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.

New photochromic switches based on helical alkenes can quickly and efficiently be accessed by Pd-catalyzed domino reactions using a modular approach; this allows a wide variability in product formation with the advantages of a convergent synthetic route. The alkenes have been synthesized in excellent enantioselectivity and their switching properties assessed by stimulation with nanosecond laser pulses at two different wavelengths in over 1000 switching cycles.

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 decomposition of CF4 (+Ar) → CF3 + F (+Ar) was studied in shock waves over the temperature range 2000-3000 K varying the bath gas concentration [Ar] between 4 × 10(-6) and 9 × 10(-5) mol cm(-3). It is shown that the reaction corresponds to the intermediate range of the falloff curve. By combination with room temperature data for the reverse reaction CF3 + F (+He) → CF4 (+He) and applying unimolecular rate theory, falloff curves over the temperature range 300-6000 K are modeled. A comparison with the reaction system CH4 (+M) ⇔ CH3 + H (+M) is made.