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 kE ≈ 2 × 105 s−1 when little buffer gas was present to ~1.9 × 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.
Highlights
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
These authors simulated trajectories that indicated the importance of a large-amplitude excursion of the Cl atom far from equilibrium geometry within the chloroalkyl complex, which led to formation of HCl +allyl fragmentation[12], but a clear distinction in the internal energy distribution of HCl produced via abstraction from that via addition-elimination of Cl +i-C4H8 was unavailable
In this work we demonstrate a new experimental method to characterize the roaming path in the atom-molecule reaction Cl +i-C4H8 →HCl +C4H7 by using a step-scan Fourier-transform infrared (FTIR) spectrometer to obtain time-resolved IR emission spectra of HCl23–25
Summary
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. Employing crossed molecular beams to investigate the dynamics of the reaction Cl +i-C4H8, Suits and coworkers detected C4H7 with a slice ion-imaging method[9,10] These authors found that the addition-elimination path occurs from an abstraction-like Cl-H-C geometry rather than a conventional three-center or four-center transition state, and this geometry is attained through roaming excursions of the Cl atom from the initially formed adduct.
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