Experimental investigation of the kinetics and mechanism of the reaction of n-propyl radicals with molecular oxygen from 297 to 635 K

Abstract

The kinetics and mechanism of the n -C 3 H 7 +O 2 reaction were studied from 297 to 635 K to gain new knowledge of the change in mechanisms of R+O 2 reactions as a function of temperature. Studies were conducted using a heated tubular reactoor coupled to a photoionization mass spectrometer. n -C 3 H 7 and C 3 H 6 concentrations were monitored in real-time experiments initiated by laser photolysis of the free-radical precursor. Overall rate constants were measured as a function of gas density (He or N 2 ) from 1.2–12×10 16 molec cm −3 at four temperatures: 297, 400, 550 and 635 K. The dependencies of the rate constants on pressure and temperature, up to 550K were those expected of the addition reaction n -C 3 H 7 +O 2 →C 3 H 7 O 2 near the high pressure limit. The fraction of the n -C 3 H 7 +O 2 reaction that proceeds by an alternate reactive route which produces C 3 H 6 +HO 2 was determined at 297 K ( 0.00 ± 0.00 0.05 ) and at 550 K (0.14±0.05). At 635 K the R ⇌ RO 2 equilibrium was observed together with a delayed production of C 3 H 6 . The ultimate fractional yield of C 3 H 6 could not be observed at this temperature. It is estimated to be over 0.8. The results are discussed in terms of both an uncoupled parallel-path reaction mechanism leading to the two possible sets of reaction products and a coupled mechanism that proceeds via a common C 3 H 7 O 2 adduct.