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The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF4 (+M) ⇄ CF3 + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database. Agreement is achieved by minor (unfortunately not separable) adjustments of reaction enthalpy and collisional energy transfer parameters.

The pressure and temperature dependences of the reactions of Fe+ with methyl halides CH3X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300-600 K. FeX+ was observed for all three halides and FeCH3+ was observed for the CH3I reaction. FeCH3X+ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX+ products come via an insertion mechanism, while the FeCH3+ can be produced from either insertion or SN2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH3I reaction, indicating that some more direct process must be important.

We report for the first time temperature dependences (from 300 to 600 K) of the reactions of Fe(+) and FeO(+) with NO and NO(2). Both ions react quickly with NO(2), and their rate constants have weak negative temperature dependences. The former is consistent with the calculated energy profile along the Fe(+) + NO(2) reaction coordinate. Ground state Fe(+) reacts with NO(2) to produce only FeO(+), while FeO(+) reacts with NO(2) to produce NO(+) exclusively. Certain source conditions produce excited Fe(+), as evidenced by production of primary NO(+), which is endothermic with the ground state by 0.35 eV. The room temperature rate constants are in agreement with previous values. For the reactions of Fe(+) and FeO(+) with NO, we find an upper limit of

The quantum dynamics of the fall of a charged particle (i.e., the capture of a charged particle) onto a stationary dipolar target is considered. Extending previous approaches for the calculation of rate coefficients in the lowest channels, we now determine rate coefficients for all channels until the quantum rate coefficients converge to their classical counterpart. The results bridge the gap between the capture of light particles (electrons) and heavy particles (ions) in the limit of sudden dynamics, when the collision time is short in comparison to the rotational period of the molecular target. The quantum-classical correspondence is discussed in terms of semiclassical numbers of channels which are open for capture in effective potentials formed by charge-dipole attraction and centrifugal repulsion. The quantum capture rate coefficients are presented through classical rate coefficients and correction factors that converge to unity for high temperatures and whose behavior at ultralow temperatures, for not too small values of the dipole moment, is determined by semiclassical numbers of capture channels.

Specific rate constants k(E) of the dissociation of the halobenzene ions C6H5X+ --C6H5+ + X* (X* = Cl, Br, and I) were measured over a range of 10(3)-10(7) s-1 by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. The experimental data were analyzed by various statistical unimolecular rate theories in order to derive the threshold energies E0. Although rigid activated complex RRKM theory fits the data in the experimentally measured energy range, it significantly underestimates E0 for chloro- and bromobenzene. Phase space theory (PST) does not fit the experimentally measured rates. A parametrized version of the variational transition state theory (VTST) as well as a simplified version of the statistical adiabatic channel model (SSACM) incorporating an energy dependent rigidity factor provide excellent fits to the experimental data and predict the correct dissociation energies. Although both approaches have just two adjustable parameters, one of which is E0, SSACM is effective and particularly simple to apply.

Experimental quantum yields of the photolysis of formaldehyde at lambda310 nm are combined with absolute and relative rate calculations for the molecular elimination H2CO --H2 + CO (1), the bond fission H2CO --H + HCO (2), and the intramolecular hydrogen abstraction H2CO --H ... HCO --H2 + CO (3) taking place in the electronic ground state. Temperature and pressure dependencies of the quantum yields are analyzed with the goal to achieve consistency between experiment and modeling. Two wavelength ranges with considerably different properties are considered: 340-360 nm, where channel 1 competes with collisional deactivation of excited molecules, and 310-340 nm, which is dominated by the competition between the formation of radical and molecular products. The close relation between photolysis and pyrolysis of formaldehyde, such as analyzed for the pyrolysis in the companion paper, is documented and an internally consistent treatment of the two reaction systems is provided. The quantum yields are modeled and represented in analytical form such that values outside the available experimental range can be predicted to some extent.

New experimental results for the thermal dissociation of formaldehyde to radical and molecular products (Proc. Combust. Inst. 2007, 31, in press) form the basis of the present analysis of the respective low-pressure rate coefficients k(Rad,0) and k(Mol,0) of the reaction. The article supersedes an earlier analysis (J. Phys. Chem. A 2005, 109, 8320) which used less accurate and more preliminary input information. In addition, refined rotational factors F(rot) are determined and specific energy and angular momentum dependent branching ratios from a more detailed analysis of photolysis quantum yields (J. Phys. Chem. A 2007, 111, 3868) are employed as well. It is emphasized again that pyrolysis and photolysis are intimately related and should be analyzed in an internally consistent manner. The combination of the new with earlier experimental results for pyrolysis rates allows one to fit the height of the energy barrier for the molecular elimination channel with improved precision. A value of E0,1 = 81.7(+/-0.5) kcal mol(-1) is obtained. In addition, employing anharmonicity factors F(anh) from the earlier work, a total average energy transferred per collision of -DeltaE/hc = 100(+/-20) cm(-1) is fitted from the experiments in the bath gas Ar. This value is consistent with the value -DeltaE/hc = 80(+/-40) cm(-1) for the bath gas N(2) such as fitted from photolysis quenching experiments (using the same molecular parameters as for the pyrolysis). Rate coefficients for the temperature range 1200-3500 K are represented in the form k(Mol,0)/[Ar] = 7.3 x 10(14) T -6.1 exp(-47300 K/T) cm(3) molecule(-1) s(-1) and k(Rad,0)/[Ar] = 2.1 x 10(12) T -5.5 exp(-47300 K/T) cm(3) molecule(-1) s(-1) (accuracy +/-25%) and recommended for use in combustion chemistry.

The rate constants of the recombination reaction of p-fluorobenzyl radicals, p-F-C6H4CH2 + p-F-C6H4CH2 (+M) --C14H12F2 (+M), have been measured over the pressure range 0.2-800 bar and the temperature range 255-420 K. Helium, argon, and CO2 were employed as bath gases (M). At pressures below 0.9 bar in Ar and CO2, and 40 bar in He, the rate constant k1 showed no dependence on the pressure and the nature of the bath gas, clearly indicating that it had reached the limiting high-pressure value of the energy-transfer (ET) mechanism (k(1,infinity)ET). A value of k(1,infinity)ET(T) = (4.3 +/- 0.5) x 10(-11) (T/300 K)(-0.2) cm3 molecule(-1) s(-1) was determined. At pressures above about 5 bar, the k1 values in Ar and CO2 were found to gradually increase in a pressure range where according to energy-transfer mechanism, they should remain at the constant value k(1,infinity)ET. The enhancement of the recombination rate constant beyond the value k(1,infinity)ET increased in the order HeArCO2, and it became more pronounced with decreasing temperature. The dependences of k1 on pressure, temperature, and the bath gas were similar to previous observations in the recombination of benzyl radicals. The effect of fluorine-substitution of the benzyl ring on k1 values is discussed. The present results confirm the significant role of radical complexes in the recombination kinetics of benzyl-type radicals in the gas-liquid transition range. The observations on a rate enhancement beyond the experimental value of k(1,infinity)ET at elevated densities up to the onset of diffusion-control are consistently explained by the kinetic contribution of a "radical-complex" mechanism which is solely based on standard van der Waals interaction between radicals and bath gases.

The combination reactions CCl 3 + CCl 3 (+ M) → C 2 Cl 6 (+ M) and CCl 3 + Br (+ M) → CCl 3 Br (+ M) (with rate constants of k 1 and k 2 , respectively) were studied at temperatures of 250 and 300 K over the pressure range of 0.01-1000 bar. Helium, argon, xenon, N 2 , CO 2 , and SF 6 were used as bath gases. CCl 3 radicals were generated via the photolysis of CCl 3 Br at 248 nm, and their absorption was monitored at 223.5 nm. The limiting "high-pressure" rate constants within the energy-transfer mechanism were determined, independent of density and the choice of the bath gas, over the pressure range of 1-10 bar, to be k 1 , ∞ (T) = (1.0 ′ 0.2) × 10 - 1 1 (T/300 K) - 0 . 1 7 cm 3 molecule - 1 s - 1 and k 2 , ∞ (T) = (2.0 ′ 0.2) x 10 - 1 1 (T/300 K) - 0 . 1 3 cm 3 molecule - 1 s - 1 . In the helium, N 2 , and argon bath gases, at pressures above ∼40 bar, the reactions became increasingly faster when the pressure was further raised until they finally started to slow at densities where diffusion-controlled kinetics dominates. This is the first detailed report of such a peculiar density dependence of combination rate constants for larger radicals with five or eight atoms. Possible origins of these pressure effects, such as the influence of the radical-complex mechanism and the density dependence of electronic quenching, are discussed.

The recombination reaction CCl3 + O2 (+M) → CCl3O2 (+M) was studied in the upper part of the falloff curve at 260, 300, and 346 K over the pressure range of 2−900 bar. CCl3 radicals were generated by photolysis of CCl3Br at 248 nm; the temporal decay of the absorption from CCl3 at 223.5 nm was monitored in the presence of O2; falloff curves of the rate constants were determined in N2 and He as bath gases. The falloff curves could well be represented by limiting high-pressure rate constants k∞ = (5.2 ± 0.2) × 10-12 (T/300 K)-1.4±0.4 cm3 molecule-1 s-1 from the present work, limiting low-pressure rate constants k0 = [N2] (1.1 ± 0.3) × 10-30 (T/300 K)-6.3±1.0 cm3 molecule-1 s-1 or k0 = [He] (4.2 ± 0.7) × 10-31 (T/300 K)-6.9±1.0 cm3 molecule-1 s-1 from the literature, and center-broadening factors Fc (N2) = (0.35 ± 0.03) (T/300 K)-0.35 or Fc (He) = (0.30 ± 0.03) (T/300 K)-0.48 derived from unimolecular rate theory. An onset of diffusion-controlled kinetics at pressures above about 300 bar of N2 was observed, ...