Reactive scattering of O( 3 P) atoms with C 2 H 5 I molecules has been studied at initial translational energies E ∼ 51 and 16 kJ mol -1 using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. A mildly forward-and backward-peaked angular distribution of IO product observed at lower initial translational energy becomes backward-scattered at higher initial translational energy. An isotropic angular distribution is observed for HOI product at lower initial translational energy which becomes forward-and backward-peaked, slightly favoring the backward direction at higher initial translational energy. Extended phase space calculations indicate that reaction leading to IO product proceeds via intersystem crossing from the lowest triplet 3 A′′ potential energy surface to a bound OIC 2 H 5 intermediate on the singlet 1 A′ potential energy surface in small impact parameter collisions at low initial translational energy. However, the emergence of backward-scattered IO product indicates the onset of direct reaction over the triplet 3 A′′ potential energy surface at higher initial translational energy. The formation of HOI reaction product arises from the singlet OIC 2 H 5 complex via a five-membered ring transition state where the H atom is abstracted from the terminal CH 3 group at the top of a barrier leading to vibrational excitation of HOI and repulsion between the reaction products in this strongly exoergic exit valley of the potential energy surface.

Reactive scattering of F atoms with C 6 H 5 Br and C 6 H 5 I molecules leading to both hydrogen and halogen atom displacement has been studied at an initial translational energy E ∼ 38 kJ mol -1 using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distributions of C 6 H 5 F reactive scattering show symmetrical forward and backward peaking, which is consistent with a long-lived collison complex, but the product translational energy distributions peak at a fraction f′ pk ∼ 0.18 of the total available energy and lie well above the predictions of phase space theory. The branching ratio ∼4 favors halogen atom over hydrogen atom displacement. The H atom displacement pathway is impeded by a potential energy barrier and occurs in competition with atom migration around the benzene ring. The halogen atom displacement occurs directly an F or H atom becomes bonded to the carbon atom adjacent to the halogen atom, following F atom or possibly H atom migration from other locations on the ring. The H atom displacement pathway for both FC 6 H 4 Br and FC 6 H 4 I reaction products shows a nominally isotropic angular distribution and a product translation energy distribution peaking at a fraction f′ pk ∼ 0.2 of the total available energy, in line with the H atom displacement dynamics previously observed for F + C 6 H 5 Cl.

Reactive scattering of O( 3 P) atoms with (CH 3 ) 2 CHI and (CH 3 ) 3 CI molecules has been studied at an initial translational energy E ∼ 51 kJ mol -1 using a supersonic beam of O atoms seeded in He buffer gas from a microwave discharge source. Sharply backward peaked angular distributions of IO product scattering are observed with distinctly higher product translational energy for the backward compared with the forward and sideways scattering. Mildly forward and backward peaked angular distributions slightly favoring the backward direction are observed for HOI product scattering with the product translational energy distribution being independent of scattering angle. The component of IO scattering with low product translational energy and the HOI scattering are both attributed to intersystem crossing from the lowest triplet 3 A′′ potential energy surface to a bound OIR intermediate on the singlet 1 A′ potential energy surface. The HOI product is formed via a five-membered ring transition state where the H atom is abstracted from a terminal CH 3 group of the alkyl radical R. The fraction of singlet OIR complexes which dissociates to HOI product compared with IO product increases from ∼0.3 for R ) C 2 H 5 to ∼0.5 for R ) (CH 3 ) 2 CH and (CH 3 ) 3 C. The backward scattered IO product is attributed to direct reaction over the triplet 3 A′′ potential energy surface and contributes a fraction ∼ 1 / 5 to the total reaction cross section largely independent of the alkyl radical.Dynamics of the O( 3 P) + (CH 3 ) 2 CHI, (CH 3 ) 3 CI Reactions

Reactive scattering of O(3P) atoms with C3H5I molecules has been studied at initial translational energies E ∼ 46 and 16 kJ mol-1 using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. Strongly backward peaked angular distributions of IO product scattering are observed at both initial translational energies with the peaking becoming sharper at higher energy. The product translational energy distributions are pitched at higher energy for the backward scattering and shift progressively toward lower energy as the IO scattering moves round into the forward direction. Backward scattering with high product translational energy is attributed to direct reaction in small impact parameter collisions b ≤ 2.5 Å over the triplet potential energy surface. Scattering with low product translational energy which becomes perceptible in the forward direction is attributed to intersystem crossing to the underlying singlet potential energy surface forming a bound OIC3H5 intermediate complex. Direct reaction over the 3A‘‘ potential energy surface plays a more significant role for the exoergic allyl iodide reaction than in the nearly thermoneutral reactions of alkyl iodide molecules, where intersystem crossing to the 1A‘ potential energy surface is the predominant reaction mechanism. The dynamical basis for this difference in mechanism is discussed in terms of the topography of the potential energy surfaces involved.