A one-dimensional photochemical model is used to analyze the photochemistries of CH4 and HCN in the primitive terrestrial atmosphere. CH4, N2, and HCN photolysis are examined. The background atmosphere and boundary conditions applied in the analysis are described. The formation of HCN as a by-product of N2 and CH4 photolysis is investigated; the effects of photodissociation and rainfall on HCN is discussed. The low and high CH4 mixing ratios and radical densities are studied.

Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere

A chemical kinetic model for the oxidation of acetylene is presented. The model is usedto calculate flame speeds for adiabatic, freely propagating flames, composition profiles for lean and moderately rich burner-stabilized flames, and induction times and exponential growth constants for shock induced ignition. These results are then compared to the corresponding experimental results currently available. Rate coefficient information is derived predominantly from reaction-specific experiments. Since the comparisons made here are generally favorable, the kinetic model is consistent with a very large body of experimental, and in some cases theoretical, information. Important aspects of the reaction mechanism are discussed.

Toward a comprehensive chemical kinetic mechanism for the oxidation of acetylene: Comparison of model predictions with results from flame and shock tube experiments

Absolute rate constants for the reactions of hydroxyl radicals with benzene (1), benzene-d6 (2), and naphthalene (3) have been obtained using excimer laser photolysis to generate OH and time resolved resonance fluorescence as its monitor. Reaction (1) was studied at total pressures between 1.5 and 112 mbar at 298 K using Ar as a diluent gas. k1 is found to increase with pressure up to –40 mbar, but to be essentially pressure independent at still higher pressures, k (298 K) = (7.0 ± 1.5) 1011 cm3/mol · s. The temperature variation of k1 as studied over the range 244 – 24 K. At temperatures below 330 K normal Arrhenius behaviour with k1 (T) = (3.8 ± 1.0) 1012 exp[-(500 ± 50) K/T] cm3/mol · s is observed, corresponding to the addition reaction (1) OH + C6H6 C6H6OH. Between 330 and 420 K a sharp decrease of k1 with increasing temperature is noted, which we attribute to reaction (1) becoming reversible. For the stabilization energy of C6H6OH we derive (79 ± 6) kJ/mol. Above 420 K a transition of k1 to a positive temperature dependence, corresponding to the abstraction reaction (1') OH + C6H6 H2O + C6H5 is noted. Rate constants for the reaction of OH with benzene-d6 (2) are essentially identical to those of reaction (1) except for temperatures above 420 K, where k2 falls considerably below k1. This isotope effect is a confirmation of the (1') abstraction route. Reaction (3) was studied at total pressures between 6 and 128 mbar (He) where it is found to be essentially pressure independent. Its temperature variation was studied between 337 and 525 K. Over this region the rate coefficient decreases from (8.8 ± 3.5) 1012 cm3/mol · s to (6.6 ± 2.6) 1011 cm3/mol · s, with the decrease with temperature being relatively weak up to 410 K, k1(T ≤ 410 K) = (2.2 ± 1.0) 1012 exp[(440 ± 50) K/T] cm3/mol · s, and stronger at still higher temperatures. This behaviour is interpreted as to correspond to the addition reaction (3) OH + C10H8 C10H8OH and its equilibration. The stabilization energy of the OH-naphthalene adduct is estimated to be (95 ± 6) kJ/mol.

Kinetics of the Reactions of OH-Radicals with Benzene, Benzene-d6 and Naphthalene

The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.

The reaction of acetylene with hydroxyl radicals.

Using a relative rate technique, rate constants have been determined for the gas-phase reactions of Cl atoms with the cholorethenes and ethane at 298 ± 2 K and 735 torr total pressure of air. Using a rate constant of 1.97 × 10−10 cm3 molecule−1 s−1 for the reaction of Cl atoms with n-butane, the following rate constants (in units of 10−11 cm3 molecule−1 s−1) were obtained: vinyl chloride, 12.7 ± 0.2; 1,1-dichloroethene, 14.0 ± 0.2; cis-1,2-dichloroethene, 9.65 ± 0.10; trans-1,2-dichloroethene, 9.58 ± 0.18; trichloroethene, 8.08 ± 0.10; tetrachloroethene, 4.13 ± 0.23; and ethane, 6.17 ± 0.08 (where the indicated error limits do not include the uncertainties in the rate constant for n-butane). A small amount of cis-trans isomerization was observed for the reactions involving the cis- and trans-1,2-dichloroethenes. These data are compared and discussed with the available literature data.

Kinetics of the gas-phase reactions of Cl atoms with chloroethenes at 298 ± 2 K and atmospheric pressure

The pressure dependence of absolute rate constants for the reaction of OH + C2H2 yields products has been examined at five temperatures ranging from 228 to 413 K. The experimental techniques which was used is flash photolysis-resonance fluoresence. OH was produced by water photolysis and hydroxyl resonance fluorescent photons were measured by multiscaling techniques. The results indicate that the low pressure bimolecular rate constant is 4 x 10 the the minus 13th power cu cm molecule (-1) s(-1) over the temperature range studied. A substantial increase in the bimolecular rate constant with an increase in pressure was observed at all temperatures except 228 K. This indicates the importance of initial adduct formation and subsequent stablization. The high pressure results are well represented by the Arrhenius expression (k sub bi) sub infinity = (6.83 + or - 1.19) x 10 to the minus 12th power exp(-646 + or - 47/T)cu cm molecule (-1) s(-1). The results are compared to previous investigated and are theoretically discussed. The implications of these results on modeling of terrestrial and planetary atmospheres and also in combustion chemistry are discussed.

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L. J. Stief

Papers

Pressure dependence of the absolute rate constant for the reaction OH + C2H2 from 228 to 413 K