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

Abstract

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.

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

“…It is interesting to note that in the case of the C s front-side attack saddle-point geometry of F − + CH 3 Cl, four atoms, H, C, F, Cl, are in the C s plane. 12,13 The double-inversion classical barrier height (see TS4 in Fig. 1) of about 29 kcal mol −1 is substantially lower than that of the front-side attack.…”

“…We use another approach based on analytical potential energy surfaces (PESs) obtained by fitting high-level ab initio energy points. 17,18 We have already developed a full-dimensional global analytical PES for the F − + CH 3 Cl reaction, 11,12 and in the present study, we apply this approach to the F − + CH 3 F system. The F − + CH 3 Cl S N 2 reaction is highly exothermic with a negative barrier, whereas the F − + CH 3 F system has a symmetric double-well potential.…”

“…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.…”

“…Moreover, our recent reaction dynamics computations revealed a new double-inversion mechanism for the F − + CH 3 Cl S N 2 reaction. 12 The first step of double inversion is a proton-abstraction induced inversion via a [XH · · · CH 2 Y] − transition state (TS), which is followed a) E-mail: czako@chem.elte.hu by a second inversion via the usual [X · · · CH 3 · · · Y] − saddle point, thereby resulting in retention of the initial configuration. Very recently we showed that the [XH · · · CH 2 Y] − TS (firstorder saddle point) exists for all the 16 reactions involving X, Y = F, Cl, Br, and I.…”

“…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.…”