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).
Highlights
Since Walden’s discovery in 1896,1 bimolecular nucleophilic substitution been widelyedrebacottihonesxpateraimteetnrathaleldyraanl dcatrhbeoonrecteincatellry.h2a−v3e5The prototypes CH3X, where X, of these reactions are Y = F, Cl, Br, I
Based on the recent knowledge accumulated on the reaction pathways of carbon-centered SN2 reactions,[16,19,25,30,32] we present a comprehensive stationary-point characterization for the X− + CH3Y and X− + SiH3Y [X, Y = F, Cl, Br, I] SN2 reactions using the explicitly correlated CCSD(T)-F12b method with the correlation-consistent aug-cc-pVnZ [n = D, T, Q] basis sets
Basis-set convergence tests are carried out, and the results show that the relative energies obtained in the present study are usually well within chemical accuracy
Summary
Since Walden’s discovery in 1896,1 bimolecular nucleophilic substitution been widely (sStuNd2i)edrebacottihonesxpateraimteetnrathaleldyraanl dcatrhbeoonrecteincatellry.h2a−v3e5. Due to the large size of the Si atom, at the silicon center, the front-side attack transition states (XYSiH3)− are usually submerged, opening barrierless retention pathways that may compete with Walden inversion.[41,47−50] Despite the interesting features of silicon-centered SN2 reactions, these systems are less studied than the carboncentered analogues. We determine the reaction enthalpies of several alternative product channels obtained by, for example, proton and halogen abstractions. These results may shape our fundamental knowledge of model ion− molecule reactions at carbon and silicon centers and guide future global potential energy surface developments and experimental and theoretical dynamics studies.
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