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To provide a deeper understanding of the kinetics of electron attachment to CF3, the six-dimensional potential energy surfaces of both CF3 and CF3− were developed by fitting ∼3000 ab initio points per surface at the AE-CCSD(T)-F12a/AVTZ level using the permutation invariant polynomial-neural network (PIP-NN) approach. The fitted potential energy surfaces for CF3 and CF3− had root mean square fitting errors relative to the ab initio calculations of 1.2 and 1.8 cm−1, respectively. The main active mode for the crossing between the two potential energy surfaces was identified as the umbrella bending mode of CF3 in C3v symmetry. The lowest energy crossing point is located at RCF = 1.306 A and θFCF = 113.6° with the energy of 0.051 eV above the minimum of the CF3 electronic surface. This value is only slightly larger than the experimental data 0.026 ± 0.01 eV determined by kinetic modeling of electron attachment to CF3. The small discrepancy between the theoretical and experimentally measured values is analyzed.

The rates of the reactions Fe(+) + N2O → FeO(+) + N2 and FeO(+) + CO → Fe(+) + CO2 are modeled by statistical rate theory accounting for energy- and angular momentum-specific rate constants for formation of the primary and secondary cationic adducts and their backward and forward reactions. The reactions are both suggested to proceed on sextet and quartet potential energy surfaces with efficient, but probably not complete, equilibration by spin-inversion of the populations of the sextet and quartet adducts. The influence of spin-inversion on the overall reaction rate is investigated. The differences of the two reaction rates mostly are due to different numbers of entrance states (atom + linear rotor or linear rotor + linear rotor, respectively). The reaction Fe(+) + N2O was studied either with (6)Fe(+) or with (4)Fe(+) reactants. Differences in the rate constants of (6)Fe(+) and (4)Fe(+) reacting with N2O are attributed to different contributions from electronically excited potential energy surfaces, such as they originate from the open-electronic shell reactants.

The quantum and classical expressions for the vibrational predissociation rates of two-degree-of-freedom triatomic van der Waals complexes are compared. The classical rate is approximated by its diffusional counterparts (Parts I and II of this series), while the expression for the quantum rate is based on the generalization of the Landau method (E. E. Nikitin and L. Pitaevski, Phys. Rev. A, 1994, 49, 695). The ratio of the classical to quantum rates is expressed by an analytical formula which shows the reason for the difference between the classical and quantum rates and which explains a similar trend of both rates with the change of the key parameters of the vibrational predissociation event. We conclude that the occasional numerical closeness of classical and quantum rates results from the delicate interplay between the classical diffusion through a narrow energy layer and quantum tunneling through the broad energy range.

The potential energy surfaces of the X 12A′- and C 12A″-states of the dissociation/recombination system NO2O+NO have been characterized by abinitio methods. Thermal rate constants for the high pressure limit of this reaction have been determined by calculating classical trajectories with simplified representations of the two abinitio potentials. The results well agree with the less accurate experimental data in this temperature range. The treatment is compared with relationships for model valence potentials which were derived earlier. It is demonstrated which parts of the potential are most relevant and to what extent the rate constants could have been predicted without specific abinitio information.

The falloff curve of the recombination O+NO+N2→NO2+N2 has been studied at temperatures between 200 and 400 K and at pressures between 2 and 200 bar. Oxygen atoms were produced by laser flash photolysis of N2O at 193 nm, NO2 was detected by light absorption at 405 nm. The results supersede earlier data from relative rate measurements. Neglecting the effects of an onset of diffusion control at high pressures, limiting rate constants krec,∞ of 4.7×10-11 and 4.0×10-11 cm3 molecule-1 s-1 at 200 and 300 K, respectively, are obtained by extrapolation. Accounting for diffusion control increases the extrapolated values by 30 to 50% such that, for example, krec,∞(300 K)≈5±1×10-11 cm3 molecule-1 s-1. Rate constants for the high pressure limiting rate constants for recombination and dissociation, as well as for the vibrational relaxation of NO by oxygen atoms, over the range 200–2700 K are compared. Most of the observed temperature dependence of k∞ can be attributed to that of the thermal population of the lowest electronic fine structure components of O and NO. The remaining temperature dependence is estimated to be weaker than T-0.5.

The thermal decomposition of biphenylene was studied by UV absorption spectroscopy in reflected shock waves over the temperature range 1470–1860 K. Rates for the first three steps of the reaction sequence were determined, the primary step being characterized by a rate constant of k=1.2×1016 exp(-374 kJ mol-1/RT) s-1. By comparison with single pulse shock tube measurements the reaction mechanism could be identified as a sequence of ring opening processes followed by fragmentation of the forming linear chains.

The ion-molecule reactions F-e+ + N2O -> FeO+ + N-2 and FeO+ + CO -> Fe+ + CO2, which catalyze the reaction CO + N2O -> CO2 + N-2, have been studied over the temperature range 120-700 K using a variable temperature selected ion flow tube apparatus. Values of the rate constants for the former two reactions were experimentally derived as k(2) (10(-11) cm(3) s(-1)) = 2.0(+/- 0.3) (T/300)(-1.5(+/- 0.2)) + 6.3(+/- 0.9) exp(-515(+/- 77)/T) and k(3) (10(-10) cm(3) s(-1)) = 3.1(+/- 0.1) (T/300)(-0.9(+/- 0.1)). Characterizing the energy parameters of the reactions by density functional theory at the B3LYP/TZVP level, the rate constants are modeled, accounting for the intermediate formation of complexes. The reactions are characterized by nonstatistical intrinsic dynamics and rotation-dependent competition between forward and backward fluxes. For Fe+ + N2O, sextet-quartet switching of the potential energy surfaces is quantified. The rate constant for the clustering reaction FeO+ + N2O + He -> FeO(N2O)(+) + He was also measured, being k(4) (10(-27) cm(6) s(-1)) = 1.1(+/- 0.1) (T/300)(-2.5(+/- 0.1)) in the low pressure limit, and analyzed in terms of unimolecular rate theory.

The data bases for low-pressure rate coefficients of the dissociation of O3 and the reverse recombination of O with O2 in the bath gases M = He, Ar, N2, CO2 and SF6 are carefully analyzed. At very high temperatures, the rate constants have to correspond solely to the energy transfer (ET) mechanism. On condition that this holds for Ar and N2 near 800 K, average energies transferred per collision of −〈ΔE〉/hc = 18 and 25 cm−1 are derived, respectively. Assuming an only weak temperature dependence of 〈ΔE〉 as known in similar systems, rate coefficients for the ET-mechanism are extrapolated to lower temperatures and compared with the experiments. The difference between measured and extrapolated rate coefficients is attributed to the radical complex (RC) mechanism. The derived rate coefficients for the RC-mechanism are rationalized in terms of equilibrium constants for equilibria of van der Waals complexes of O (or O2) with the bath gases and with rate coefficients for oxygen abstraction from these complexes. The latter are of similar magnitude as rate coefficients for oxygen isotope exchange which provides support for the present interpretation of the reaction in terms of a superposition of RC- and ET-mechanisms. We obtained rate coefficients for the ET-mechanism of kETrec,0/[Ar] = 2.3 × 10−34(T/300)−1.5 and kETrec,0/[N2] = 3.5 × 10−34(T/300)−1.5 cm6 molecule−2 s−1 and rate coefficients for the RC-mechanism of kRCrec,0/[Ar] = 1.7 × 10−34(T/300)−3.2 and kRCrec,0/[N2] = 2.5 × 10−34(T/300)−3.3 cm6 molecule−2 s−1. The data bases for M = He, CO2 and SF6 are less complete and only approximate separations of RC- and ET-mechanism were possible. The consequences of the present analysis for an analysis of isotope effects in ozone recombination are emphasized.

The combination reaction C6H5CH2 + C6H5CH2 (+M) → C14H14 (+M) was studied over the pressure range 0.03–900 bar and the temperature range 250–400 K. Helium, argon, xenon, N2 and CO2 were employed as bath gases. Benzyl radicals were generated by H abstraction from toluene by Cl atoms formed via laser photolysis of Cl2 at 308 nm. Benzyl radicals were monitored by time-resolved transient UV absorption at 253 nm. The observed combination rates were pressure independent over the range 0.04–0.45 bar in CO2 and 0.03–5 bar in argon which identifies a limiting “high-pressure” value of rate constant within the energy-transfer mechanism, kET∞(T) = (4.1 ± 0.3) × 10−11 (T/300 K)−0.23 cm3 molecule−1 s−1. Although the reaction has clearly reached the “high-pressure limit” (kET∞) at pressures below 1 bar, a further increase of the rate constants was observed when the pressure of the bath gas was raised above ∼5 bar. At much higher pressures finally the rate constants decrease when diffusion-controlled kinetics takes over. The degree of enhancement of the combination rates beyond the “high-pressure limit” was found to depend on the bath gas, increasing in the order He < N2 ≈ Ar < CO2. The enhancement was most prominent at low temperatures. Measurements of transient UV absorption spectra of benzyl radicals, over the pressure range 5–30 bar of CO2 at 300 K, confirmed that an only small pressure-dependent solvatochromic shift of benzyl radicals cannot be responsible for the observation of enhanced rate constants. Instead, the results clearly point toward a significant effect of van der Waals clustering, i.e. of “radical-complex” formation, on the combination reaction kinetics in the gas–liquid transition range. An analysis in terms of statistical adiabatic channel/classical trajectory calculation (SACM/CT) appears to provide a consistent description.

The combination reaction of two benzyl radicals has been studied in a wide range of pressures (0.01–1000 bar) and temperatures (250–400 K) in various bath gases (Ar, N2 and CO2). The measured second-order combination rate constants of benzyl radicals were independent of the pressure and the bath gas below 1 bar, as expected for a limiting “high pressure” rate constant of a termolecular combination process. However, the reaction becomes steadily faster when the pressure in Ar is further raised until it finally starts to decrease when densities corresponding to diffusion controlled kinetics are reached. Such a unique pressure dependence was more strongly accentuated in CO2 and at lower temperatures. Our results seem to provide first clear indications for a contribution of the radical–solvent interaction in the combination reaction kinetics of such large radicals as benzyl in the gas to liquid transition range.