Calculations of the active mode and energetic barrier to electron attachment to CF₃ and comparison with kinetic modeling of experimental results

Abstract

To provide a deeper understanding of the kinetics of electron attachment to CF, the six-dimensional potential energy surfaces of both CF and CF were developed by fitting ∼3000 ab initio points per surface at the AE-CCSD(T)-F12a/AVTZ level using the permutation invariant polynomial-neural network (PIP-NN) approach. The fitted potential energy surfaces for CF and CF had root mean square fitting errors relative to the ab initio calculations of 1.2 and 1.8 cm, respectively. The main active mode for the crossing between the two potential energy surfaces was identified as the umbrella bending mode of CF in C symmetry. The lowest energy crossing point is located at R = 1.306 Å and θ = 113.6° with the energy of 0.051 eV above the minimum of the CF electronic surface. This value is only slightly larger than the experimental data 0.026 ± 0.01 eV determined by kinetic modeling of electron attachment to CF. The small discrepancy between the theoretical and experimentally measured values is analyzed.

Anion-Initiated Trifluoromethylation by TMSCF3: Deconvolution of the Siliconate–Carbanion Dichotomy by Stopped-Flow NMR/IR

“…Extensive calculations were conducted to test for direct nucleophilic reactivity of the pentacoordinate anions B and D . All calculations revealed that direct transfer of CF 3 from the silicon center to an electrophile requires concomitant inversion of the CF 3 , with a prohibitively large barrier (>100 kcal mol –1 ; in line with the barrier for inversion of the free CF 3 anion) . The pentacoordinate siliconate anions thus act as reservoirs, not active nucleophiles, liberating free (non-silicon-coordinated) CF 3 – via dissociation.…”

“…[X – ] 0 sets the initial concentration of the siliconate ([ D ] 0 = (1 – x EI )[X – ] 0 ), 60 which, in the absence of endogenous inhibitors, is essentially constant throughout the reaction. The insurmountable barrier for CF 3 inversion 67 means that, independent of the identity of the electrophile, E , or proton donor, R–H, the siliconate is unable to effect direct anionic trifluoromethyl transfer, Figure 10 i. In all cases, the reaction must proceed via a dissociative pathway, Figure 10 ii, in which M + plays a key role: the stronger the association of M + with CF 3 , the more favorable k –3 .…”

“…Extensive calculations were conducted to test for direct nucleophilic reactivity of the pentacoordinate anions B and D. All calculations revealed that direct transfer of CF 3 from the silicon center to an electrophile requires concomitant inversion of the CF 3 , with a prohibitively large barrier (>100 kcal mol −1 ; in line with the barrier for inversion of the free CF 3 anion). 67 The pentacoordinate siliconate anions thus act as reservoirs, not active nucleophiles, liberating free (non-silicon-coordinated) CF 3 − via dissociation. The transition state for addition of the CF 3− anion(oid) to the ketone formally involves movement between a nonclassical hydrogen-bonded complex and the addition product, a process that occurs with low calculated barrier (7.5 kcal mol −1 ) and well represents the process that occurs once the two species are in contact.…”