Imaging Nucleophilic Substitution Dynamics

Mikosch, Jochen; Trippel, Sebastian; Eichhorn, Christoph; Otto, R.; Lourderaj, Upakarasamy; Zhang, J. X.; Hase, William L.; Weidemüller, Matthias; Wester, Roland

doi:10.1126/science.1150238

Cited by 317 publications

(415 citation statements)

References 30 publications

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“…First, we compute the structures and harmonic frequencies at the AE-CCSD(T)/aug-cc-pCVQZ and FC-CCSD(T)/aug-cc-pVTZ levels of theory, respectively, for all the minima and saddle points shown in Fig. 1, except for TS 3 , TS 4 , TS 5 and MIN 4 , where the structures are obtained at the AE-CCSD(T)/aug-ccpCVTZ level. Second, the benchmark FPA relative energies are obtained by considering (a) extrapolation to the complete basis set limit using AE-CCSD(T)/ aug-cc-pCVnZ (n ¼ Q(4) and 5) energies, (b) post-CCSD(T) correlation effects up to CCSDT(Q) based on AE-CCSDT/aug-cc-pCVDZ and FC-CCSDT(Q)/aug-ccpVDZ energy computations and (c) scalar relativistic effects at the second-order Douglas-Kroll AE-CCSD(T)/aug-cc-pCVQZ level of theory.…”

Section: Methodsmentioning

confidence: 99%

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

2015

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Stereo-specific reaction mechanisms play a fundamental role in chemistry. The back-side attack inversion and front-side attack retention pathways of the bimolecular nucleophilic substitution (S N 2) reactions are the textbook examples for stereo-specific chemical processes. Here, we report an accurate global analytic potential energy surface (PES) for the F À þ CH 3 Cl S N 2 reaction, which describes both the back-side and front-side attack substitution pathways as well as the proton-abstraction channel. Moreover, reaction dynamics simulations on this surface reveal a novel double-inversion mechanism, in which an abstraction-induced inversion via a FH Á Á Á CH 2 Cl À transition state is followed by a second inversion via the usual [F Á Á Á CH 3 Á Á Á Cl] À saddle point, thereby opening a lower energy reaction path for retention than the front-side attack. Quasi-classical trajectory computations for the F À þ CH 3 Cl(n 1 ¼ 0, 1) reactions show that the front-side attack is a fast direct, whereas the double inversion is a slow indirect process.

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Section: Methodsmentioning

confidence: 99%

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

2015

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“…However, recent theoretical and experimental investigations showed that the dynamics is not so simple. [3][4][5][6][7][8][9][10][11][12] Besides the direct rebound mechanism described above, direct stripping (X − approaches the side of CH 3 Y and strips off CH 3 ), front-side attack (direct replacement of Y − without inversion), and indirect mechanisms (complex formations, roundabout, 6 and barrier recrossing) can occur. Moreover, our recent reaction dynamics computations revealed a new double-inversion mechanism for the F − + CH 3 Cl S N 2 reaction.…”

Section: Introductionmentioning

confidence: 99%

“…13 For Y = Cl, our recent reaction dynamics simulations showed that double inversion occurs at low collision energies where the front-side attack pathway is closed. 12 Atomic-level simulations of S N 2 reactions usually employ the direct dynamics approach, 3,6,8,[14][15][16] in which the electronic structure computations are performed on-the-fly at each time step of a quasiclassical trajectory (QCT). We use another approach based on analytical potential energy surfaces (PESs) obtained by fitting high-level ab initio energy points.…”

Section: Introductionmentioning

confidence: 99%

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F− + CH3F SN2 and proton-abstraction reactions

Szabó

Telekes

Czakó

2015

The Journal of Chemical Physics

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Articles you may be interested inWe develop a full-dimensional global analytical potential energy surface (PES) for the F − + CH 3 F reaction by fitting about 50 000 energy points obtained by an explicitly correlated composite method based on the second-order Møller-Plesset perturbation-F12 and coupled-cluster singles, doubles, and perturbative triples-F12a methods and the cc-pVnZ-F12 [n = D, T] basis sets. The PES accurately describes the (a) back-side attack Walden inversion mechanism involving the pre-and post-reaction (b) ion-dipole and (c) hydrogen-bonded complexes, the configuration-retaining (d) front-side attack and (e) double-inversion substitution pathways, as well as (f) the proton-abstraction channel. The benchmark quality relative energies of all the important stationary points are computed using the focal-point analysis (FPA) approach considering electron correlation up to coupled-cluster singles, doubles, triples, and perturbative quadruples method, extrapolation to the complete basis set limit, core-valence correlation, and scalar relativistic effects. The FPA classical(adiabatic) barrier heights of (a), (d), and (e) are −0.45(−0.61), 46.07(45.16), and 29.18(26.07) kcal mol −1 , respectively, the dissociation energies of (b) and (c) are 13.81(13.56) and 13.73(13.52) kcal mol −1 , respectively, and the endothermicity of (f) is 42.54(38.11) kcal mol −1 . Quasiclassical trajectory computations of cross sections, scattering (θ) and initial attack (α) angle distributions, as well as translational and internal energy distributions are performed for the F − + CH 3 F(v = 0) reaction using the new PES. Apart from low collision energies (E coll ), the S N 2 excitation function is nearly constant, the abstraction cross sections rapidly increase with E coll from a threshold of ∼40 kcal mol −1 , and retention trajectories via double inversion are found above E coll = ∼30 kcal mol −1 , and at E coll = ∼50 kcal mol −1 , the front-side attack cross sections start to increase very rapidly. At low E coll , the indirect mechanism dominates (mainly isotropic backward-forward symmetric θ distribution and translationally cold products) and significant long-range orientation effects (isotropic α distribution) and barrier recrossings are found. At higher E coll , the S N 2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation). C 2015 AIP Publishing LLC. [http://dx

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“…The experiment is based on our previous work on negative ion-molecule reactions [34,35]. Crossed-beam imaging studies of ion-molecule reactions have recently also been reported for reactions of C + with NH 3 [36].…”

mentioning

confidence: 99%

Differential Scattering Cross-Sections for the Different Product Vibrational States in the Ion-Molecule ReactionAr++N2

et al. 2013

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The charge transfer reaction Ar + + N2 → Ar + N + 2 has been investigated in a crossed beam experiment in combination with three-dimensional velocity map imaging. Angular differential stateto-state cross sections were determined as a function of the collision energy. We found that scattering into the first excited vibrational level dominates as expected, but only for scattering in the forward direction. Higher vibrational excitations up to v ′ = 6 have been observed for larger scattering angles. For decreasing collision energy, scattering into higher scattering angles becomes increasingly important for all kinematically allowed quantum states. Our detailed measurements indicate that a quantitative agreement between experiment and theory for this basic ion-molecule reaction now comes within reach.Gas phase studies of ion-molecule reactions have provided insight into a multitude of chemical processes in environments where ions and neutrals coexist. Ion-molecule reactions determine the abundance of many of the complex species that can be detected in interstellar molecular clouds and in planetary atmospheres [1,2]. The conceptually simple charge transfer reactions are particularly interesting, as they were found to explain the X-ray emission from comets [3,4] and may serve as a possible acceleration mechanism of cosmic rays due to strong shock waves in supernova remnants [5]. Charge transfer is also of cosmological importance in that it shapes the hydrogen chemistry in the early universe [6]. Laboratory measurements and theoretical calculations are needed to provide the basis for modeling these processes. In addition charge transfer reactions lead to characteristic light emission from excited states that are useful to determine the parameters of laboratory or technical plasmas, such as temperature, velocity, electron density and charge states of ions [7,8]. To reach very low collision energies, charge transfer has been studied near threshold in half collisions [9]. Recently, ultracold charge transfer reactions have become of interest in studies of cold atom-ion collisions, which are carried out to investigate quantum mechanical phenomena in scattering processes at very low energies [10][11][12].Charge transfer reactions in gas phase evolve in many cases not on a single potential energy surface and are therefore often accompanied by a breakdown of the BornOppenheimer approximation. Since the shape of the molecule changes upon charge transfer, state to state electron transfer rates are often controlled by intramolecular vibrational motion [13][14][15]. While in many cases the outcome of a reaction at high collision energies is well described by the properties of the isolated molecule the results at low energies are not explained by a Frank-Condon treatment [16,17]. This behavior may be explained by molecular bond distortions due to the electric field of the incoming ion [18] and to short range repulsive interactions between the projectile and the target [19].A model system of a gas phase non-Born-Oppenheimer reaction ...

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Imaging Nucleophilic Substitution Dynamics

Cited by 317 publications

References 30 publications

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F− + CH3F SN2 and proton-abstraction reactions

Differential Scattering Cross-Sections for the Different Product Vibrational States in the Ion-Molecule ReactionAr++N2

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