DECOMPOSITION OF THE 1,3-BUTADIENE RADICAL CATION ON THE MICROSECOND TO PICOSECOND TIME SCALE
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The decomposition of ionized 1, 3-butadiene to give C3H3+and -CH3has been studied over the time interval of approximately 10-11-10-5s using the complementary techniques of field ionization kinetics (FIK) and mass-analyzed ion kinetic energy spectroscopy (MIKES). By employing both carbon-13 and deuterium labeled 1, 3-butadiene, it has been established that (1) hydrogen atom randomization takes place very rapidly and is complete by approximately 10-11s, (2) carbon atom randomization occurs at a slower rate which is competitive with the decomposition reaction. The hydrogen atom randomization is postulated to take place by isomerization to other acyclic C4H6 ions without change in the carbon skeleton. The near lack of carbon atom interchange at short times ( 10-9-10-11s) is explained by invoking a high-energy formation of a methylcyclopropene which occurs as a rate-determining step. For more slowly decomposing (metastable) C4H6ions, the ring-opening reaction of this intermediate to re-form the 1, 3-butadiene cation becomes more competitive with dissociation, and this explains the nearly complete carbon atom randomization at longer times. 1979, American Chemical Society. All rights reserved.