Eduardo Carrascosa

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.

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.

Imaging dynamic fingerprints of competing E2 and S N 2 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.

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.

Photoswitching an Isolated Donor–Acceptor Stenhouse Adduct

Donor-acceptor Stenhouse adducts (DASAs) are a new class of photoswitching molecules with excellent fatigue resistance and synthetic tunability. Here, tandem ion mobility mass spectrometry coupled with laser excitation is used to characterize the photocyclization reaction of isolated, charge-tagged DASA molecules over the 450-580 nm range. The experimental maximum response at 530 nm agrees with multireference perturbation theory calculations for the S1 ← S0 transition maximum at 533 nm. Photocyclization in the gas phase involves absorption of at least two photons; the first photon induces Z-E isomerization from the linear isomer to metastable intermediate isomers, while the second photon drives another E-Z isomerization and 4π-electrocyclization reaction. Cyclization is thermally reversible in the gas phase with collisional excitation.

Photoisomerization of Protonated Azobenzenes in the Gas Phase

Because of their high photoisomerization efficiencies, azobenzenes and their functionalized derivatives are used in a broad range of molecular photoswitches. Here, the photochemical properties of the trans isomers of protonated azobenzene (ABH+) and protonated 4-aminoazobenzene (NH2ABH+) cations are investigated in the gas phase using a tandem ion mobility spectrometer. Both cations display a strong photoisomerization response across their S1 ← S0 bands, with peaks in their photoisomerization yields at 435 and 525 nm, respectively, red-shifted with respect to the electronic absorption bands of the unprotonated AB and NH2AB molecules. The experimental results are interpreted with the aid of supporting electronic structure calculations considering the relative stabilities and geometries of the possible isomers and protomers and vertical electronic excitation energies.

Preferential Isomer Formation observed in H3+ + CO by Crossed Beam Imaging

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.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.

Isomer-specific product formation in the proton transfer reaction of HOCO+ with CO

The proton transfer reaction HOCO++CO → HCO+/HOC+ has been studied using crossed-beam velocity map imaging. Angular and energy differential cross sections were obtained for collision energies from 0.3 to 2.3 eV. Scattering in forward direction together with a prominent scattering angle-dependent internal excitation is found at all collision energies. The exothermic HCO+ product appears to be very dominant even at energies above the energy threshold for the formation of metastable HOC+ ion. To determine the HOC+ contribution for different angular ranges, a model has been developed. We obtain an upper limit for the HOC+ product isomer fraction of <2%. In theoretical calculations, we find the CO2-catalysed isomerisation channel to be energetically accessible. However, it may not have a strong impact on the isomer ratio. Chemical dynamics simulations are needed to shed more light on this question.

Nucleophilic substitution with two reactive centers: The CN− + CH3I case

The nucleophilic substitution reaction CN(-) + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle- and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.

Reversible Photoisomerization of the Isolated Green Fluorescent Protein Chromophore

Fluorescent proteins have revolutionized the visualization of biological processes, prompting efforts to understand and control their intrinsic photophysics. Here we investigate the photoisomerization of deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI-), the chromophore in green fluorescent protein and in Dronpa protein, where it plays a role in switching between fluorescent and nonfluorescent states. In the present work, isolated HBDI- molecules are switched between the Z and E forms in the gas phase in a tandem ion mobility mass spectrometer outfitted for selecting the initial and final isomers. Excitation of the S1 ← S0 transition provokes both Z → E and E → Z photoisomerization, with a maximum response for both processes at 480 nm. Photodetachment is a minor channel at low light intensity. At higher light intensities, absorption of several photons in the drift region drives photofragmentation, through channels involving CH3 loss and concerted CO and CH3CN loss, although isomerization remains the dominant process.