DIRECT INVESTIGATIONS ON THE THERMAL UNIMOLECULAR ISOMERIZATION REACTION OF 1-PENTYL RADICALS

摘要

The unimolecular isomerization reaction of the 1-pentyl radical (· CH{sub}2CH{sub}2CH{sub}2CH{sub}2CH{sub}3) to the 2-pentyl radical (CH{sub}3 CHCH{sub}2CH{sub}2CH{sub}3) has been directly investigated for the first time by laser photolysis-photoionization mass spectrometry in the temperature range 440-520 K and the pressure range 1-7 Torr in He buffer gas. 1-Pentyl radicals were generated by 193-nm photolysis of 1-chloropentane and the isomerization products, 2-pentyl radicals, were selectively ionized and detected utilizing the lower ionization potential of the 2-pentyl radical than that of the 1-pentyl radical. The rate constant was found to be in the fall-off region, and the limiting high-pressure rate constant was evaluated to be k{sub}1{sup}∞(1-pentyl→ 2-pentyl)/s{sup}-1=1.916×10{sup}10 exp(-8873/T), for 440-520 K by an RRKM/master-equation analysis including semiclassical tunneling correction. By an extrapolation using the derived RRKM model, the rate expression for a wider temperature range was also evaluated. Because of the large tunneling effect, the usual modified Arrhenius formula could not be applied as a rate expression, and the high-pressure limiting rate constant was expressed as a sum of two Arrhenius forms as k{sub}1{sup}∞/s{sup}-1= 2.432×10{sup}3 T{sup}2.324 exp(-8183/T)+ 9.111×10{sup}5 exp(-5303/T), for 300-1300 K. The RRKM calculation indicates the assumption of the high-pressure limit is valid up to 800-900 K at pressures above 1 atm of N2. The fall-off behavior of the isomerization reaction with a tight transition state is also discussed.