Intramolecular Vibrational Energy Redistribution Involving the Torsion in CF3CH3: A Molecular Dynamics Study

On the Chaperon Mechanism: Application to ClO + ClO (+N2) → ClOOCl (+N2)

The dynamics of the ClO + ClO (+N(2)) radical complex (or chaperon) mechanism is studied by electronic structure methods and quasi-classical trajectory calculations. The geometries and frequencies of the stationary points on the potential energy surface (PES) are optimized at the B3LYP/6-311+G(3df) level of theory, and the energies are refined at the CCSD(T)/6-311+G(3df) (single-point) level of theory. Basis set superposition error (BSSE) corrections are applied to obtain 1.5 kcal mol(-1) for the binding energy of the ClO.N(2) van der Waals (VDW) complex. A model PES is developed and used in quasi-classical trajectory calculations to obtain the capture rate constant and nascent energy distributions of ClOOCl* produced via the chaperon mechanism. A range of VDW binding energies from 1.5 to 9.0 kcal mol(-1) are investigated. The anisotropic PES for the ClO.N(2) complex and a separable anharmonic oscillator approximation are used to estimate the equilibrium constant for formation of the VDW complex. Rate constants, branching ratios to produce ClOOCl, and nascent energy distributions of excited ClOOCl* are discussed with respect to the VDW binding energy and temperature. Interestingly, even for weak VDW binding energies, the N(2) usually carries away enough energy to stabilize the nascent ClOOCl*, although the VDW equilibrium constant is small. For stronger binding energies, the stabilization efficiency is reduced, but the capture rate constant is increased commensurately. The resulting rate constants for forming ClOOCl* from the title reaction are only weakly dependent on the VDW binding energy and temperature. As a result, the relative importance of the chaperon mechanism is mostly dependent on the VDW equilibrium constant. For the calculated ClO.N(2) binding energy of 1.5 kcal mol(-1), the VDW equilibrium constant is small, and the chaperon mechanism is only important at very high pressures.

Dissociation of 1,1,1-Trifluoroethane Behind Reflected Shock Waves: Shock Tube/Time-of-Flight Mass Spectrometry Experiments

The dissociation of 1,1,1,-trifluoroethane, a potential non-RRKM reaction, has been studied at 600 and 1200 Torr and high temperatures (1500-1840 K) using a new shock tube/time-of-flight mass spectrometer (ST/TOF-MS). These data obtained by an independent method are in good agreement with the laser schlieren, LS, experiments of Kiefer et al. [J. Phys. Chem. A 2004, 108, 2443-2450] and extend the range of that experimental dataset. The data have been simulated by both standard RRKM calculations and the non-RRKM model reported by Kiefer et al. but with <DeltaE(down)> = 750 cm(-1). Both the RRKM and non-RRKM calculations provide equally good fits to the ST/TOF-MS data. Neither model simulates the combined ST/TOF-MS and LS datasets particularly well. However, the non-RRKM model predicts a pressure dependency closer to that observed in the experiments than the RRKM model.

Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500-2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late observations. We thus include a laser schlieren study (1717-2332 K, 75 < P < 482 torr in 10 and 4% vinyl fluoride in Kr) of this dissociation. This latter work also includes a set of experiments using shock-tube time-of-flight mass spectrometry (4% vinyl fluoride in neon, 1500-1980 K, 500 < P < 1300 torr). These time-of-flight experiments confirm the theoretical expectation that the only reaction in vinyl fluoride is HF elimination. The dissociation experiments are augmented by laser schlieren measurements of vibrational relaxation (1-20% C(2)H(3)F in Kr, 415-1975 K, 5 < P < 50 torr, and 2 and 5% C(2)H(4)F(2) in Kr, 700-1350 K, 6 < P < 22 torr). These experiments exhibit very rapid relaxation, and incubation delays should be negligible in dissociation. An RRKM model of dissociation in 1,1-difluoroethane based on a G3B3 calculation of barrier and other properties fits the experiments but requires a very large DeltaE(down) of 1600 cm(-1), similar to that found in a previous examination of 1,1,1-trifluoroethane. Dissociation of vinyl fluoride is complicated by the presence of two parallel HF eliminations, both three-center and four-center. Structure calculations find nearly equal barriers for these, and TST calculations show almost identical k(infinity). An RRKM fit to the observed falloff again requires an unusually large DeltaE(down) and the experiments actually support a slightly reduced barrier. These large energy-transfer parameters now seem routine in these large fluorinated species. It is perhaps a surprising result for which there is as yet no explanation.