Kinetics of elementary steps in the reactions of atomic bromine with isoprene and 1,3-butadiene under atmospheric conditions.

Laser flash photolysis of CF(2)Br(2) has been coupled with time-resolved detection of atomic bromine by resonance fluorescence spectroscopy to investigate the gas-phase kinetics of early elementary steps in the Br-initiated oxidations of isoprene (2-methyl-1,3-butadiene, Iso) and 1,3-butadiene (Bu) under atmospheric conditions. At T ≥ 526 K, measured rate coefficients for Br + isoprene are independent of pressure, suggesting that hydrogen transfer (1a) is the dominant reaction pathway. The following Arrhenius expression adequately describes all kinetic data at 526 K ≤ T ≤ 673 K: k(1a)(T) = (1.22 ± 0.57) × 10(-11) exp[(-2100 ± 280)/T] cm(3) molecule(-1) s(-1) (uncertainties are 2σ and represent precision of the Arrhenius parameters). At 271 K ≤ T ≤ 357 K, kinetic evidence for the reversible addition reactions Br + Iso ↔ Br-Iso (k(1b), k(-1b)) and Br + Bu ↔ Br-Bu (k(3b), k(-3b)) is observed. Analysis of the approach to equilibrium data allows the temperature- and pressure-dependent rate coefficients k(1b), k(-1b), k(3b), and k(-3b) to be evaluated. At atmospheric pressure, addition of Br to each conjugated diene occurs with a near-gas-kinetic rate coefficient. Equilibrium constants for the addition/dissociation reactions are obtained from k(1b)/k(-1b) and k(3b)/k(-3b), respectively. Combining the experimental equilibrium data with electronic structure calculations allows both second- and third-law analyses of thermochemistry to be carried out. The following thermochemical parameters for the addition reactions 1b and 3b at 0 and 298 K are obtained (units are kJ mol(-1) for Δ(r)H and J mol(-1) K(-1) for Δ(r)S; uncertainties are accuracy estimates at the 95% confidence level): Δ(r)H(0)(1b) = -66.6 ± 7.1, Δ(r)H(298)(1b) = -67.5 ± 6.6, and Δ(r)S(298)(3b) = -93 ± 16; Δ(r)H(0)(3b) = -62.4 ± 9.0, Δ(r)H(298)(3b) = -64.5 ± 8.5, and Δ(r)S(298)(3b) = -94 ± 20. Examination of the effect of added O(2) on Br kinetics under conditions where reversible adduct formation is observed allows the rate coefficients for the Br-Iso + O(2) (k(2)) and Br-Bu + O(2) (k(4)) reactions to be determined. At 298 K, we find that k(2) = (3.2 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1) independent of pressure (uncertainty is 2σ, precision only; pressure range is 25-700 Torr) whereas k(4) increases from 3.2 to 4.7 × 10(-13) cm(3) molecule(-1) s(-1) as the pressure increases from 25 to 700 Torr. Our results suggest that under atmospheric conditions, Br-Iso and Br-Bu react with O(2) to produce peroxy radicals considerably more rapidly than they undergo unimolecular decomposition. Hence, the very fast addition reactions appear to control the rates of Br-initiated formation of Br-Iso-OO and Br-Bu-OO radicals under atmospheric conditions. The peroxy radicals are relatively weakly bound, so conjugated diene regeneration via unimolecular decomposition reactions, though unimportant on the time scale of the reported experiments (milliseconds), is likely to compete effectively with bimolecular reactions of peroxy radicals under relatively warm atmospheric conditions as well as in 298 K competitive kinetics experiments carried out in large chambers.

Kinetic and mechanistic study of the reactions of atomic chlorine with CH3CH2Br, CH3CH2CH2Br, and CH2BrCH2Br.

A laser flash photolysis-resonance fluorescence technique has been employed to investigate the reactions of atomic chlorine with three alkyl bromides (R-Br) that have been identified as short-lived atmospheric constituents with significant ozone depletion potentials (ODPs). Kinetic data are obtained through time-resolved observation of the appearance of atomic bromine that is formed by rapid unimolecular decomposition of radicals generated via abstraction of a ß-hydrogen atom. The following Arrhenius expressions are excellent representations of the temperature dependence of rate coefficients measured for the reactions Cl + CH(3)CH(2)Br (eq 1 ) and Cl + CH(3)CH(2)CH(2)Br (eq 2 ) over the temperature range 221-436 K (units are 10(-11) cm(3) molecule(-1) s(-1)): k(1)(T) = 3.73 exp(-378/T) and k(2)(T) = 5.14 exp(+21/T). The accuracy (2σ) of rate coefficients obtained from the above expressions is estimated to be ±15% for k(2)(T) and +15/-25% for k(1)(T) independent of T. For the relatively slow reaction Cl + CH(2)BrCH(2)Br (eq 3 ), a nonlinear ln k(3) vs 1/T dependence is observed and contributions to observed kinetics from impurity reactions cannot be ruled out; the following modified Arrhenius expression represents the temperature dependence (244-569 K) of upper-limit rate coefficients that are consistent with the data: k(3)(T) ≤ 3.2 × 10(-17)T(2) exp(-184/T) cm(3) molecule(-1) s(-1). Comparison of Br fluorescence signal strengths obtained when Cl removal is dominated by reaction with R-Br with those obtained when Cl removal is dominated by reaction with Br(2) (unit yield calibration) allows branching ratios for ß-hydrogen abstraction (k(ia)/k(i), i = 1,2) to be evaluated. The following Arrhenius-type expressions are excellent representations of the observed temperature dependences: k(1a)/k(1) = 0.85 exp(-230/T) and k(2a)/k(2) = 0.40 exp(+181/T). The accuracy (2σ) of branching ratios obtained from the above expressions is estimated to be ±35% for reaction 1 and ±25% for reaction 2 independent of T. It appears likely that reactions 1 and 2 play a significant role in limiting the tropospheric lifetime and, therefore, the ODP of CH(3)CH(2)Br and CH(3)CH(2)CH(2)Br, respectively.

Kinetic and mechanistic study of the reactions of O(1D2) with HCN and CH3CN.

A laser flash photolysis-resonance fluorescence (LFP-RF) technique is employed to investigate the kinetics and mechanism of the reactions of O((1)D(2)) with HCN [reaction (1)] and CH(3)CN [reaction (2)] as a function of temperature over the range 193-430 K. The experiments involve time-resolved RF detection of O((3)P(J)) or H((2)S(1/2)) following LFP of O(3)/X/He mixtures (X=HCN or CH(3)CN), some of which also contain N(2), H(2), and/or NO(2). Measured rate coefficients for total removal of O((1)D(2)) by HCN and CH(3)CN are well-described by the following Arrhenius expressions (units are 10(-10) cm(3) molecule(-1) s(-1)): k(1)(T)=1.08exp(+105/T) and k(2)(T)=2.54exp(-24/T). Temperature-dependent product yields of O((3)P(J)), k(1a)/k(1) and k(2a)/k(2) are well-described by the following Arrhenius-type expressions: k(1a)/k(1)=0.150exp(+200/T) and k(2a)/k(2)=0.0269 exp(+137/T). The H((2)S(1/2)) yield from reaction (2) is found to be 0.16±0.03 independent of temperature (200-423 K). Large 298 K yields of H((2)S(1/2)), 0.68±0.12 produced per O((1)D(2)) destroyed by HCN, are observed for reaction (1). However, observed kinetics suggest that only about half of detected H((2)S(1/2)) is generated as a primary product of the O((1)D(2))+HCN reaction, with the remainder generated via a fast secondary reaction. The implications of the reported kinetic and mechanistic results for understanding the atmospheric chemistry of HCN and CH(3)CN are discussed.

Reactive and nonreactive quenching of O(1D) by the potent greenhouse gases SO2F2, NF3, and SF5CF3.

A laser flash photolysis-resonance fluorescence technique has been employed to measure rate coefficients and physical vs. reactive quenching branching ratios for O((1)D) deactivation by three potent greenhouse gases, SO(2)F(2)(k(1)), NF(3)(k(2)), and SF(5)CF(3)(k(3)). In excellent agreement with one published study, we find that k(1)(T) = 9.0 x 10(-11) exp(+98/T) cm(3) molecule(-1) s(-1) and that the reactive quenching rate coefficient is k(1b) = (5.8 +/- 2.3) x 10(-11) cm(3) molecule(-1) s(-1) independent of temperature. We find that k(2)(T) = 2.0 x 10(-11) exp(+52/T) cm(3) molecule(-1) s(-1) with reaction proceeding almost entirely (approximately 99%) by reactive quenching. Reactive quenching of O((1)D) by NF(3) is more than a factor of two faster than reported in one published study, a result that will significantly lower the model-derived atmospheric lifetime and global warming potential of NF(3). Deactivation of O((1)D) by SF(5)CF(3) is slow enough (k(3) < 2.0 x 10(-13) cm(3) molecule(-1) s(-1) at 298 K) that reaction with O((1)D) is unimportant as an atmospheric removal mechanism for SF(5)CF(3). The kinetics of O((1)D) reactions with SO(2) (k(4)) and CS(2) (k(5)) have also been investigated at 298 K. We find that k(4) = (2.2 +/- 0.3) x 10(-10) and k(5) = (4.6 +/- 0.6) x 10(-10) cm(3) molecule(-1) s(-1); branching ratios for reactive quenching are 0.76 +/- 0.12 and 0.94 +/- 0.06 for the SO(2) and CS(2) reactions, respectively. All uncertainties reported above are estimates of accuracy (2sigma) and rate coefficients k(i)(T) (i = 1,2) calculated from the above Arrhenius expressions have estimated accuracies of +/- 15% (2sigma).

Experimental and theoretical study of the carbon-13 and deuterium kinetic isotope effects in the Cl and OH reactions of CH3F.

A laser flash photolysis-resonance fluorescence technique has been employed to determine absolute rate coefficients for the CH3F + Cl reaction in N2 bath gas in the temperature range of 200-700 K and pressure range of 33-133 hPa. The data were fitted to a modified Arrhenius expression k(T) = 1.14 x 10(-12) x (T/298)2.26 exp{-313/T}. The OH and Cl reaction rates of (13)CH3F and CD3F have been measured by long-path FTIR spectroscopy relative to CH3F at 298 +/- 2 K and 1013 +/- 10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least-squares spectral fitting method including line data from the HITRAN database and measured infrared spectra as references. The relative reaction rates defined by alpha = k(light)/k(heavy) were determined to be k(OH+CH3F)/k(OH+CD3F) = 4.067 +/- 0.018, k(OH+CH3F)/k(OH+(13)CH3F) = 1.067 +/- 0.006, k(Cl+CH3F)/k(Cl+CD3F) = 5.11 +/- 0.07, and k(Cl+CH3F)/k(Cl+(13)CH3F) = 1.016 +/- 0.006. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3F have been further investigated by quantum chemistry methods and variational transition state theory.

Kinetics studies of aqueous phase reactions of Cl atoms and Cl2(-) radicals with organic sulfur compounds of atmospheric interest.

A laser flash photolysis-long path UV-visible absorption technique has been employed to investigate the kinetics of aqueous phase reactions of chlorine atoms (Cl) and dichloride radicals (Cl2(-)) with four organic sulfur compounds of atmospheric interest, dimethyl sulfoxide (DMSO; CH3S(O)CH3), dimethyl sulfone (DMSO2; CH3(O)S(O)CH3), methanesulfinate (MSI; CH3S(O)O-), and methanesulfonate (MS; CH3(O)S(O)O-). Measured rate coefficients at T = 295 +/- 1 K (in units of M(-1) s(-1)) are as follows: Cl + DMSO, (6.3 +/- 0.6) x 10(9); Cl2(-) + DMSO, (1.6 +/- 0.8) x 10(7); Cl + DMSO2, (8.2 +/- 1.6) x 10(5); Cl2(-) + DMSO2, (8.2 +/- 5.5) x 10(3); Cl2(-) + MSI, (8.0 +/- 1.0) x 10(8); Cl + MS, (4.9 +/- 0.6) x 10(5); Cl2(-) + MS, (3.9 +/- 0.7) x 10(3). Reported uncertainties are estimates of accuracy at the 95% confidence level and the rate coefficients for MSI and MS reactions with Cl2(-) are corrected to the zero ionic strength limit. The absorption spectrum of the DMSO-Cl adduct is reported; peak absorbance is observed at 390 nm and the peak extinction coefficient is found to be 5760 M(-1) cm(-1) with a 2sigma uncertainty of +/-30%. Some implications of the new kinetics results for understanding the atmospheric sulfur cycle are discussed.

Rates and mechanisms for the reactions of chlorine atoms with iodoethane and 2-iodopropane.

The reaction of Cl atoms with iodoethane has been studied via a combination of laser flash photolysis/resonance fluorescence (LFP-RF), environmental chamber/Fourier transform (FT)IR, and quantum chemical techniques. Above 330 K, the flash photolysis data indicate that the reaction proceeds predominantly via hydrogen abstraction. The following Arrhenius expressions (in units of cm3 molecule(-1) s(-1)) apply over the temperature range 334-434 K for reaction of Cl with CH3CH2I (k4(H)) and CD3CD2I (k4(D)): k4(H) = (6.53 +/- 3.40) x 10(-11) exp[-(428 +/- 206)/T] and k4(D) = (2.21 +/- 0.44) x 10(-11) exp[-(317 +/- 76)/T]. At room temperature and below, the reaction proceeds both via hydrogen abstraction and via reversible formation of an iodoethane/Cl adduct. Analysis of the LFP-RF data yields a binding enthalpy (0 K) for CD3CD2I x Cl of 57 +/- 10 kJ mol(-1). Calculations using density functional theory show that the adduct is characterized by a C-I-Cl bond angle of 84.5 degrees; theoretical binding enthalpies of 38.2 kJ/mol, G2'[ECP(S)], and 59.0 kJ mol(-1), B3LYP/ECP, are reasonably consistent with the experimentally derived result. Product studies conducted in the environmental chamber show that hydrogen abstraction from both the -CH2I and -CH3 groups occur to a significant extent and also provide evidence for a reaction of the CH3CH2I x Cl adduct with CH3CH2I, leading to CH3CH2Cl formation. Complementary environmental chamber studies of the reaction of Cl atoms with 2-iodopropane, CH3CHICH3, are also presented. As determined by relative rate methods, the reaction proceeds with an effective rate coefficient, k6, of (5.0 +/- 0.6) x 10(-11) cm3 molecule(-1) s(-1) at 298 K. Product studies indicate that this reaction also occurs via two abstraction channels (from the CH3 groups and from the -CHI- group) and via reversible adduct formation.