New Insights into the Remarkable Difference between CH₅^– and SiH₅^–

Abstract

It has long been known that there is a fundamental difference in the electronic structures of CH 5 − and SiH 5 − , two isoelectronic molecules. The former is a saddle point for the S N 2 exchange reaction H − + CH 4 → [CH 5 − ] ‡ → CH 4 + H − , while the latter is a stable molecule that is bound relative to SiH 4 + H − . SCGVB calculations indicate that this difference is the result of a dramatic difference in the nature of the axial electron pairs in these anions. In SiH 5 − , the axial pairs represent two stable bondsa weak recoupled pair bond dyad. In CH 5 − , the axial electron pairs represent an intermediate transition between the electron pairs in the reactants and those in the products. Furthermore, the axial orbitals at the saddle point in CH 5 − are highly overlapping, giving rise to strong Pauli repulsion and a high barrier for the S N 2 exchange reaction.

Detailed quasiclassical dynamics of the F− + SiH3Cl multi-channel reaction

“…The dynamics and mechanisms of bimolecular nucleophilic substitution (S N 2) reactions of halide ions (X À ) with methylhalide molecules (CH 3 Y) have been extensively investigated, [1][2][3][4][5][6][7][8] however, analogous S N 2 reactions at silicon (Si) centers are less thoroughly studied. Previous theoretical work on the H À /F À + SiH 4 and X À + SiH 3 Y systems was restricted to stationarypoint characterizations [9][10][11][12][13][14][15][16][17][18][19] and on-the-fly direct dynamics simulations [20][21][22] until 2023 when we developed a fulldimensional coupled-cluster-quality analytical potential energy surface (PES) for the F À + SiH 3 Cl reaction, 23 which, to the best of our knowledge, was the first PES for a Si-centered S N 2 reaction. The analytical PES allows the efficient computations of a large number of quasiclassical trajectories (QCTs) providing statistically-converged cross sections and unprecedented insights into the mechanisms of the system of interest.…”