High-Level Systematic Ab Initio Comparison of Carbon- and Silicon-Centered S_N2 Reactions

Abstract

We characterize the stationary points along the Walden inversion, front-side attack, and double-inversion pathways of the X– + CH3Y and X– + SiH3Y [X, Y = F, Cl, Br, I] SN2 reactions using chemically accurate CCSD­(T)-F12b/aug-cc-pVnZ [n = D, T, Q] levels of theory. At the carbon center, Walden inversion dominates and proceeds via prereaction (X–···H3CY) and postreaction (XCH3···Y–) ion-dipole wells separated by a usually submerged transition state (X–H3C–Y)−, front-side attack occurs over high barriers, double inversion is the lowest-energy retention pathway for X = F, and hydrogen- (F–···HCH2Y) and halogen-bonded (X–···YCH3) complexes exist in the entrance channel. At the silicon center, Walden inversion proceeds through a single minimum (X–SiH3–Y)−, the front-side attack is competitive via a usually submerged transition state separating pre- and postreaction minima having X–Si–Y angles close to 90°, double inversion occurs over positive, often high barriers, and hydrogen- and halogen-bonded complexes are not found. In addition to the SN2 channels (Y– + CH3X/SiH3X), we report reaction enthalpies for proton abstraction (HX + CH2Y–/SiH2Y–), hydride substitution (H– + CH2XY/SiH2XY), XH···Y– complex formation (XH···Y– + 1CH2/1SiH2), and halogen abstraction (XY + CH3 –/SiH3 – and XY– + CH3/SiH3).

Hydrogen Bonding versus Halogen Bonding: Spectroscopic Investigation of Gas‐Phase Complexes Involving Bromide and Chloromethanes

“…[74,75] The two structures optimised for the Br À •••CH 3 Cl complex have the same binding motifs as S N 2 reaction adducts. [76,77] The hydrogen bond motif observed for the Br À •••CH 2 Cl 2 and Br À •••CHCl 3 complexes has been shown to also exist in triangular hydrogen and halogen bonded complexes involving CBr 4 . [78] The halogen bond motif for Br À •••CHCl 3 is similar to that seen for the Br À •••CHBr 3 complex, [79] while Br À •••CH 2 Cl 2 (XB) has been studied previously.…”

“…Three structures have been described for the Br − ⋅⋅⋅CH 4 complex, but only the hydrogen bonded complex is a minimum structure, [73] and has been confirmed by experimental IR studies [74,75] . The two structures optimised for the Br − ⋅⋅⋅CH 3 Cl complex have the same binding motifs as S N 2 reaction adducts [76,77] . The hydrogen bond motif observed for the Br − ⋅⋅⋅CH 2 Cl 2 and Br − ⋅⋅⋅CHCl 3 complexes has been shown to also exist in triangular hydrogen and halogen bonded complexes involving CBr 4 [78] .…”

Nucleophilic Radiofluorination Using Tri-tert-Butanol Ammonium as a Bifunctional Organocatalyst: Mechanism and Energetics

“…The relative Gibbs free energies of the pre-reaction complexes in Figure 2 , with respect to those of free reactants, are worth mentioning: the (stationary) pre-reaction complexes are predicted to lie higher than free reactants, in contrast with most S N 2 reactions we have studied. Although some theoretical studies [ 41 , 42 , 43 ] have suggested that pre-reaction complexes could not be obtained in some cases, the origin of the ‘well-skipping’ S N 2 reactions predicted here is not clear.…”

Spectroscopic Study of the Br−+CH3I→I−+CH3Br SN2 Reaction

“…[b] Ervin and co‐workers [45] . [c] Czakó and co‐workers [46] . [d] Bickelhaupt and co‐workers [15] .…”

“…Ervin and co‐workers calculated the barrier heights at the CCSD(T) level of theory using two different basis sets, one of which is the LanL2DZ basis set, and the other of which is the SDD basis set [45] . Czakó and co‐workers used CCSD(T)‐F12b/AVQZ theory in their respective study, [46] whereas Bickelhaupt and co‐workers used ZORA‐OLYP/TZ2P theory, [15] and Radom and co‐workers used G2(+) theory [47] . Compared to the CCSD(T)/CBS barrier heights reported in this study, the G2(+) barrier heights are in good agreement, with the largest difference being the $${{{\rm D}}_{0}}$$ of the $${{{\rm B}{\rm r}}^{-}\cdots {{\rm H}}_{3}{\rm C}{\rm I}}$$ complex at around 6.9 kJ $${{{\rm m}{\rm o}{\rm l}}^{-1}}$$ .…”