Imaging Proton Transfer and Dihalide Formation Pathways in Reactions of F^– + CH₃I

Abstract

Ion–molecule reactions of the type X– + CH3Y are commonly assumed to produce Y– through bimolecular nucleophilic substitution (SN2). Beyond this reaction, additional reaction products have been observed throughout the last decades and have been ascribed to different entrance channel geometries differing from the commonly assumed collinear approach. We have performed a crossed beam velocity map imaging experiment on the F– + CH3I reaction at different relative collision energies between 0.4 and 2.9 eV. We find three additional channels competing with nucleophilic substitution at high energies. Experimental branching ratios and angle- and energy differential cross sections are presented for each product channel. The proton transfer product CH2I– is the main reaction channel, which competes with nucleophilic substitution up to 2.9 eV relative collision energy. At this level, the second additional channel, the formation of IF– via halogen abstraction, becomes more efficient. In addition, we present the first evidence for an [FHI]− product ion. This [FHI]− product ion is present only for a narrow range of collision energies, indicating possible dissociation at high energies. All three products show a similar trend with respect to their velocity- and scattering angle distributions, with isotropic scattering and forward scattering of the product ions occurring at low and high energies, respectively. Reactions leading to all three reaction channels present a considerable amount of energy partitioning in product internal excitation. The internally excited fraction shows a collision energy dependence only for CH2I–. A similar trend is observed for the isoelectronic OH– + CH3I system. The comparison of our experimental data at 1.55 eV collision energy with a recent theoretical calculation for the same system shows a slightly higher fraction of internal excitation than predicted, which is, however, compatible within the experimental accuracy.

Proton transfer dynamics modified by CH-stretching excitation

“…This coincides with the opening of two halide abstraction channels. 27 Vibrational excitation again leads to an increase in the proton transfer to S N 2 ratio at 2.5 eV total energy, although this increase is not as strong compared to the data at 1.2 eV collision energy. We still observe a significant increase by a factor of 1.5.…”

“…A direct, forward scattered mechanism, often referred to as stripping and an indirect, isotropically scattered mechanism around zero product velocity. We know from previous studies, 27 that the general trend with additional collision energy in this reaction leads to more forward, direct scattering. For the vibrationally excited reaction, the additional energy instead leads only to an indirect scattering signature (see Fig.…”

“…The points for excited reactions are scaled to 100% excitation in the neutral beam. The grey points 27 and the light blue (ground state) and pink points (vibrationally excited) 28 are from previous studies by our group. Points shown in blue are for ground state and in red for vibrationally excited reactions.…”

High-level ab initio potential energy surface and dynamics of the F−+ CH3I SN2 and proton-transfer reactions

“…The branching ratio between the S N 2 and proton-abstraction channels was recently measured by Wester and co-workers. 14 In the E coll range of 30–50 kcal mol –1 the abstraction channel was found to contribute about 20% to the total reactivity. Without ZPE constraint the present simulation overestimates the abstraction cross sections and gives about 60% contribution.…”

Imaging the dynamics of ion–molecule reactions

“…In a recent experimental work, we have investigated the branching ratio and reaction dynamics of these pathways in F À + CH 3 I as a function of relative collision energy. 166 Formation of a protonated dihalide anion [FHI] À was identified for the first time during this study. While the S N 2 channel is the dominant product at all studied relative collision energies, all other channels contribute to a considerable extent at energies above 1 eV.…”