When Rate Constants Are Not Enough.

Real-world chemical systems consisting of multiple isomers and multiple reaction channels often react significantly prior to attaining a steady state energy distribution (SED). Detailed elementary reaction models, which implicitly require SED conditions, may be invalid when non-steady-state energy distributions (NSED) exist. NSED conditions may result in reaction rates and product yields that are different from those expected for SED conditions, although this problem is to some extent reduced by using phenomenological models and rate constants. The present study defines pragmatic diagnostics useful for identifying NSED conditions in stochastic master equation simulations. A representative example is presented for each of four classes of common combustion species: RO2 radicals, aliphatic hydrocarbons, alkyl radicals, and polyaromatic radicals. An example selected from the seminal work of Tsang et al. demonstrates that stochastic simulations and eigenvalue methods for solving the master equation predict the same NSED effects. NSED effects are common under relatively moderate combustion conditions, and accurate simulations may require a master equation analysis.

Fidelity of northern pine snakes (Pituophis m. melanoleucus) to natural and artificial hibernation sites in the New Jersey Pine Barrens.

Environmental managers require information on whether human-made hibernacula are used by rare snakes before constructing large numbers of them as mitigation measures. Fidelity of northern pine snakes (Pituophis m. melanoleucus) was examined in a 6-year study in the New Jersey Pine Barrens to determine whether they used natural and artificial hibernacula equally. Pine snakes used both artificial (human-made) and natural (snake-adapted) hibernacula. Most natural hibernacula were in abandoned burrows of large mammals. Occupancy rates were similar between natural and artificial hibernacula. Only 6 of 27 radio-tracked snakes did not shift hibernacula between years, whereas 78% shifted sites at least once, and fidelity from one year to the next was 42%. For snakes that switched hibernacula (n = 21), one switched among artificial hibernacula, 14 (65%) switched among natural hibernacula, and 6 (29%) switched from artificial to natural hibernacula. Data indicate that most pine snakes switch among hibernacula, mainly selecting natural hibernacula, suggesting that artificial dens are used, but protecting natural hibernacula should be a higher conservation priority.

Shock tube measurements of the rate constant for the reaction ethanol + OH.

The overall rate constant for the reaction ethanol + OH → products was determined experimentally from 900 to 1270 K behind reflected shock waves. Ethan(18)ol was utilized for these measurements in order to avoid the recycling of OH radicals following H-atom abstraction at the ß-site of ethanol. Similar experiments were also performed with unlabeled ethan(16)ol in order to infer the rate constant that excludes reactivity at the ß-site. The two data sets were used to directly infer the branching ratio for the reaction at the ß-site. Experimental data in the current study and in previous low-temperature studies for the overall rate constant are best fit by the expression koverall = 5.07 × 10(5) T[K](2.31) exp(608/T[K]) cm(3) mol(-1) s(-1), valid from 300 to 1300 K. Measurements indicate that the branching ratio of the ß-site is between 20 and 25% at the conditions studied. Pseudo-first-order reaction conditions were generated using tert-butylhydroperoxide (TBHP) as a fast source of (16)OH with ethanol in excess. (16)OH mole fraction time-histories were measured using narrow-line width laser absorption near 307 nm. Measurements were performed at the linecenter of the R22(5.5) transition in the A-X(0,0) band of (16)OH that does not overlap with any absorption features of (18)OH, thus producing a measurement of the (16)OH mole fraction that is insensitive to the presence of (18)OH.

Master equation modeling of the unimolecular decompositions of hydroxymethyl (CH2OH) and methoxy (CH3O) radicals to formaldehyde (CH2O) + H.

α-Hydroxyalkyl radical intermediates (RCHOH, R = H, CH3, etc.) are common to the combustion of nearly all oxygenated fuels. Despite their importance in modeling the combustion phenomena of these compounds through detailed kinetic models, the unimolecular decomposition kinetics remains uncertain for even the simplest α-hydroxyalkyl radical, hydroxymethyl (CH2OH). In this study, RRKM/master equation simulations were carried out for CH2OH decomposition to formaldehyde + H between N2 pressures of 0.01-100 atm and temperatures ranging from 1000 to 1800 K. These simulations were guided by methoxy (CH3O) decomposition calculations between pressures of 0.01-100 atm and temperatures ranging from 600 to 1200 K, in both helium and nitrogen. Excellent agreement of the methoxy results was observed for all regions where experimental data exist. Rates were parametrized as a function of both density and temperature within the Troe formalism. Temperature- and pressure-dependent uncertainty estimates are provided, with the largest source of uncertainty being tunneling contributions at very low pressures and at the lowest temperatures. In the regimes relevant to combustion, uncertainties range from factors of 1.4-2 for CH3O decomposition, and from 1.5-2.6 for CH2OH decomposition. The results of this study are expected to have an impact on the high temperature combustion modeling of methanol, as formation rates to CH2O + H from CH2OH are notably different from previous estimates under some conditions.

Shock tube measurements of the tert-butanol + OH reaction rate and the tert-C4H8OH radical ß-scission branching ratio using isotopic labeling.

The overall rate constant for the reaction tert-butanol + OH → products was determined experimentally behind reflected shock waves by using (18)O-substituted tert-butanol (tert-butan(18)ol) and tert-butyl hydroperoxide (TBHP) as a fast source of (16)OH. The data were acquired from 900 to 1200 K near 1.1 atm and are best fit by the Arrhenius expression 1.24 × 10(-10) exp(-2501/T [K]) cm(3) molecule(-1) s(-1). The products of the title reaction include the tert-C4H8OH radical that is known to have two major ß-scission decomposition channels, one of which produces OH radicals. Experiments with the isotopically labeled tert-butan(18)ol also lead to an experimental determination of the branching ratio for the ß-scission pathways of the tert-C4H8OH radical by comparing the measured pseudo-first-order decay rate of (16)OH in the presence of excess tert-butan(16)ol with the respective decay rate of (16)OH in the presence of excess tert-butan(18)ol. The two decay rates of (16)OH as a result of reactions with the two forms of tert-butanol differ by approximately a factor of 5 due to the absence of (16)OH-producing pathways in experiments with tert-butan(18)ol. This indicates that 80% of the (16)OH molecules that react with tert-butan(16)ol will reproduce another (16)OH molecule through ß-scission of the resulting tert-C4H8(16)OH radical. (16)OH mole fraction time histories were measured using narrow-line-width laser absorption near 307 nm. Measurements were performed at the line center of the R22(5.5) transition in the A-X(0,0) band of (16)OH, a transition that does not overlap with any absorption features of (18)OH, hence yielding a measurement of (16)OH mole fraction that is insensitive to any production of (18)OH.

Experimental determination of the high-temperature rate constant for the reaction of OH with sec-butanol.

The overall rate constant for the reaction of OH with sec-butanol [CH(3)CH(OH)CH(2)CH(3)] was determined from measurements of the near-first-order OH decay in shock-heated mixtures of tert-butylhydroperoxide (as a fast source of OH) with sec-butanol in excess. Three kinetic mechanisms from the literature describing sec-butanol combustion were used to examine the sensitivity of the rate constant determination to secondary kinetics. The overall rate constant determined can be described by the Arrhenius expression 6.97 × 10(-11) exp(-1550/T[K]) cm(3) molecule(-1) s(-1), valid over the temperature range of 888-1178 K. Uncertainty bounds of ±30% were found to adequately account for the uncertainty in secondary kinetics. To our knowledge, the current data represent the first efforts toward an experimentally determined rate constant for the overall reaction of OH with sec-butanol at combustion-relevant temperatures. A rate constant predicted using a structure-activity relationship from the literature was compared to the current data and previous rate constant measurements for the title reaction at atmospheric-relevant temperatures. The structure-activity relationship was found to be unable to correctly predict the measured rate constant at all temperatures where experimental data exist. We found that the three-parameter fit of 4.95 × 10(-20)T(2.66) exp(+1123/T[K]) cm(3) molecule(-1) s(-1) better describes the overall rate constant for the reaction of OH with sec-butanol from 263 to 1178 K.

The reaction OH + C2H4: an example of rotational channel switching.

The low-temperature data for the reaction between OH and C(2)H(4) is treated canonically as either a two-well or one-well problem using the "Multiwell" suite of codes, in which a "well" refers to a minimum in the potential energy surface. The former is analogous to the two transition state model of Greenwald et al. [Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein, S. J. J. Phys. Chem. A2005, 109, 6031], while the latter reflects the dominance of the so-called "inner transition state". External rotations are treated adiabatically, causing changes in the magnitude of effective barriers as a function of temperature. Extant data are well-described with either model using only the average energy transferred in a downward direction, upon collision, ΔE(d)(T), as a fitting parameter. The best value for the parameters describing the rate coefficient as a function of temperature (200 < T/K < 400) (Data at lower temperature is too sparse to yield a recommendation.) and pressure in the form used in the NASA/JPL format [Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17, Jet Propulsion Laboratory, 2011] are k(0) = 1.0 × 10(-28)(T/300)(-3.5) cm(6) molecule(-2) s(-1) and k(∞) to 8.0 × 10(-12)(T/300)(-2.3) cm(3) molecule(-1) s(-1).

High-temperature rate constant determination for the reaction of OH with iso-butanol.

This work presents the first direct experimental study of the rate constant for the reaction of OH with iso-butanol (2-methyl-1-propanol) at temperatures from 907 to 1147 K at near-atmospheric pressures. OH time-histories were measured behind reflected shock waves using a narrow-linewidth laser absorption method during reactions of dilute mixtures of tert-butylhydroperoxide (as a fast source of OH) with iso-butanol in excess. The title reaction's overall rate constant (OH + iso-butanol →(k(overall)) all products) minus the rate constant for the ß-radical-producing channel (OH + iso-butanol →(k(ß)) 1-hydroxy-2-methyl-prop-2-yl radical + H(2)O) was determined from the pseudo-first-order rate of OH decay. A two-parameter Arrhenius fit of the experimentally determined rate constant in the current temperature range yields the expression (k(overall) - k(ß)) = 1.84 × 10(-10) exp(-2350/T[K]) cm(3) molecule(-1) s(-1). A recommendation for the overall rate constant, including k(ß), is made, and comparisons of the results to rate constant recommendations from the literature are discussed.

Rate constant measurements for the overall reaction of OH + 1-butanol → products from 900 to 1200 K.

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.

Rate constant for the reaction C2H5 + HBr → C2H6 + Br.

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ∼4 kJ mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.

Reactions of OH with butene isomers: measurements of the overall rates and a theoretical study.

Reactions of hydroxyl (OH) radicals with 1-butene (k(1)), trans-2-butene (k(2)), and cis-2-butene (k(3)) were studied behind reflected shock waves over the temperature range 880-1341 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and monitored by narrow-line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time histories were modeled using a comprehensive C(5) oxidation mechanism, and rate constants for the reaction of OH with butene isomers were extracted by matching modeled and measured OH concentration time histories. We present the first high-temperature measurement of OH + cis-2-butene and extend the temperature range of the only previous high-temperature study for both 1-butene and trans-2-butene. With the potential energy surface calculated using CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d), the rate constants and branching fractions for the H-abstraction channels of the reaction of OH with 1-butene were calculated in the temperature range 300-1500 K. Corrections for variational and tunneling effects as well as hindered-rotation treatments were included. The calculations are in good agreement with current and previous experimental data and with a recent theoretical study.

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

Reaction rates of hydroxyl (OH) radicals with ethylene (C2H4) and propene (C3H6) 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, (CH3)3-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 k1 = 2.23 × 104 (T)(2.745) exp(-1115 K/T) for 600-2000 K and k2 = 1.94 × 106 (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: k3 (1/s) = 8.13 × 10⁻¹² (T)(7.83) exp(-14598 K/T).

High-temperature measurements and a theoretical study of the reaction of OH with 1,3-butadiene.

The reaction of hydroxyl (OH) radicals with 1,3-butadiene (C(4)H(6)) was studied behind reflected shock waves over the temperature range 1011-1406 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and were monitored by narrow line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0,0) band near 306.7 nm. OH time histories were modeled using a comprehensive 1,3-butadiene oxidation mechanism, and rate constants for the reaction of OH with 1,3-butadiene were extracted by matching modeled and measured OH concentration time histories. Detailed error analyses yielded an uncertainty estimate of +/-13% at 1200 K for the rate coefficient of the target reaction. The current data extends the temperature range of the only previous high-temperature study for this reaction. The rate coefficient and the branching fractions for the H-abstraction channels of the target reaction were also calculated over the temperature range 250-2500 K using variational transition-state theory based on QCISD(T)/cc-pVinfinityZ//B3LYP/6-311++G(d,p) quantum chemistry. The calculations are in good agreement with the experimental results above 1200 K.

Pressure dependent reactions for atmospheric and combustion models.

In order to characterize reactions as functions of temperature, pressure (molecular density) and the nature of the species that constitute that molecular density, master equation solutions are required. In this tutorial review, the application of the Multiwell suite of codes to some reactions of interest in atmospheric and combustion chemistry is discussed, with attention given to the details of the molecular and energy transfer values. Uncertainties in data and in structural and energetic molecular parameters combine to assure the need for optimization and collaborative processing of the entire data base when modeling practical systems.

Evaluation of data for atmospheric models: master equation/RRKM calculations on the combination reaction, BrO + NO2 --> BrONO2, a conundrum.

Experimental data for the title reaction were modeled using master equation (ME)/RRKM methods based on the Multiwell suite of programs. The starting point for the exercise was the empirical fitting provided by the NASA (Sander, S. P.; Finlayson-Pitts, B. J.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Orkin, V. L.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15; Jet Propulsion Laboratory: Pasadena, California, 2006)1 and IUPAC (Atkinson, R.; Baulch, D. L.; Cox, R. A.; R. F. Hampson, J.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 2000, 29, 167)2 data evaluation panels, which represents the data in the experimental pressure ranges rather well. Despite the availability of quite reliable parameters for these calculations (molecular vibrational frequencies (Parthiban, S.; Lee, T. J. J. Chem. Phys. 2000, 113, 145)3 and a value (Orlando, J. J.; Tyndall, G. S. J. Phys. Chem. 1996, 100, 19398)4 of the bond dissociation energy, D298(BrO-NO2) = 118 kJ mol-1, corresponding to DeltaH0o = 114.3 kJ mol-1 at 0 K) and the use of RRKM/ME methods, fitting calculations to the reported data or the empirical equations was anything but straightforward. Using these molecular parameters resulted in a discrepancy between the calculations and the database of rate constants of a factor of ca. 4 at, or close to, the low-pressure limit. Agreement between calculation and experiment could be achieved in two ways, either by increasing DeltaH0o to an unrealistically high value (149.3 kJ mol-1) or by increasing DeltaEd, the average energy transferred in a downward collision, to an unusually large value (>5000 cm-1). The discrepancy could also be reduced by making all overall rotations fully active. The system was relatively insensitive to changing the moments of inertia in the transition state to increase the centrifugal effect. The possibility of involvement of BrOONO was tested and cannot account for the difficulties of fitting the data.

Shock tube study of the reaction of CH with N2: overall rate and branching ratio.

We have studied the reaction between CH and N2, (1) CH + N2 --> products, in shock tube experiments using CH and NCN laser absorption. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1 nm. The overall rate coefficient of the CH + N2 reaction was measured between 1943 and 3543 K, in the 0.9-1.4 atm pressure range, using a CH perturbation approach. CH profiles recorded upon shock-heating dilute mixtures of ethane in argon and acetic anhydride in argon were perturbed by the addition of nitrogen. The perturbation in the CH concentration was principally due to the reaction between CH and N2. Rate coefficients for the overall reaction were inferred by kinetically modeling the perturbed CH profiles. A least-squares, two-parameter fit of the current overall rate coefficient measurements was k1 = 6.03 x 1012 exp(-11150/T [K]) (cm3 mol-1 s-1). The uncertainty in k1 was estimated to be approximately +/-25% and approximately +/-35% at approximately 3350 and approximately 2100 K, respectively. At high temperatures, there are two possible product channels for the reaction between CH and N2, (1a) CH + N2 --> HCN + N and (1b) CH + N2 --> H + NCN. The large difference in the rates of the reverse reactions enabled inference of the branching ratio of reaction 1, k1b/(k1b + k1a), in the 2228-2905 K temperature range by CH laser absorption in experiments in a nitrogen bath. The current CH measurements are consistent with a branching ratio of 1 and establish NCN and H as the primary products of the CH + N2 reaction. A detailed and systematic uncertainty analysis, taking into account experimental and mechanism-induced contributions, yields a conservative lower bound of 0.70 for the branching ratio. NCN was also detected by continuous-wave, narrow-line-width laser absorption at 329.13 nm. The measured NCN time histories were used to infer the rate coefficient of the reaction between H and NCN, H + NCN --> HCN + N, and to estimate an absorption coefficient for the NCN radical.

The reaction Cl+NO2-->ClONO and ClNO2.

The extant data (see Int. J. Chem. Kinet. 1984, 16, 1311; Int. J. Chem. Kinet. 1988, 20, 811; and J. Phys. Chem. 1996, 100, 4019)1-3 for the reaction of Cl atoms with NO2 has been examined and compared with calculated values using RRKM/ME methods through the use of the coded suite "Multiwell" (see Int. J. Chem. Kinet. 2001, 33, 232; MultiWell-2.08 Software; University of Michigan: Ann Arbor, MI, 2007; http://aoss.engin.umich.edu/multiwell/)4,5 and ab initio quantum calculations in the literature (see J. Phys. Chem. 1994, 98, 111; J. Phys. Chem. A 2005, 109, 4736; and Chemphyschem 2004, 5, 1864).6-8 The data are in, or very near, the low-pressure limit and therefore sensitive to the density of states of the excited ClONO and ClNO2 molecules as well as collisional energy transfer, centrifugal effects, and anharmonicity corrections. The data were underpredicted by a factor of 2.6 using accepted prescriptions for centrifugal corrections, collision frequency, and ignoring anharmonicity. The data could be fit by either making all rotational degrees of freedom active or artificially increasing the collision frequency, or maybe some of each. This last could also be complemented or supplemented by a multiplicative factor ascribed to anharmonicity.

High-temperature shock tube measurements of methyl radical decomposition.

We have studied the two-channel thermal decomposition of methyl radicals in argon, involving the reactions CH3 + Ar --> CH + H2 + Ar (1a) and CH3 + Ar --> CH2 + H + Ar (1b), in shock tube experiments over the 2253-3527 K temperature range, at pressures between 0.7 and 4.2 atm. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1311 nm. The collision-broadening coefficient for CH in argon, 2gamma(CH-Ar), was measured via repeated single-frequency experiments in the ethane pyrolysis system behind reflected shock waves. The measured 2gamma(CH-Ar) value and updated spectroscopic and molecular parameters were used to calculate the CH absorption coefficient at 431.1311 nm (23194.80 cm(-1)), which was then used to convert raw traces of fractional transmission to quantitative CH concentration time histories in the methyl decomposition experiments. The rate coefficient of reaction 1a was measured by monitoring CH radicals generated upon shock-heating highly dilute mixtures of ethane, C2H6, or methyl iodide, CH3I, in an argon bath. A detailed chemical kinetic mechanism was used to model the measured CH time histories. Within experimental uncertainty and scatter, no pressure dependence could be discerned in the rate coefficient of reaction 1a in the 0.7-4.2 atm pressure range. A least-squares, two-parameter fit of the current measurements, applicable between 2706 and 3527 K, gives k(1a) (cm(3) mol(-1) s(-1)) = 3.09 x 1015 exp[-40700/T (K)]. The rate coefficient of reaction 1b was determined by shock-heating dilute mixtures of C2H6 or CH3I and excess O2 in argon. During the course of reaction, OH radicals were monitored using the well-characterized R(1)(5) line of the OH A-X (0,0) band at 306.6871 nm (32606.52 cm(-1)). H atoms generated via reaction 1b rapidly react with O2, which is present in excess, forming OH. The OH traces are primarily sensitive to reaction 1b, reaction 9 (H + O2 --> OH + O) and reaction 10 (CH3 + O2 --> products), where the rate coefficients of reactions 9 and 10 are relatively well-established. No pressure dependence could be discerned for reaction 1b between 1.1 and 3.9 atm. A two-parameter, least-squares fit of the current data, valid over the 2253-2975 K temperature range, yields the rate expression k(1b) (cm(3) mol(-1) s(-1)) = 2.24 x 10(15) exp[-41600/T (K)]. Theoretical calculations carried out using a master equation/RRKM analysis fit the measurements reasonably well.

Evaluating data for atmospheric models, an example: IO + NO2 = IONO2.

Data for the title reaction have been fit to the different formalisms used by the NASA and IUPAC data evaluation panels. The data are well represented by either formalism. Reported values for the bond dissociation energy at 0 K, D0(IO-NO2) vary from about 95 to 135 kJ mol(-1), with uncertainty ranges of about 20 kJ mol(-1). Master equation/RRKM methods were employed in an attempt to reconcile these values with the data. This was possible within reasonable bounds and suggests a value in the neighborhood of 150 kJ mol(-1). As always, there are sufficient assumptions and unknowns in such an attempt, that this value is somewhat uncertain, but the true value is not expected to be too far from this result. Thus, it is possible to evaluate data of the type addressed here in a manner reasonably consistent with the basic understanding of pressure dependent rate coefficients for use in atmospheric or other models of "engineering" problems. There are, however, strict limits on our ability to know specific details. It is possible that true anharmonicity corrections that include stretch-bend interactions as well as effects due to averaging rotational contributions could combine to lower this value by as much as 10 kJ mol(-1). In addition collision and energy transfer parameters are somewhat uncertain.