## Abstract

Extensive classical chemical dynamics simulations of gas-phase X ^{-} + CH _{3}Y→ XCH _{3} + Y ^{-} S _{N}2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S _{N}2 reactions exhibit important nonstatistical atomic-level dynamics. The X ^{-} + CH _{3}Y→ X ^{-}-CH _{3}Y association rate constant is less than the capture model as a result of inefficient energy transfer from X ^{-}+ CH _{3}Y relative translation to CH _{3}Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X ^{-}-CH _{3}Y complex and higher frequency CH _{3}Y intramolecular modes, resulting in non-RRKM kinetics for X ^{-}-CH _{3}Y unimolecular decomposition. Recrossings of the [X - CH _{3} - Y] ^{-} central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S _{N}2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH _{3} +Y ^{-} reaction products. Besides the indirect, complex forming atomic-level mechanism for the S _{N}2 reaction, direct mechanisms promoted by X ^{-} + CH _{3}Y relative translational or CH _{3}Y vibrational excitation are possible, e.g., the roundabout mechanism.

Original language | English |
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Pages (from-to) | 3061-3080 |

Number of pages | 20 |

Journal | Journal of Physical Chemistry A |

Volume | 116 |

Issue number | 12 |

DOIs | |

State | Published - Mar 29 2012 |

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