COMPARISON OF THE KINETICS OF O(P-3) REACTIONS WITH THE 4 BUTENES OVER WIDE TEMPERATURE RANGES

Rate coefficients for reactions of ground-state oxygen atoms with isob utene, trans-2-butene, and cis-2-butene have been measured using the h igh-temperature photochemistry (HTP) technique. The oxygen atoms were generated by flash photolysis of either O-2 or CO2 and the relative at om concentrations were monitored by resonance fluorescence. We find, i n cm(3) molecule(-1) s(-1) units, for isobutene, k(284-1015 K) = 2.0 x 10(-15). (T/K)(1.28) exp(543 K/T); trans-2-butene, k(292-1022 K) = 3. 9 X 10(-17)(T/K)(1.87) exp(743 K/T); and cis-2-butene, k(292-1086 K) = 2.4 X 10(-17)(T/K)(1.94) exp(716 K/T). The 2 sigma precision limits a re around +/-10%, and the corresponding 2 sigma accuracy limits are es timated to be about +/-25%. Conventional transition-state-theory calcu lations for the electrophilic O-addition to the double bond yield at h igh temperatures rate coefficients smaller than observed. A method is developed for estimating rate coefficients for H abstraction. When the results are added to those of the addition, good agreement with exper iments is attained. The differences between the k(T) expressions for t he present three reactions and the corresponding 1-butene reaction are discussed in terms of the electronic effects of alkyl substituents on the double bond, and the greater curvature of the addition reaction A rrhenius plot and faster H abstraction for 1-butene. Based on these ob servations, a method is presented for estimating rate coefficients for O-atom reactions with higher olefins over wide temperature ranges.