 were estimated to be +/-30%.     

Kinetics of the O + HCNO reaction

The kinetics of the O + HCNO reaction were investigated by a relative rate technique using infrared diode laser absorption spectroscopy. Laser photolysis (355 nm) of NO2 was used to produce O atoms, followed by O atom reactions with CS2, NO2, and HCNO, and infrared detection of OCS product from the O + CS2 reaction. Analysis of the experiment data yields a rate constant of k1= (9.84 +/- 3.52) x 10-12 exp[(-195 +/- 120)/T)] (cm3 molecule-1 s-1) over the temperature range 298-375 K, with a value of k1 = (5.32 +/- 0.40) x 10-12 cm3 molecule-1 s-1 at 298 K. Infrared detection of product species indicates that CO producing channels, probably CO + NO + H, dominate the reaction.     

Spectroscopic and kinetic study of the gas-phase CH3I-Cl and C2H5I-Cl adducts

Time-resolved UV-visible absorption spectroscopy has been coupled with UV laser flash photolysis of Cl2/RI/N2/X mixtures (R = CH3 or C2H5; X = O2, NO, or NO2) to generate the RI-Cl radical adducts in the gas phase and study the spectroscopy and reaction kinetics of these species. Both adducts were found to absorb strongly over the wavelength range 310-500 nm. The spectra were very similar in wavelength dependence with lambda(max) approximately 315 nm for both adducts and sigma(max) = (3.5 +/- 1.2) x 10(-17) and (2.7 +/- 1.0) x 10(-17) cm(2) molecule(-1) (base e) for CH3I-Cl and C2H5I-Cl, respectively (uncertainties are estimates of accuracy at the 95% confidence level). Two weaker bands with lambda max approximately 350 and 420 nm were also observed. Over the wavelength range 405-500 nm, where adduct spectra are reported both in the literature and in this study, the absorption cross sections obtained in this study are a factor of approximately 4 lower than those reported previously [Enami et al. J. Phys. Chem. A 2005, 109, 1587 and 6066]. Reactions of RI-Cl with O2 were not observed, and our data suggest that upper limit rate coefficients for these reactions at 250 K are 1.0 x 10(-17) cm(3) molecule(-1) s(-1) for R = CH3 and 2.5 x 10(-17) cm(3) molecule(-1) s(-1) for R = C2H5. Their lack of reactivity with O2 suggests that RI-Cl adducts are unlikely to play a significant role in atmospheric chemistry. Possible reactions of RI-Cl with RI could not be confirmed or ruled out, although our data suggest that upper limit rate coefficients for these reactions at 250 K are 3 x 10(-13) cm(3) molecule(-1) s(-1) for R = CH3 and 5 x 10(-13) cm(3) molecule(-1) s(-1) for R = C2H5. Rate coefficients for CH3I-Cl reactions with CH3I-Cl (k9), NO (k22), and NO2 (k24), and C2H5I-Cl reactions with C2H5I-Cl (k14), NO (k23), and NO2 (k25) were measured at 250 K. In units of 10(-11) cm(3) molecule(-1) s(-1), the rate coefficients were found to be 2k9 = 35 +/- 12, k22 = 1.8 +/- 0.4, k24 = 3.3 +/- 0.6, 2k14 = 40 +/- 16, k23 = 1.8 +/- 0.3, and k25 = 4.0 +/- 0.9, where the uncertainties are estimates of accuracy at the 95% confidence level.