New experimental evidence to support roaming in the reaction Cl plus isobutene (i-C4H8)

Abstract

The reaction Cl + isobutene (i-C4H8) was reported by Suits et al. to proceed via, in addition to abstraction, an addition-elimination path following a roaming excursion of Cl; a near-zero translational energy release and an isotropic angular distribution observed at a small collision energy characterized this mechanism. We employed a new experimental method to further characterize this roaming mechanism through observation of the internal distribution of HCl (v, J) and their temporal behavior upon irradiation of a mixture of Cl2C2O2 and i-C4H8 in He or Ar buffer gas. With 1-3 Torr buffer gas added to approach the condition of small collision energy, the intensities of emission of HCl (v = 1, 2) and the HCl production rates increased significantly; Ar shows a more significant effect than He because Ar quenches Cl more efficiently to reduce the collisional energy and facilitate the roaming path. According to kinetic modeling, the rate of addition-elimination (roaming) increased from k(E) approximate to 2 x 10(5) s(-1) when little buffer gas was present to similar to 1.9 x 106 s(-1) when 2-3 Torr of Ar was added, and the branching ratio for formation of [HCl (v = 2)]/[ HCl (v = 1)] increased from 0.02 +/- 0.01 for abstraction to 0.06 +/- 0.01 for roaming.