R. Grice

Reactive scattering of a supersonic chlorine atom beam: Cl + CH3I

Reactive scattering of Cl atoms with CH3I molecules has been studied over the range of initial translational energy E = 62-23 kJ mol-1, using a supersonic beam of Cl atoms seeded in He and Ne buffer gas. Laboratory angular and velocity distributions of ICl product were measured which indicate that reaction occurs via a long-lived collision complex with forward scattering appearing only at the highest collision energy. The dependence of the total reaction cross section on initial translational energy was also determined. The observed reactive scattering arises from an endoergic dissociation of the collision complex. The results are interpreted by a model which accounts for the effect of angular momentum conservation on this endoergic dissociation compared with the favoured dissociation of the complex back to reactants.

Angular distributions of reactive scattering arising from persistent complexes with dissociation at arbitrary angles to the inertial axes of the transition state

Angular distributions of reactive scattering arising from a long-lived collision complex dissociating via a transition state that approximates to a linear rotator are predicted for H atom displacement occurring over a distribution of bending angles β with respect to the rotator axis. A conical angular distribution may be generated when only a narrow range of bending angles contributes to the reactive scattering. However, the angular distribution may take on quite different character as the distribution of bending angles becomes broader. Thus, an isotropic angular distribution is generated by a bending angle distribution which follows the spherical polar weighting P(β) ∝ sin β. Forward and backward scattering is generated by bending angle distributions favouring less strongly bent displacement, while sideways scattering is generated by a bending angle distribution favouring more strongly bent displacement. The experimental angular distribution for OH + CO reactive scattering which shows mild forward and ba...

Angular distributions of reactive scattering arising from persistent complexes with oblate symmetric top transition states

Microcanonical transition state theory has been employed to predict the angular distribution of reactive scattering arising from a long-lived collision complex dissociating via a transition state which approximates to a prolate symmetric top. Forward and backward peaked angular distributions arise only when the orientation of the product angular momentum becomes unconstrained with respect to the direction of the final relative velocity vector as the transition state dissociates to products. If, as assumed in previous models, the orientation of the product angular momentum is constrained to that prevailing at the transition state so that the final relative velocity vector is coincident with the symmetry axis of the transition state, then a conical angular distribution ensues. Explicit formulae for each of these limiting cases are presented. The physical basis for the ubiquity of forward and backward peaking of reactive scattering compared with the rarity of conical angular distributions is examined in the ...

Reactive scattering of a supersonic methyl radical beam: CH3 + ICl, IBr, I2, Br2

Reactive scattering of CH3 radicals with ICl, IBr, I2 and Br2 molecules has been studied at an initial translational energy E ∼ 30kJ mol-1, using a supersonic beam of CH3 radicals seeded in He buffer gas. Laboratory angular and velocity distributions of CH3I and CH3Br product have been measured which show a gradation from short-lived complex dynamics for ICl to rebound dynamics for I2 and Br2 with intermediate behaviour for IBr. This variation in dynamics is attributed to the form of the potential energy surface which shifts from a shallow hollow for CH3 + ICl to repulsion in the exit valley for CH3 + I2, Br2, with both features being present for the intermediate case of CH3 + IBr.

Reactive scattering of a supersonic chlorine atom beam: Cl + C2H3Br

Reactive scattering of Cl atoms with C2H3Br molecules has been studied at an initial translational energy E = 60 kJ mol-1, using a supersonic beam of Cl atoms seeded in He buffer gas. Laboratory angular and velocity distributions of C2H3Cl product have been measured which show reaction proceeding via a short-lived collision complex, with a lifetime ∼ 2 ± 1 rotational periods. The forward and backward scattering has a higher product translational energy, peaking at E′pk ∼ 35 kJ mol-1 than the sideways scattering peaking at E′pk ∼ 20 kJ mol-1. An extended version of the Halpern Strutinski model is presented to give an interpretation of the data whereby forward and backward scattering arises mainly from Cl atom attack at the CH2 end of the C2H3Br molecule followed by migration, but sideways scattering may include a contribution from Cl atom attack at the CHBr end of the molecule.

Reactive scattering of O(3P) atoms with alkyl and allyl iodide molecules

Abstract Differential cross sections have been measured for the reactions of O( 3 P) atoms with C 2 H 5 I, (CH 3 ) 2 CHI, (CH 3 ) 3 CI and C 3 H 5 I molecules at an initial translational energy E ≈ 36 kJ mol −1 using a supersonic beam of O atoms seeded in He buffer gas. The product angular distribution for O+C 2 H 5 I shows mild peaking in the forward and backward direction with slightly favours the backward direction, and a product translational energy distribution in accord with the predictions of phase space theory. The propensity toward backward scattering and the fraction of the total available energy disposed into product translation both increase progressively along the series of reactions. This trend is attributed to reaction at small impact parameters b ⩽2.5 A over a weakly attractive triplet potential energy surface, which splits into 3 A′ and 3 A″ Renner—Teller components in bent OIR configurations.

Reactive scattering of a supersonic oxygen-atom beam: O + C2H4, C2H2

Reactive scattering of O atoms with C2H4 and C2H2 molecules has been studied at an initial translational energy E≈ 27 kJ mol–1 using a supersonic beam of O atoms seeded in He. Reactive scattering of C2H3O and C2HO radicals is found to be nearly isotropic but slightly favours forward and sideways scattering, respectively. High product translational energies E′av≈ 30 kJ mol–1 are observed in both cases. The total reaction cross-sections were measured as a function of initial translational energy, using a He/Ne buffer-gas mixture to vary the O-atom velocity. The threshold energy E0= 5 ± 3 kJ mol–1 for O + C2H4 is in good agreement with the activation energy for the overall reaction of O atoms with C2H4 but the threshold energy E0= 8 ± 3 kJ mol–1 for O + C2H2 is slightly lower than the activation energy Ea= 12.5 kJ mol–1. Hence the H-atom displacement reactions represent important reaction pathways for the O + C2H4, C2H2 reactions which are initiated by bonding of the electrophilic O atom to the unsaturated bonds of the hydrocarbon molecules. The structures of the triplet intermediates for these reactions are compared with that proposed for the O + H2S reaction.

Reactive scattering via a persistent collision complex as a function of impact parameter

Abstract A recently proposed microcanonical transition state theory for angular distributions of reactive scattering arising from a persistent collision complex is extended to the full range of impact parameters. At larger impact parameters the angular distribution is governed by the precessional motion of the symmetric top transition state for complex dissociation while at smaller impact parameters the angular distribution is determined by the disposition of the product orbital angular momentum estimated from phase space theory. The extended theory predicts an angular distribution for F+C 2 H 5 I reactive scattering which is in excellent agreement with recent experimental measurements at an initial translational energy E ≈ 38 kJ mol −1 .

Precessional motion of asymmetric top complexes in reactive scattering

Differential reaction cross sections have been calculated from the classical precessional motion of a bent triatomic long-lived complex which constitutes an asymmetric top. The angular distributions are similar to those predicted by the Halpern Strutinski model for precession of a symmetric top but the singularities of this model are rounded off and the abrupt fall to zero is replaced by a smooth decline. Consideration of the stability of asymmetric top precession suggests that vibrational-rotational energy exchange may separate the motion of short-lived complexes rotating close to the plane of collision from the motion of those rotating far from the plane of collision.

Transition state dynamics of F + ICl, C2H5I reactive scattering

Reactive scattering of F atoms with ICl and C2H5I molecules has been studied at an initial translational energy E ∼ 12 kJ mol-1 using a supersonic beam of F atoms seeded in Ne buffer gas. The centre-of-mass angular distribution of IF scattering for F + ICl, measured previously at E ∼ 34 kJ mol-1, shows peaking mainly in the forward direction, which is consistent with reaction via a short-lived complex with a lifetime of half a rotational period. The microcanonical theory for dissociation of a persistent complex via a prolate symmetric top transition state accounts for the higher energy F + ICl scattering arising from the full range of impact parameters. The angular distribution for F + C2H5I shows peaking in the forward and backward directions which is consistent with a complex lifetime greater than one rotational period. The extended microcanonical theory accounts for scattering by a range of transition state configurations generated by internal rotation about the extended C-I bond for collisions at the ...