## Abstract

Chemical dynamics simulations, based on both an analytic potential energy surface (PES) and direct dynamics, were used to investigate the intrinsic non-RRKM dynamics of the Cl^{-}-CH_{3}Br ion-dipole complex, an important intermediate in the Cl^{-}+CH_{3}Br S_{N}2 nucleophilic substitution reaction. This intermediate may dissociate to Cl ^{-}+CH_{3}Br or isomerize to the ClCH_{3}-Br ^{-} ion-dipole complex. The decomposition of microcanonical ensembles of the Cl^{-}-CH_{3}Br intermediate were simulated, and the ensuing populations vs. Time of the excited intermediate and Cl ^{-}+CH_{3}Br and ClCH_{3}-Br^{-} products were fit with multi-exponential functions. The intrinsic non-RRKM dynamics is more pronounced for the simulations with the analytic PES than by direct dynamics, with the populations for the former and latter primarily represented by tri-and bi-exponential functions, respectively. For the analytic PES and direct dynamics simulations, the intrinsic non-RRKM dynamics is more important for the isomerization pathway to form ClCH_{3}-Br^{-} than for dissociation to Cl^{-}+CH_{3}Br. Since the decomposition probability of Cl^{-}-CH_{3}Br is non-exponential, the Cl ^{-}-CH_{3}Br unimolecular rate constant depends on pressure, with both high and low pressure limits. The high pressure limit is the RRKM rate constant and for the simulations with the analytic PES the rate constant decreased by a factor of 3.0, 5.6, and 4.3 in going from the high to low pressure limit for total energies of 40, 60, and 80 kcal/mol. For the direct dynamics simulations these respective factors are 2.4, 1.4, and 1.2. A separable phase space model with intermolecular and intramolecular complexes describes some of the simulation results, but overall models advanced for intrinsic non-RRKM dynamics give incomplete representations of the intermediate and product populations vs. Time determined from the simulations.

Original language | English |
---|---|

Pages (from-to) | 1361-1379 |

Number of pages | 19 |

Journal | Zeitschrift fur Physikalische Chemie |

Volume | 227 |

Issue number | 11 |

DOIs | |

State | Published - Nov 2013 |