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

✍ Scribed by John H. Kiefer; Rammohan Sathyanarayana

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 243 KB
• Volume: 29
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/(sici)1097-4601(1997)29:9<705::aid-kin8>3.0.co;2-y

✦ Synopsis

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF 3 Cl, CF 3 Br, and CF 3 I have been studied in the shock tube with the laserschlieren technique. Vibrational relaxation was resolved in pure CF 3 Cl and CF 3 Br (400-484 K and 400-500 K, respectively), and in the mixtures; 2% CF 3 Cl/Kr (500-1000 K), 10% CF 3 Cl/Kr (440-670 K), 4% CF 3 Br/Kr (450-850 K), and 2% CF 3 I/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 s-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 CF 3 -X fission in these mixtures were measured over 1800-3000 K, for CF 3 Cl; 1600-2500 K, P Ͻ 0.55 atm, in CF 3 Br; and 1260-2100 K, in CF 3 I. Rates for dissociation were P Ͻ 0.55 atm, P Ͻ 0.34 atm, derived from a full profile modeling using a secondary mechanism of six CF 3 reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with for CF 3 Cl, for CF 3 Br, and

for CF 3 F. All these transfers are on the large side, similar to those found Ϫ 1 ͗⌬E͘ ϭ800 cm down