Gas-Phase Identity S_N2 Reactions of Halide Anions and Methyl Halides with Retention of Configuration

Abstract

High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X -+ CH 3 X, for X ) F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X ) F), 193.8 (X ) Cl), 178.9 (X ) Br), and 171.4 kJ mol -1 (X ) I), substantially higher than the corresponding barriers for back-side attack (-8.0 for X ) F, 11.5 for X ) Cl, 5.8 for X ) Br, and 6.5 kJ mol -1 for X ) I). The difference between the overall barrier for back-side attack and front-side attack is smallest for X ) I (164.9 kJ mol -1 ). Central barrier heights for front-side attack decrease in the following order: 241.0 (X ) F), 237.8 (X ) Cl), 220.0 (X ) Br), and 207.4 kJ mol -1 (X ) I). The minimum energy pathways for both back-side and front-side S N 2 reactions are found to involve the same ion-molecule complex (X -‚‚‚H 3 CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH 3 Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side S N 2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side S N 2 reaction with retention of configuration in going from X ) F to X ) I. Ion-molecule complexes CH 3 -X‚‚‚X -, which may be pre-reaction complexes in direct collinear halophilic attack, were found for X ) Br and I but not for X ) F and Cl. The calculated complexation energies (∆H comp ) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol -1 for X ) Br and I, respectively) than those for the corresponding pre-reaction complexes for S N 2 attack at carbon (41.1 and 36.0 kJ mol -1 for X ) Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH 3 X (X ) F-I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.

Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation

“…Therefore, the backside attack channel is energetically favoured, the calculated barriers being calculated to be lower by about 200 kJ mole À1 than for the front side attack. 162 In the reaction…”

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

“…Besides the well-known Walden inversion mechanism, there is a front-side attack pathway, which goes over a high energy barrier and results in retention of configuration. This front-side attack mechanism is much less studied than the Walden inversion [10][11][12][13] . Due to the high barrier of the former, the S N 2 reactions are known to proceed via Walden inversion at low collision energies (E coll ) and the front-side attack pathway may open at higher E coll .…”

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

“…(12)) is dramatically enhanced as the ion῎molecule collision energy is increased in the SIFT-drift field, 39) stimulating fundamental interest in the mechanism of gas phase nucleophilic substitution. A high-level ab initio theoretical study 44) suggested this behavior as evidence for the occurrence of a novel "front-side" S N 2 mechanism, in which 37 Cl ῌ approaches the carbon atom from the same side as 35 Cl ῌ leaves (Fig. 4b).…”

“…4b). At the G2(῏) level of theory 44) the energy threshold for the front-side attack was predicted to be 46.3 kcal mol ῌ1 . However, guided ion beam experiments under single collision conditions 45) measured the threshold for the translational activation to be 11 ῐ 4 kcal mol ῌ1 , significantly lower than the theoretical front-side threshold.…”