Atomistic dynamics of elimination and nucleophilic substitution disentangled for the F- + CH3CH2Cl reaction.

Chemical reaction dynamics are studied to monitor and understand the concerted motion of several atoms while they rearrange from reactants to products. When the number of atoms involved increases, the number of pathways, transition states and product channels also increases and rapidly presents a challenge to experiment and theory. Here we disentangle the dynamics of the competition between bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2) in the polyatomic reaction F- + CH3CH2Cl. We find quantitative agreement for the energy- and angle-differential reactive scattering cross-sections between ion-imaging experiments and quasi-classical trajectory simulations on a 21-dimensional potential energy hypersurface. The anti-E2 pathway is most important, but the SN2 pathway becomes more relevant as the collision energy is increased. In both cases the reaction is dominated by direct dynamics. Our study presents atomic-level dynamics of a major benchmark reaction in physical organic chemistry, thereby pushing the number of atoms for detailed reaction dynamics studies to a size that allows applications in many areas of complex chemical networks and environments.

Stretching vibration is a spectator in nucleophilic substitution.

How chemical reactions are influenced by reactant vibrational excitation is a long-standing question at the core of chemical reaction dynamics. In reactions of polyatomic molecules, where the Polanyi rules are not directly applicable, certain vibrational modes can act as spectators. In nucleophilic substitution reactions, CH stretching vibrations have been considered to be such spectators. While this picture has been challenged by some theoretical studies, experimental insight has been lacking. We show that the nucleophilic substitution reaction of F- with CH3I is minimally influenced by an excitation of the symmetric CH stretching vibration. This contrasts with the strong vibrational enhancement of the proton transfer reaction measured in parallel. The spectator behavior of the stretching mode is supported by both quasi-classical trajectory simulations and the Sudden Vector Projection model.

Conservation of direct dynamics in sterically hindered SN2/E2 reactions.

Nucleophilic substitution (SN2) and base-induced elimination (E2), two indispensable reactions in organic synthesis, are commonly assumed to proceed under stereospecific conditions. Understanding the way in which the reactants pre-orient in these reactions, that is its stereodynamics, is essential in order to achieve a detailed atomistic picture and control over such processes. Using crossed beam velocity map imaging, we study the effect of steric hindrance in reactions of Cl- and CN- with increasingly methylated alkyl iodides by monitoring the product ion energy and scattering angle. For both attacking anions the rebound mechanism, indicative of a direct SN2 pathway, is found to contribute to the reaction at high relative collision energies despite being increasingly hindered. An additional forward scattering mechanism, ascribed to a direct E2 reaction, also contributes at these energies. Inspection of the product energy distributions confirms the direct and fast character of both mechanisms as opposed to an indirect reaction mechanism which leads to statistical energy redistribution in the reaction complex. This work demonstrates that nonstatistical dynamics and energetics govern SN2 and E2 pathways even in sterically hindered exchange reaction systems.

Imaging dynamic fingerprints of competing E2 and SN2 reactions.

The competition between bimolecular nucleophilic substitution and base-induced elimination is of fundamental importance for the synthesis of pure samples in organic chemistry. Many factors that influence this competition have been identified over the years, but the underlying atomistic dynamics have remained difficult to observe. We present product velocity distributions for a series of reactive collisions of the type X- + RY with X and Y denoting the halogen atoms fluorine, chlorine and iodine. By increasing the size of the residue R from methyl to tert-butyl in several steps, we find that the dynamics drastically change from backward to dominant forward scattering of the leaving ion relative to the reactant RY velocity. This characteristic fingerprint is also confirmed by direct dynamics simulations for ethyl as residue and attributed to the dynamics of elimination reactions. This work opens the door to a detailed atomistic understanding of transformation reactions in even larger systems.The competition between chemical reactions critically affects our natural environment and the synthesis of new materials. Here, the authors present an approach to directly image distinct fingerprints of essential organic reactions and monitor their competition as a function of steric substitution.

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The proton transfer reaction H3(+) + CO is one of the cornerstone chemical processes in the interstellar medium. Here, the dynamics of this reaction have been investigated using crossed beam velocity map imaging. Formyl product cations are found to be predominantly scattered into the forward direction irrespective of the collision energy. In this process, a high amount of energy is transferred to internal product excitation. By fitting a sum of two distribution functions to the measured internal energy distributions, the product isomer ratio is extracted. A small HOC(+) fraction is obtained at a collision energy of 1.8 eV, characterized by an upper limit of 24% with a confidence level of 84%. At lower collision energies, the data indicate purely HCO(+) formation. Such low values are unexpected given the previously predicted efficient formation of both HCO(+) and HOC(+) isomers for thermal conditions. This is discussed in light of the direct reaction dynamics that are observed.

Influence of the leaving group on the dynamics of a gas-phase SN2 reaction.

In addition to the nucleophile and solvent, the leaving group has a significant influence on SN2 nucleophilic substitution reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains unclear. Here, we uncover the influence of the leaving group on the gas-phase dynamics of SN2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F(-) + CH3Cl and compared it to F(-) + CH3I. For the two leaving groups, Cl and I, we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial and the differences are attributed to the relative orientation of the approaching reactants. This effect occurs on short timescales and may also prevail in solution-phase conditions.

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

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.

Rotationally inelastic scattering of methyl radicals with Ar and N2.

The rotationally inelastic scattering of methyl radical with Ar and N2 is examined at collision energies of 330 ± 25 cm(-1) and 425 ± 50 cm(-1), respectively. Differential cross sections (DCSs) were measured for different final n' rotational levels (up to n' = 5) of the methyl radicals, averaged over k' sub-levels, using a crossed molecular beam machine with velocity map imaging. For Ar as a collision partner, we present a newly constructed ab initio potential energy surface and quantum mechanical scattering calculations of state-resolved DCSs. These computed DCSs agree well with the measurements. The DCSs for both Ar and N2 collision partners are strongly forward peaked for all spectroscopic lines measured. For scattering angles below 60°, the theoretical CD3-Ar DCSs show diffraction oscillations that become less pronounced as n' increases, but these oscillations are not resolved experimentally. Comparisons are drawn with our recently reported DCSs for scattering of methyl radicals with He atoms.

High resolution spatial map imaging of a gaseous target.

Electrostatic ion imaging with the velocity map imaging mode is a widely used method in atomic and molecular physics and physical chemistry. In contrast, the spatial map imaging (SMI) mode has received very little attention, despite the fact that it has been proposed earlier [A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997)]. Here, we present a detailed parametric characterization of SMI both by simulation and experiment. One-, two- and three-dimensional imaging modes are described. The influence of different parameters on the imaging process is described by means of a Taylor expansion. To experimentally quantify elements of the Taylor expansion and to infer the spatial resolution of our spectrometer, photoionization of toluene with a focused laser beam has been carried out. A spatial resolution of better than 4 µm out of a focal volume of several mm in diameter has been achieved. Our results will be useful for applications of SMI to the characterization of laser beams, the overlap control of multiple particle or light beams, and the determination of absolute collision cross sections.