Sebastian Trippel

Single solvent molecules can affect the dynamics of substitution reactions

Solvents have a profound influence on chemical reactions in solution and have long been used to control their outcome. Such effects are generally considered to be governed by thermodynamics; however, little is known about the steric effects of solvent molecules. Here, we probe the influence of individual solvent molecules on reaction dynamics and present results on the atomistic dynamics of a microsolvated chemical reaction--the fundamentally important nucleophilic substitution reaction. We study the reaction of OH(-) with CH(3)I using a technique that combines crossed-beam imaging with a cold source of microsolvated reactants. Our results reveal several distinct reaction mechanisms for different degrees of solvation; surprisingly, the classical co-linear substitution mechanism only dominates the dynamics for mono-solvated reactants. We analyse the relative importance of the different mechanisms using ab initio calculations and show that the steric characteristics are at least as relevant as the energetics in understanding the influence of solvent molecules in such microsolvated reactions.

Imaging charge transfer in iodomethane upon x-ray photoabsorption

Tightly tracking charge migration Electron transfer dynamics underlie many chemical and biochemical reactions. Erk et al. examined the charge migration between individual carbon and iodine atoms during dissociation of iodomethane (ICH3) molecules (see the Perspective by Pratt). After initiating scission of the C-I bond with a relatively low-energy laser pulse, they introduced a higher-energy x-ray pulse to instigate ionization and charge migration. Delaying the arrival time of the x-ray pulse effectively varied the separation distance being probed as the fragments steadily drifted apart. The experimental approach should also prove useful for future studies of charge transfer dynamics in different molecular or solid-state systems. Science, this issue p. 288; see also p. 267 A free-electron laser enables precise tracking of electron movement between segments of a dissociating molecule. [Also see Perspective by Pratt] Studies of charge transfer are often hampered by difficulties in determining the charge localization at a given time. Here, we used ultrashort x-ray free-electron laser pulses to image charge rearrangement dynamics within gas-phase iodomethane molecules during dissociation induced by a synchronized near-infrared (NIR) laser pulse. Inner-shell photoionization creates positive charge, which is initially localized on the iodine atom. We map the electron transfer between the methyl and iodine fragments as a function of their interatomic separation set by the NIR–x-ray delay. We observe signatures of electron transfer for distances up to 20 angstroms and show that a realistic estimate of its effective spatial range can be obtained from a classical over-the-barrier model. The presented technique is applicable for spatiotemporal imaging of charge transfer dynamics in a wide range of molecular systems.

Indirect Dynamics in a Highly Exoergic Substitution Reaction

The highly exoergic nucleophilic substitution reaction F(-) + CH3I shows reaction dynamics strikingly different from that of substitution reactions of larger halogen anions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH3F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl(-) + CH3I [Science 2008, 319, 183-186] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747-2752]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F(-)-HCH2I hydrogen-bonded complex, a direct rebound, and a direct stripping reaction. The indirect mechanism is found to contribute about one-half of the overall substitution reaction rate at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, because the barrier for the F(-)-HCH2I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.

Inverse Temperature Dependent Lifetimes of Transient SN2 Ion-Dipole Complexes

The association and collisional stabilization of the S(N)2 entrance channel complex [Cl(-)...CH3Cl]* is studied in a low-temperature radiofrequency ion trap. The temperature dependence of the ternary rate coefficient is measured and a much stronger inverse temperature dependence than expected from a simple statistical calculation is found. From these data the lifetime of the transient S(N)2 complex has been derived as a function of temperature. It is suggested that the inverse temperature dependent rates of nonsymmetric S(N)2 reactions are related to the observed inverse temperature dependence of the transient ion-dipole complexes.

ABSOLUTE PHOTODETACHMENT CROSS-SECTION MEASUREMENTS FOR HYDROCARBON CHAIN ANIONS

Absolute photodetachment cross sections have been measured for the hydrocarbon chain anions C(n)H(-), n = 2, 4, and 6, which are relevant for an understanding of molecular clouds in the interstella ...

Spatial separation of state- and size-selected neutral clusters

We demonstrate the spatial separation of the prototypical indole(H

Spatially-controlled complex molecules and their applications

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Reaction dynamics of temperature-variable anion water clusters studied with crossed beams and by direct dynamics

O) clusters from the various species present in the supersonic expansion of mixtures of indole and water. The major molecular constituents of the resulting molecular beam are H

Absolute photodetachment cross section measurements of the O− and OH− anion

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Velocity map imaging of ion–molecule reactive scattering: The Ar++ N2 charge transfer reaction

O, indole, indole(H