Ab initio molecular dynamics studies on substitution vs electron transfer reactions of substituted ketyl radical anions with chloroalkanes: how do the two products form in a borderline mechanism?

Abstract

We present a qualitative analysis, based on ab initio molecular dynamics (MD) calculations, of the S N 2/ ET mechanistic spectrum for three reactions: (1) HC(CN)=O . À CH 3 Cl, (2) HC(CN)=O . À (CH 3 ) 2 CHCl and (3) H 2 C=O . À CH 3 Cl, passing through their S N 2-like transition states. Finite temperature (298 K) direct MD simulations indicate that the trajectories for reaction (1) appear to have a propensity towards S N 2 products, the propensity for trajectories for reaction (2) seems to be towards ET products, whereas trajectories for reaction (3) appear to show no particular propensity towards either ET or S N 2 products. The mechanistic diversity is consistent with the electron-donating ability of the ketyl species and steric bulkiness of chloroalkanes. We find that the trajectories have characteristics that reflect strongly the types of process [S N 2 trajectories in reactions (1) and (3) vs ET trajectories in reactions (2) and (3)]. Trajectories that lead to S N 2 products are simple with C-C bond formation and C-Cl bond breaking essentially completed within 50 fs. By contrast, trajectories leading to ET products are more complex with a sudden electron reorganization taking place within 15-30 fs and the major bonding changes and electron and spin reorganizations completed after 250 fs.

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH−(H2O) with CH3Cl

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Nevertheless, fewer studies have addressed the role of solvation in these reactions. 16,17 From an experimental point of view, O'Hair et al investigated solvent effects on the S N 2 reaction of fluoride ion with methyl chloride and measured the product ions formed by the reaction F − ͑H 2 O͒ +CH 3 Cl.…”

“…[1][2][3] From the theoretical points of view, ab initio, classical, and quantum chemical dynamics calculations [4][5][6][7][8][9][10][11][12] were obtained by the ab initio method. The reaction rates for several systems were also investigated on the basis of transition state theory using the ab initio data.…”

A Single Transition State Serves Two Mechanisms. The Branching Ratio for CH2O•- + CH3Cl on Improved Potential Energy Surfaces

“…A well-known example of the latter case is the reaction between ketyl radical anions and alkyl halides in which both electron transfer (ET) products and substitution at carbon (Sub(C)) products can be formed via the same, tightly bound transition state. [3][4][5][6][7][8][9][10][11][12][13][14] Such cases are especially attractive, since a slight change of the potential energy surface around the transition state may vary the branching ratio of the products.…”

“…In the other case, the reaction can proceed via a tightly bound transition state, which may also lead to substitution products. 3,4,[7][8][9][10][11][12][13][14][15] Early mechanistic studies were carried out by Garst on the reactions of benzophenone ketyls with alkyl bromides, 16,17 where it was shown that, along with an ET mechanism that leads to radical formation, one also obtains alkylation products. More recent mechanistic studies were carried out by Screttas and Michascrettas, 18 who used 13 C spin transfer NMR techniques to try to quantify the inner sphere tightly bound transition states.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14] Electron transfer can occur through two pathways. In one case, an electron can be transferred via an outer sphere species which involves a weak interaction and surface hopping.…”