Vibrational relaxation and dissociation in the perfluoromethyl halides, CF3Cl, CF3Br, andCF3I

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF3Cl, CF3Br, and CF3I have been studied in the shock tube with the laser-schlieren technique. Vibrational relaxation was resolved in pure CF3Cl and CF3Br (400–484 K and 400–500 K, respectively), and in the mixtures; 2% CF3Cl/Kr (500–1000 K), 10% CF3Cl/Kr (440–670 K), 4% CF3Br/Kr (450–850 K), and 2% CF3I/Kr (620–860 K). Relaxation in the pure gases is extremely rapid, but shows a well-resolved, accurately exponential decay which provides very precise relaxation times in close agreement with ultrasonic results. Relaxation times as short as 0.1 μs-atm can be resolved, showing the method has a resolution within a factor 2–3 of the best ultrasonic methods. Relaxation dilute in rare gas shows a complex double exponential behavior consistent with a two-stage series process. Rates of CF3(SINGLEBOND)X fission in these mixtures were measured over 1800–3000 K, P<0.55 atm, for CF3Cl; 1600–2500 K, P<0.55 atm, in CF3Br; and 1260–2100 K, P<0.34 atm, in CF3I. Rates for dissociation were derived from a full profile modeling using a secondary mechanism of six CF3 reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with (ΔE)all=−270 cm−1 for CF3Cl, 〈ΔE〉down=0.3 T for CF3Br, and 〈ΔE〉down=800 cm−1 for CF3F. All these transfers are on the large side, similar to those found in other halogenated methanes.