A. Miyoshi, J. Widjaja, Noboru Yamauchi
2002
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Abstract
The unimolecular isomerization reaction of the 1-pentyl radical (· CH 2 CH 2 CH 2 CH 2 CH 3 ) to the 2-pentyl radical (CH 3 ·CHCH 2 CH 2 CH 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 1 ∞ (1-pentyl→2-pentyl)/ s −1 =1.916×10 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 1 ∞ / s −1 =2.432×10 3 T 2.324 exp (−8183/ T )+9.111×10 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 N 2 . The fall-off behavior of the isomerization reaction with a tight transition state is also discussed.