Imaging Proton Transfer and Dihalide Formation Pathways in Reactions of F(-) + CH3I.

Eduardo Carrascosa,Björn Bastian,Roland Wester,Jennifer Meyer,Tim Michaelsen,Jochen Mikosch,Martin Stei

doi:10.1021/acs.jpca.5b11181

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.

Highlights

The bimolecular nucleophilic substitution (SN2) reaction is one of the fundamental reaction types in physical organic chemistry and has been the subject of extensive experimental and theoretical studies for more than a century.[1−9] The simplest representation of a nucleophilic substitution consists of a methyl halide reacting with an atomic halogen anion: X− + CH3Y → CH3X + Y− (X, Y = F, Cl, Br, I) (1)
After the reactants have crossed, the field plates of the velocity map imaging (VMI) spectrometer are activated, and if created, a product ion is extracted perpendicular to the scattering plane onto a time- and position-sensitive detector, consisting of a microchannel plate (MCP), a phosphor screen, and a photomultiplier
The dominating product at all energies is I−. This anion is essentially produced via the classical nucleophilic substitution reaction, a process that is exothermic by 1.84 eV

Summary

Introduction

The energetics of nucleophilic substitution in the gas phase has been studied very precisely for decades and is described by a minimum energy pathway consisting of two potential energy wells separated by a potential energy barrier that corresponds to the transition state.[3,10] The barrier has been the subject of many studies because it acts as a hindrance to product formation. This hindrance occurs at thermal conditions even for submerged barriers due to the low density of states at this transition state. This mechanism is commonly called backside attack of the nucleophile

Methods

Results

Conclusion