Theoretical study of thermal dissociation and recombination reactions of XONO2 (X = F, Cl, Br, or I)

Determination of the Rate Coefficients for the Reactions IO + NO2 + M (Air) → IONO2 + M and O(3P) + NO2 → O2 + NO Using Laser-Induced Fluorescence Spectroscopy

Laser-induced fluorescence spectroscopy via excitation of the A2pi(3/2) <-- X2pi(3/2) (2,0) band at 445 nm was used to monitor IO in the presence of NO2 following its generation in the reactions O(3P) + CF3I and O(3P) + I2. Both photolysis of O3 (248 nm) and NO2 (351 nm) were used to initiate the production of IO. The rate coefficients for the thermolecular reaction IO + NO2 + M --> IONO2 + M were measured in air, N2, and O2 over the range P = 18-760 Torr, covering typical tropospheric conditions, and were found to be in the falloff region. No dependence of k1 upon bath gas identity was observed, and in general, the results are in good agreement with recent determinations. Using a Troe broadening factor of F(B) = 0.4, the falloff parameters k0(1) = (9.5 +/- 1.6) x 10(-31) cm6 molecule(-2) s(-1) and k(infinity)(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) were determined at 294 K. The temporal profile of IO at elevated temperatures was used to investigate the thermal stability of the product, IONO2, but no evidence was observed for the regeneration of IO, consistent with recent calculations for the IO-NO2 bond strength being approximately 100 kJ mol(-1). Previous modeling studies of iodine chemistry in the marine boundary layer that utilize values of k1 measured in N2 are hence validated by these results conducted in air. The rate coefficient for the reaction O(3P) + NO2 --> O2 + NO at 294 K and in 100 Torr of air was determined to be k2 = (9.3 +/- 0.9) x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recommended values. All uncertainties are quoted at the 95% confidence limit.

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.