Kinetic studies of the reactions of O2(b 1sigma(g)+) with several atmospheric molecules

Thermal rate coefficients for the removal (reaction + quenching) of O2(1sigma(g)+) by collision with several atmospheric molecules were determined to be as follows: O3, k3(210-370 K) = (3.63 +/- 0.86) x 10(-11) exp((-115 +/- 66)/T); H2O, k4(250-370 K) = (4.52 +/- 2.14) x 10(-12) exp((89 +/- 210)/T); N2, k5(210-370 K) = (2.03 +/- 0.30) x 10(-15) exp((37 +/- 40)/T); CO2, k6(298 K) = (3.39 +/- 0.36) x 10(-13); CH4, k7(298 K) = (1.08 +/- 0.11) x 10(-13); CO, k8(298 K) = (3.74 +/- 0.87) x 10(-15); all units in cm3 molecule(-1) s(-1). O2(1sigma(g)+) was produced by directly exciting ground-state O2(3sigma(g)-) with a 762 nm pulsed dye laser. The reaction of O2(1sigma(g)+) with O3 was used to produce O(3P), and temporal profiles of O(3P) were measured using VUV atomic resonance fluorescence in the presence of the reactant to determine the rate coefficients for removal of O2(1sigma(g)+). Our results are compared with previous values, where available, and the overall trend in the O2(1sigma(g)+) removal rate coefficients and the atmospheric implications of these rate coefficients are discussed. Additionally, an upper limit for the branching ratio of O2(1sigma(g)+) + CO to give O(3P) + CO2 was determined to be < or = 0.2% and this reaction channel is shown to be of negligible importance in the atmosphere.     

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)).     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

An experimental and theoretical study of stereoselectivity of furan-maleic anhydride and furan-maleimide diels-alder reactions

The stereoselectivity of the reaction of furan (1) with maleic anhydride (2) and maleimide (3) was studied experimentally and theoretically. Although the two reactions are highly similar with regard to their preference for endo and exo steroisomers, notable differences were experimentally observed and explained on the basis of calculated reaction-free energies and transition-state barriers. The experimental values of rate constants (k(1+2endo) = (1.75 +/- 0.48) x 10(-5); mol(-1) l s(-1); k(1+2exo) = (3.10 +/- 0.55) x 10(-5); mol(-1) l s(-1); k(1+3endo) = (1.93 +/- 0.082) x 10(-5); mol(-1) l s(-1), k(1+3exo) = (1.38 +/- 0.055) x 10(-5); mol(-1) l s(-1) all at 300 K) and the observed reaction course clearly confirm that neither of these reactions are prototypical examples of Diels-Alder [4 + 2] cycloadditions, whose dominant preference is for endo isomers. However, only by comparing their energetics-calculated at the CCSD(T) level of theory-with the analogous reactions involving cyclopentadiene (8) as a diene can these observations be understood. The low thermodynamic stability of furan [4 + 2] adducts opens retro-Diels-Alder reaction channels and overrules the very small kinetic preference (calculated and measured here) of initial formation for endo stereoisomers. On a macroscopic scale "an irregular"-thermodynamically more stable-exo stereoisomer was consequently observed as a dominant species.     

Atmospheric chemistry of hydrofluoroethers: Reaction of a series of hydrofluoroethers with OH radicals and Cl atoms, atmospheric lifetimes, and global warming potentials

The kinetics of the OH radical and Cl atom reactions with nine fluorinated ethers have been studied by the relative rate method at 298 K and 1013 hPa using gas chromatography-mass spectroscopy (GC-MS) detection: k(OH + CH3CH2OCF3) = (1.55 +/- 0.25) x 10(-13), k(OH + CF3CH2OCH3) = (5.7 +/- 0.8) x 10(-13),k(OH + CF3CH2OCHF2) = (9.1 +/- 1.1) x 10(-15), k(OH + CF3CHFOCHF2) = (6.5 +/- 0.8) x 10(-15), k(OH + CHF2CHFOCF3) = (6.8 +/- 1.1) x 10(-15), k(OH + CF3CHFOCF3) < 1 x 10(-15), k(OH + CF3CHFCF2OCHF2) = (1.69 +/- 0.26) x 10(-14), k(OH + CF3CHFCF2OCH2CH3) = (1.47 +/- 0.13) x 10(-13), k(OH + CF3CF2CF2OCHFCF3) < 1 x 10(-15), k(Cl + CH3CH2OCF3) = (2.2 +/- 0.8) x 10(-12), k(Cl + CF3CH2OCH3) = (1.8 +/- 0.9) x 10(-11), k(Cl + CF3CH2OCHF2) = (1.5 +/- 0.4) x 10(-14), k(Cl + CF3CHFOCHF2) = (1.1 +/- 1.9) x 10(-15), k(Cl + CHF2CHFOCF3) = (1.2 +/- 2.0) x 10(-15), k(Cl + CF3CHFOCF3) < 3 x 10(-15), k(Cl + CF3CHFCF2OCHF2) < 6 x 10(-16), k(Cl + CF3CHFCF2OCH2CH3) = (3.1 +/- 1.1) x 10(-12), and k(Cl + CF3CF2CF2OCHFCF3) < 3 x 10(-15) cm3 molecule(-1) s(-1). The error limits include three standard deviations (3 sigma) from the statistical data analyses, as well as the errors in the rate coefficients of the reference compounds that are used. Infrared absorption cross sections and estimates of the trophospheric lifetimes and the global warming potentials of the fluorinated ethers are presented. The atmospheric degradation of the compounds is discussed.     

Binding nucleophiles to [Fe4Y4Cl4](2-) (Y = S or Se) can increase or suppress the rate of proton transfer to the cluster

In the proton transfer reactions between [Fe 4Y 4Cl 4] (2-) (Y = S or Se) and [pyrH] (+) (pyr = pyrrolidine) in the presence of a variety of nucleophiles (L = I (-), Br (-), PhS (-), EtS (-) or ButNC), initial binding of the nucleophile can occur to generate [Fe 4Y 4Cl 4(L)] ( n- ). The subsequent rate of proton transfer depends markedly on the nature of L. Stopped-flow kinetic studies show that proton transfer from [pyrH] (+) to [Fe 4Y 4Cl 4] (2-) { (S) k 4 = (2.1 +/- 0.5) x 10 (4) dm (3) mol (-1) s (-1); (Se) k 4 = (8.0 +/- 0.5) x 10 (3) dm (3) mol (-1) s (-1)} is increased by prior binding of L = PhS (-) or Bu ( t )NC to form [Fe 4Y 4Cl 4(L)] (n-) ( (S) k 7 (L) approximately 1 x 10 (6) dm (3) mol (-1) s (-1)), but prior binding of L = I (-), Br (-), or EtS (-) to the clusters inhibits the rate of proton transfer {e.g. (S) k 7 (I) = (6.0 +/- 0.8) x 10 (2) dm (3) mol (-1) s (-1); (Se) k 7 (I) = (4.5 +/- 0.5) x 10 (2) dm (3) mol (-1) s (-1)}. This behavior is correlated with the bonding characteristics of L and the effect this has on bond length reorganization within the cluster upon proton transfer.     

Rate coefficients for the reactions of OH with CF3CH2CH3 (HFC-263fb), CF3CHFCH2F (HFC-245eb), and CHF2CHFCHF2 (HFC-245ea) between 238 and 375 K

Rate coefficients for reaction of the hydroxyl radical (OH) with three hydrofluorocarbons (HFCs) CF3CH2CH3, HFC-263fb, (k1); CF3CHFCH2F, HFC-245eb, (k2); and CHF2CHFCHF2, HFC-245ea, (k3); which are suggested as potential substitutes to chlorofluorocarbons (CFCs), were measured using pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) between 235 and 375 K. The Arrhenius expressions obtained are k1(T) = (4.36 +/- 0.72) x 10(-12) exp[-(1290 +/- 40)/T] cm3 molecule(-1) s(-1); k2(T) = (1.23 +/- 0.18) x 10(-12) exp[-(1250 +/- 40)/T] cm3 molecule(-1) s(-1); k3(T) = (1.91 +/- 0.42) x 10(-12) exp[-(1375 +/- 100)/T] cm3 molecule(-1) s(-1). The quoted uncertainties are 95% confidence limits and include estimated systematic errors. The present results are discussed and compared with rate coefficients available in the literature. Our results are also compared with those calculated using structure activity relationships (SAR) for fluorinated compounds. The IR absorption cross-sections at room temperature for these compounds were measured over the range of 500 to 4000 cm-1. The global warming potentials (GWPs) of CF3CH2CH3(HFC-263fb), CF3CHFCH2F(HFC-245eb), and CHF2CHFCHF2(HFC-245ea) were calculated to be 234, 962, and 723 for a 20-year horizon; 70, 286, and 215 for a 100-year horizon; and 22, 89, and 68 for a 500-year horizon; and the atmospheric lifetimes of these compounds are 0.8, 2.5, and 2.6 years, respectively. It is concluded that these compounds are acceptable substitutes for CFCs in terms of their impact on Earth's climate.     

A kinetic study of Ca-containing ions reacting with O, O2, CO2 and H2O: implications for calcium ion chemistry in the upper atmosphere

A series of gas-phase reactions involving molecular Ca-containing ions was studied by the pulsed laser ablation of a calcite target to produce Ca(+) in a fast flow of He, followed by the addition of reagents downstream and detection of ions by quadrupole mass spectrometry. Most of the reactions that were studied are important for describing the chemistry of meteor-ablated calcium in the earth's upper atmosphere. The following rate coefficients were measured: k(CaO(+) + O --> Ca(+) + O(2)) = (4.2 +/- 2.8) x 10(-11) at 197 K and (6.3 +/- 3.0) x 10(-11) at 294 K; k(CaO(+) + CO --> Ca(+) + CO(2), 294 K) = (2.8 +/- 1.5) x 10(-10); k(Ca(+).CO(2) + O(2) --> CaO(2)(+) + CO(2), 294 K) = (1.2 +/- 0.5) x10(-10); k(Ca(+).CO(2) + H(2)O --> Ca(+).H(2)O + CO(2)) = (13.0 +/- 4.0) x 10(-10); and k(Ca(+).H(2)O + O(2) --> CaO(2)(+) + H(2)O, 294 K) = (4.0 +/- 2.5) x 10(-10) cm(3) molecule(-1) s(-1). The quoted uncertainties are a combination of the 1sigma standard errors in the kinetic data and the systematic errors in the models used to extract the rate coefficients. Rate coefficients were also obtained for the following recombination (also termed association) reactions in He bath gas: k(Ca(+).CO(2) + CO(2) --> Ca(+).(CO(2))(2), 294 K) = (2.6 +/- 1.0) x 10(-29); k(Ca(+).H(2)O + H(2)O --> Ca(+).(H(2)O)(2)) = (1.6 +/- 1.1) x 10(-27); and k(CaO(2)(+) + O(2) --> CaO(2)(+).O(2)) < 1 x 10(-31) cm(6) molecule(-2) s(-1). These recombination rate coefficients, as well as those for the ligand-switching reactions listed above, were then interpreted using a combination of high level quantum chemistry calculations and RRKM theory using an inverse Laplace transform solution of the master equation. The surprisingly slow reaction between CaO(+) and O was explained using quantum chemistry calculations on the lowest (2)A', (2)A' and (4)A' potential energy surfaces. These calculations indicate that reaction mostly occurs on the (2)A' surface, leading to production of Ca(+)((2)S) + O(2)((1)Delta(g)). The importance of this reaction for controlling the lifetime of Ca(+) in the upper mesosphere and lower thermosphere is then discussed.     

A kinetic study of the reactions FeO+ + O, Fe+.N2 + O, Fe+.O2 + O and FeO+ + CO: implications for sporadic E layers in the upper atmosphere

These gas-phase reactions were studied by pulsed laser ablation of an iron target to produce Fe(+) in a fast flow tube, with detection of the ions by quadrupole mass spectrometry. Fe(+).N(2) and Fe(+).O(2) were produced by injecting N(2) and O(2), respectively, into the flow tube. FeO(+) was produced from Fe(+) by addition of N(2)O, or by ligand-switching from Fe(+).N(2) following the addition of atomic O. The following rate coefficients were measured: k(FeO(+) + O --> Fe(+) + O(2), 186-294 K) = (3.2 +/- 1.5) x 10(-11); k(Fe(+).N(2) + O --> FeO(+)+ N(2), 294 K) = (4.6 +/- 2.5) x 10(-10); k(Fe(+).O(2) + O --> FeO(+) + O(2), 294 K) = (6.3 +/- 2.7) x 10(-11); and k(FeO(+) + CO --> Fe(+) + CO(2), 294 K) = (1.59 +/- 0.34) x 10(-10) cm(3) molecule(-1) s(-1), where the quoted uncertainties are a combination of the 1sigma standard errors in the kinetic data and the systematic experimental errors. The surprisingly slow reaction between FeO(+) and O is examined using ab initio quantum calculations of the relevant potential energy surfaces. The importance of this reaction for controlling the lifetime of sporadic E layers is then demonstrated using a model of the upper mesosphere and lower thermosphere.     

HPLC determination of andrographolide in rat whole blood: study on the pharmacokinetics of andrographolide incorporated in liposomes and tablets

A sensitive and simple high-performance liquid chromatographic method with UV detection was developed and validated for the determination of andrographolide in rat whole blood. Carbamazepine was employed as internal standard and the blood sample was extracted with chloroform. Chromatographic separations were achieved on a Chromasil ODS column (250 x 4.6 mm, 5 microm). The mobile phase was consisted of methanol-water (52:48, v/v) and delivered at 0.8 mL/min. The detection wavelength was set at 225 nm. The calibration curve had a good linearity in the range 0.053-530 microg/mL in rat whole blood with its correlation coefficient being 0.996. The extraction recovery of andrographolide was ranged from 65.7 to 72.6%. The intra-day and inter-days repeatabilities were below 4.2% in terms of the percentage of relative standard deviation (RSD). The method was used to provide data on the pharmacokinetics of the drug in rats. The data obtained was processed using the 3P87 pharmacokinetic program. The results showed that the disposition of andrographolide after intravenous administration of liposomal andrographolide conformed to a two-compartment open model with alpha = 4.75 x 10(-2) +/- 2.41 x 10(-3) min(-1), beta = 3.16 x 10(-3) +/- 1.58 x 10(-4) min(-1), V(c) = 174.67 +/- 13.97 mL, k(21) = 1.60 x 10(-2) +/- 8.12 x 10(-4) min(-1), k(10) = 9.38 x 10(-3) +/- 5.62 x 10(-4) min(-1), k(12) = 2.53 x 10(-2) +/- 1.27 x 10(-3) min(-1) and AUC(0-infinity) = 1525.47 +/- 92.35 microg min/mL. For the intragastric administration of andrographolide tablets, the disposition of andrographolide followed a one-compartment open model with k(e) = 6.78 x 10(-3) +/- 3.53 x 10(-4) min(-1), k(a) = 3.69 x 10(-2) +/- 4.68 x 10(-3) min(-1), T(max) = 59.69 +/- 3.61 min, C(max) = 1.62 +/- 0.11 microg/mL, V(c) = 1056.90 +/- 83.42 mL, AUC(0-infinity) = 348.75 +/- 24.41 microg min/mL.     

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.     

Atmospheric chemistry of the Z and E isomers of CF3CF=CHF; kinetics, mechanisms, and products of gas-phase reactions with Cl atoms, OH radicals, and O3

Smog chamber/FTIR techniques were used to study the atmospheric chemistry of the Z and E isomers of CF3CF=CHF, which we refer to as CF3CF=CHF(Z) and CF3CF=CHF(E). The rate constants k(Cl + CF3CF=CHF(Z)) = (4.36 +/- 0.48) x 10-11, k(OH + CF3CF=CHF(Z)) = (1.22 +/- 0.14) x 10-12, and k(O3 + CF3CF=CHF(Z)) = (1.45 +/- 0.15) x 10-21 cm3 molecule-1 s-1 were determined for the Z isomer of CF3CF=CHF in 700 Torr air diluent at 296 +/- 2 K. The rate constants k(Cl + CF3CF=CHF(E)) = (5.00 +/- 0.56) x 10-11, k(OH + CF3CF=CHF(E)) = (2.15 +/- 0.23) x 10-12, and k(O3 + CF3CF=CHF(E)) = (1.98 +/- 0.15) x 10-20 cm3 molecule-1 s-1 were determined for the E isomer of CF3CF=CHF in 700 Torr air diluent at 296 +/- 2 K. Both the Cl-atom and OH-radical-initiated atmospheric oxidation of CF3CF=CHF give CF3C(O)F and HC(O)F in molar yields indistinguishable from 100% for both the Z and E isomer. CF3CF=CHF(Z) has an atmospheric lifetime of approximately 18 days and a global warming potential (100 year time horizon) of approximately 6. CF3CF=CHF(E) has an atmospheric lifetime of approximately 10 days and a global warming potential (100 year time horizon) of approximately 3. CF3CF=CHF has a negligible global warming potential and will not make any significant contribution to radiative forcing of climate change.     

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.     

Kinetics and mechanism of methane, methanol, and dimethyl ether C-H activation with electrophilic platinum complexes

The relative rates of C-H activation of methane, methanol, and dimethyl ether by [(N-N)PtMe(TFE-d(3))](+) ((N-N) = ArN=C(Me)-C(Me)=NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d(3) = CF(3)CD(2)OD) (2(TFE)) were determined. Methane activation kinetics were conducted by reacting 2(TFE)-(13)C with 300-1000 psi of methane in single-crystal sapphire NMR tubes; clean second-order behavior was obtained (k = 1.6 +/- 0.4 x 10(-3) M(-1) s(-1) at 330 K; k = 2.7 +/- 0.2 x 10(-4) M(-1) s(-1) at 313 K). Addition of methanol to solutions of 2(TFE) rapidly establishes equilibrium between methanol (2(MeOD)) and trifluoroethanol (2(TFE)) adducts, with methanol binding preferentially (K(eq) = 0.0042 +/- 0.0006). C-H activation gives [(N-N)Pt(CH(2)OD)(MeOD)](+) (4), which is unstable and reacts with [(RO)B(C(6)F(5))(3)](-) to generate a pentafluorophenyl platinum complex. Analysis of kinetics data for reaction of 2 with methanol yields k = 2.0 +/- 0.2 x 10(-3) M(-1) s(-1) at 330 K, with a small kinetic isotope effect (k(H)/k(D) = 1.4 +/- 0.1). Reaction of dimethyl ether with 2(TFE) proceeds similarly (K(eq) = 0.023 +/- 0.002, 313 K; k = 5.5 +/- 0.5 x 10(-4) M(-1) s(-1), k(H)/k(D) = 1.5 +/- 0.1); the product obtained is a novel bis(alkylidene)-bridged platinum dimer, [(diimine)Pt(mu-CH(2))(mu-(CH(OCH(3)))Pt(diimine)](2+) (5). Displacement of TFE by a C-H bond appears to be the rate-determining step for all three substrates; comparison of the second-order rate constants (k((methane))/k((methanol)) = 1/1.3, 330 K; k((methane))/k((dimethy)(l e)(ther)) = 1/2.0, 313 K) shows that this step is relatively unselective for the C-H bonds of methane, methanol, or dimethyl ether. This low selectivity agrees with previous estimates for oxidations with aqueous tetrachloroplatinate(II)/hexachloroplatinate(IV), suggesting a similar rate-determining step for those reactions.     

Rate coefficients for the reaction of OH with a series of unsaturated alcohols between 263 and 371 K

Rate coefficients for the gas-phase reactions of OH radicals with four unsaturated alcohols, 3-methyl-3-buten-1-ol (k1), 2-buten-1-ol (k2), 2-methyl-2-propen-1-ol (k3) and 3-buten-1-ol (k4), were measured using two different techniques, a conventional relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. The Arrhenius rate coefficients (in units of cm(3) molecule(-1) s(-1)) over the temperature range 263-371 K were determined from the kinetic data obtained as k1 = (5.5 +/- 1.0) x 10(-12) exp [(836 +/- 54)/T]; k2 = (6.9 +/- 0.9) x 10(-12) exp [(744 +/- 40)/T]; k3 = (10 +/- 1) x 10(-12) exp [(652 +/- 27)/T]; and k4 = (4.0 +/- 0.4) x 10(-12) exp [(783 +/- 32)/T]. At 298 K, the rate coefficients obtained by the two methods for each of the alcohols studied were in good agreement. The results are presented and compared with those obtained previously for the same and related reactions of OH radicals. Reactivity factors for substituent groups containing the hydroxyl group are determined. The atmospheric implications for the studied alcohols are considered briefly.     

A highly sensitive method for time-resolved detection of O(1D) applied to precise determination of absolute O(1D) reaction rate constants and O(3P) yields

We demonstrate detection, in the gas-phase, of O(1D2) at concentrations down to 10(7) cm(-3) and develop this new method for time-resolved kinetic studies allowing both the total removal rate of O(1D2), of up to 1.5 x 10(6) s(-1), and the fraction quenched to O(3P(J)) by species X, k(q)/k(X), to be determined precisely from a single time profile: at 295 K we find, k(O(1D2) + N2O) = (1.43 +/- 0.08) x 10(-10) cm3 s(-1) with k(q)/k(N2O) = 0.056 +/- 0.009; k(O(1D2) + C2H2) = (3.1 +/- 0.2) x 10(-10) cm3 s(-1) with k(q)/k(C2H2) = 0.020 +/- 0.010; k(q)/k(H2O) < 0.003 for O(1D2) + H2O.     

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.     

Reversibility of electron transfer in tryptophan-tyrosine peptide in acidic aqueous solution studied by time-resolved CIDNP

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) has been used to study electron transfer reactions in tryptophan-tyrosine peptide under strongly acidic conditions. It is demonstrated that a decrease in pH from 2.4 to 1.6 reduces the overall efficiency of intramolecular electron transfer from the tyrosine residue to the oxidized tryptophan residue. A detailed analysis of the CIDNP kinetics revealed that the rate constant of this reaction k(f) stays unchanged upon pH variation, whereas the rate constant of electron transfer in the opposite direction k(r) increases with decreasing pH. The values of the rate constants extracted from model simulations are as follows: k(f) = (5.5 +/- 0.5) x 10(5) s(-1); k(r) = (5.5 +/- 1.0) x 10(4) s(-1) at pH 2.4, (1.2 +/- 0.2) x 10(5) s(-1) at pH 2.0, and (3.2 +/- 0.4) x 10(5) s(-1) at pH 1.6. The pH dependence of log K = log(k(f)/k(r)) is linear and allows for the determination of the difference between the one-electron reduction potentials of the tryptophanyl and tyrosyl radicals in the peptide. The efficiency of IET in acidic aqueous solution containing 10 M urea-d(4) was estimated.     

Determination of the rate constant and product channels for the radical-radical reaction NCO(X 2Pi) + C2H5(X 2A') at 293 K

The rate constant and product branching ratios for the reaction of the cyanato radical, NCO(X (2)Pi), with the ethyl radical, C(2)H(5)(X (2)A'), have been measured over the pressure range of 0.28 to 0.59 kPa and at a temperature of 293 +/- 2 K. The total rate constant, k(1), increased with pressure, P(kPa), described by k(1) = (1.25 +/- 0.16) x 10(-10) + (4.22 +/- 0.35) x 10(-10)P cm(3) molecule(-1) s(-1). Three product channels were observed that were not pressure dependent: (1a) HNCO + C(2)H(4), k(1a) = (1.1 +/- 0.16) x 10(-10), (1b) HONC + C(2)H(4), k(1b) = (2.9 +/- 1.3) x 10(-11), (1c) HCN + C(2)H(4)O, k(1c) = (8.7 +/- 1.5) x 10(-13), with units cm(3) molecule(-1) s(-1) and uncertainties of one-standard deviation in the scatter of the data. The pressure dependence was attributed to a forth channel, (1d), forming recombination products C(2)H(5)NCO and/or C(2)H(5)OCN, with pressure dependence: (1d) k(1d) = (0.090 +/- 1.3) x 10(-11) + (3.91 +/- 0.27) x 10(-10)P cm(3) molecule(-1) s(-1). The radicals were generated by the 248 nm photolysis of ClNCO in an excess of C(2)H(6). Quantitative infrared time-resolved absorption spectrophotometry was used to follow the temporal dependence of the reactants and the appearance of the products. Five species were monitored, HCl, NCO, HCN, HNCO, and C(2)H(4), providing a detailed picture of the chemistry occurring in the system. Other rate constants were also measured: ClNCO + C(2)H(5), k(10) = (2.3 +/- 1.2) x 10(-13) , NCO + C(2)H(6), k(2) = (1.6 +/- 0.11) x 10(-14), NCO + C(4)H(10), k(4) = (5.3 +/- 0.51) x 10(-13), with units cm(3) molecule(-1) s(-1) and uncertainties of one-standard deviation in the scatter of the data.     

A kinetic and mechanistic study of the reactions of OH radicals and Cl atoms with 3,3,3-trifluoropropanol under atmospheric conditions

Product distribution studies of the OH radical and Cl atom initiated oxidation of CF3CH2CH2OH in air at 1 atm and 298 +/- 5 K have been carried out in laboratory and outdoor atmospheric simulation chambers in the presence and absence of NOx. The results show that CF3CH2CHO is the only primary product and that the aldehyde is fairly rapidly removed from the system. In the absence of NOx the major degradation product of CF3CH2CHO is CF3CHO, and the combined yields of the two aldehydes formed from CF3CH2CH2OH are close to unity (0.95 +/- 0.05). In the presence of NOx small amounts of CF3CH2C(O)O2NO2 were also observed (<15%). At longer reaction times CF3CHO is removed from the system to give mainly CF2O. The laser photolysis-laser induced fluorescence technique was used to determine values of k(OH + CF3CH2CH2OH) = (0.89 +/- 0.03) x 10(-12) and k(OH + CF3CH2CHO) = (2.96 +/- 0.04) x 10(-12) cm3 molecule(-1) s(-1). A relative rate method has been employed to measure the rate coefficients k(OH + CF3CH2CH2OH) = (1.08 +/- 0.05) x 10(-12), k(OH + C6F13CH2CH2OH) = (0.79 +/- 0.08) x 10(-12), k(Cl + CF3CH2CH2OH) = (22.4 +/- 0.4) x 10(-12), and k(Cl + CF3CH2CHO) = (25.7 +/- 0.4) x 10(-12) cm3 molecule(-1) s(-1). The results from this investigation are discussed in terms of the possible importance of emissions of fluorinated alcohols as a source of fluorinated carboxylic acids in the environment.