High-level ab initio potential energy surface and dynamics of the F⁻+ CH₃I S_N2 and proton-transfer reactions

Abstract

The first analytical ab initio potential energy surface reveals the dynamics and different mechanisms of the F– + CH3I reaction.

Deciphering Front-Side Complex Formation in SN2 Reactions via Dynamics Mapping

“…As shown in Figure 1a, the F − + CH 3 I reaction has a front-side complex with a potential energy minimum of −22.8 kcal mol −1 , relative to the reactants, and the equivalent FSC minimum for the F − + CH 3 Cl reaction is only −2.7 kcal mol −1 , whereas the corresponding values on our chemically accurate analytical potential energy surfaces (PESs) are −22.6 and −3.7 kcal mol −1 , respectively. 13 Considering all the possible Nu − --YCH 3 [Nu=F,Cl,Br,I; Y=Cl,Br,I] frontside minima (shown in Figure S1) the [F--ICH 3 ] − has the deepest minimum, followed by the F − --BrCH 3 and Cl − --ICH 3 complexes characterized by a potential energy minimum of −10.7 and −9.2 kcal mol −1 , respectively. Note that the aforementioned OH − --ICH 3 6 halogens (Y=Cl,I).…”

“…With analytical potential energy functions at hand for both F − + CH 3 Y [Y=Cl,I] reactions 8,13 we have a unique starting point to obtain statistically accurate spatial probability distributions. To shed light on the effective dynamics of these reactions, the trajectory orthogonal projection (TOP) method has been applied to the entrance channel of all the reactive S N 2 trajectories by imposing the following constraints on the internal coordinates of CH 3 Y: r C-I < 3.5 Å, and max(r C-H ) < 2.5 Å in order to avoid the interference with the S N 2 exit-channel and the proton-abstraction pathway.…”

“…Moreover, the dominant direct roundabout mechanism hinders the formation of ion-dipole and H-bonded complexes, and results in backward scattered products. 12,13 An approach for investigating the time-scale of nucleophile capture is to consider the fraction of the trajectory spent in the FSC region. 22 Considering the shape of the interaction potential, as well as the spatial distribution of the nucleophile, the life time of the individual capture events is calculated from the trajectory integration times in the ‫ݎ‬ Nu ష Y < 0 region, where ‫ݎ‬ Nu ష Y is the distance of the trapped Nu − from the Y halogen atom after orthogonal projection to the {Y,C} line.…”

“…In case of the F − + CH 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 the time-scale of the indirect mechanism. 8,12,13 After dissociation of the F − --ICH 3 intermediate complex, the F − ion leaves the front-side region being trapped for a short time in the back-side region of the pre-reaction well. Once its relative orientation to the permanent dipole becomes appropriate, the substitution event can take place by simultaneous C-F bond formation and C-I bond rupture.…”

“…17 Moreover, FSCs were proposed to explain the indirect nature of several prototypical S N 2 reactions, e.g. OH − + CH 3 I, [17][18][19] and F − + CH 3 I, 13 but the exact causes of the observed differences in the mechanism remained unclear. Our aim is to provide a detailed characterization of the structure and energetics of the Nu − --YCH 3 [Nu=F,Cl,Br,I; Y=Cl,Br,I] front-side complex minima and to explore the fascinating mechanistic roles of these intermediate ion-dipole complexes on the example of the prototypical F − --YCH 3 [Y=Cl,I] S N 2 reactions.…”