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There has been considerable debate and speculation about the role of weakly bound complexes in radical−molecule reactions in the gas phase, especially in atmospheric chemistry. Among the significant number of potentially important molecular aggregates in chemical reactions, water complexes are of particular interest. Beyond the well-known energy transfer role of water in complex-forming reactions, it has been shown that water may also have a catalytic effect on the kinetics of radical-molecule reactions because of reduced reaction barrier heights for the complexes. Here we report studies of the reaction of OH radicals and propionaldehyde in the presence and absence of water vapor between 300 and 60 K in Laval nozzle expansions. Water accelerates the overall reaction at low temperatures but much less pronounced than for the reaction of OH with acetaldehyde reported recently. Quantum chemical calculations help us to understand this behavior, which can be rationalized in terms of the stability of intermediate reaction complexes and the effect of water aggregation on the barrier separating prereactive complexes and products. [ABSTRACT FROM AUTHOR]

The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF 4 (+M) ⇄ CF 3 + 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.

Monitoring UV absorption signals of SiF 2 and SiF, the thermal dissociation reactions of SiF 4 and SiF 2 were studied in shock waves. Rationalizing the experimental observations by standard unimolecular rate theory in combination with quantum-chemical calculations of the reaction potentials, rate constants for the thermal dissociation reactions of SiF 4 , SiF 3 , and SiF 2 and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.

The thermal dissociation reactions of C 2 F 4 and C 2 F 6 were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C 2 F 4 , CF 2 , CF, and C 2 were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C 2 F 4 , CF 3 , and CF 2 , the kinetics of secondary reactions were investigated. For example, the reaction CF 2 + CF 2 → CF + CF 3 was identified. Its rate constant of 10 10 cm 3 mol -1 s -1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF 2 + CF 2 → C 2 F 4 , suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF 2 (+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C 2 radicals was shown to involve the reaction CF + CF → C 2 F + F (modeled rate constant 8.0 × 10 12 ( T /3500 K) 1.0 exp(-4400 K/ T ) cm 3 mol -1 s -1 ) and the subsequent dissociation C 2 F (+Ar) → C 2 + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 10 16 ( T /3500 K) -4.0 exp(-56880 K/ T ) cm 3 mol -1 s -1 ). This mechanism was validated by monitoring the dissociation of C 2 at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.

The thermal dissociation reaction CF 3 (+Ar) → CF 2 + F (+Ar) was studied in incident and reflected shock waves by monitoring UV absorption signals of the primary dissociation product CF 2 . CF 3 radicals were produced by thermal decomposition of CF 3 I. Accounting for secondary reactions of F atoms, rate constants for the unimolecular dissociation were derived. Experimental parts of the falloff curves were obtained over the ranges 1544-2106 K and 1.0 × 10 -5 ≤ [Ar] ≤ 9.3 × 10 -5 mol cm -3 . Theoretical modeling allowed for a construction of the full falloff curves connecting the limiting low-pressure rate constants k 0 = [Ar] 2.5 × 10 18 ( T /2000 K) -5.1  exp (-42 450 K/ T ) cm 3 mol -1 s -1 with the limiting high-pressure rate constants k ∞ = 1.6 × 10 16 ( T /2000 K) -1.3  exp (-43 250 K/ T ) s -1 (center broadening factors of F cent = 0.25, 0.22, and 0.20 at 1500, 2000, and 2500 K, respectively, were used). The influence of simplifications of falloff expressions and of limiting rate constants on the representation of experimental data is discussed.

The unimolecular dissociation of CH 2 F 2 leading to CF 2 + H 2 , CHF + HF, or CHF 2 + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400-1800 K, monitoring the formation of CF 2 . It is shown that the energetically most favorable dissociation channel leading to CF 2 + H 2 has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH 2 F 2 → CHF + HF is followed by the reaction CHF + HF → CF 2 + H 2 . The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH 2 F 2 * intermediates, as assumed before.

The reaction of CF 2 with H 2 was studied in shock waves by monitoring UV absorption signals. CF 2 was prepared by thermal dissociation of C 2 F 4 (or of c-C 3 F 6 ). The rate constant of the reaction CF 2 + H 2 → CHF + HF near 2000 K was found to be close to 10 11 cm 3 mol -1 s -1 , consistent with earlier information on the reverse reaction CHF + HF → CF 2 + H 2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C 2 F 4 between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF 2 near 2000 K were measured between 210 and 300 nm and compared with room-temperature data. Additional superimposed absorption signals were recorded during the reaction and identified by their time dependence and by quantum-chemical calculations employing time-dependent density functional theory. A previously unknown absorption spectrum of CHF radicals near 200 nm was identified and its wavelength dependence determined. Further strong absorptions between 190 and 300 nm were attributed to CH 2 F and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C 2 radicals formed at later stages of the reaction.

The thermal dissociation of CH 2 F 2 and the reaction of CF 2 with H 2 was studied in shock waves over the temperature range 1800-2200 K, monitoring the absorption-time profiles at 248 nm. Besides contributions from CF 2 , the signals showed strong absorptions from secondary reaction products, probably mostly CH 2 F formed by the reaction CHF + H 2 → CH 2 F + H. Rate constants of a series of possible secondary reactions were modeled, including falloff curves for the thermal dissociations of CHF, CHF 2 , and CH 2 F and rate constants of the reactions CHF + CH 2 F 2 → CHF 2 + CH 2 F, CHF + H 2 → CH 2 F + H, H + CH 2 F 2 → CHF 2 + H 2 , H + CF 2 → CF + HF, and H + CF → C + HF. On this basis, concentration-time profiles were simulated and compared with experimental absorption-time profiles.

The thermal dissociation of C3F6 was studied between 1330 and 2210 K in shock waves monitoring the UV absorption of CF2. CF2 yields of about 2.6 per parent C3F6 were obtained at reactant concentrations of 500-1000 ppm in the bath gas Ar. These yields dropped to about 1.8 when reactant concentrations were lowered to 60 ppm. The increase of the CF2 yield with increasing concentration was attributed to bimolecular reactions between primary and secondary dissociation products. Quantum-chemical and kinetic modeling calculations helped to estimate the contributions from the various primary dissociation steps. It was shown that the measurements correspond to unimolecular reactions in their falloff range. Falloff representations of the rate constants are given, leading to an overall high pressure rate constant k∞ = 2.0 × 10(17)(-104 kcal mol(-1)/RT) s(-1) and a relative rate of about 2/3:1/3 for the reactions C3F6 → CF3CF + CF2 versus C3F6 → C2F3 + CF3.

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent, with a C-C bond energy of 67.5 (±0.5) kcal mol(-1). High-pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface, such as demonstrated by quantum-chemical calculations of the potential energy surface.