## Abstract

Different levels of ab initio theory are used to calculate geometries, vibrational frequencies, and energies for stationary points on the H + C_{2}H_{4} ⇌ C_{2}H_{5} potential energy surface. Frequencies and geometries calculated for C_{2}H_{4} at the QCISD/6-311G** and MRCI/cc-pVDZ levels of theory are in very good agreement with experiment. The ab initio normal modes for the H- - -C_{2}H_{4} transition state are similar to those for C_{2}H_{4}, and ratios between the C_{2}H_{4} experimental anharmonic frequencies and ab initio harmonic frequencies are used to estimate anharmonic frequencies for the transition state. Nine of the 10 frequencies assigned for C_{2}H_{5} from experiment are consistent with the ab initio calculations. The CC stretch was apparently misassigned and reassigned here. The ab initio calculations do not give definitive values for the H + C_{2}H_{4} → C_{2}H_{5} barrier height and heat of reaction. Using these two energy terms as adjustable parameters in concert with the above stationary point geometries and frequencies, transition state theory (TST) rate constants are calculated for H + C_{2}H_{4} → C_{2}H_{5} recombination and C_{2}H_{5} → H + C_{2}H_{4} dissociation in the high-pressure limit. AH + C_{2}H_{4} → C_{2}H_{5} barrier height of 3.0-3.1 kcal/mol gives TST recombination rate constants in excellent agreement with experiment. Because of the uncertainty in the experimental C_{2}H_{5} → H + C_{2}H_{4} rate constant, a definitive value could not be deduced for the dissociation barrier from the TST fits to the experimental dissociation rate constants. Some of the experimentally determined dissociation A factors are more than an order of magnitude smaller than the TST value.

Original language | English |
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Pages (from-to) | 5354-5361 |

Number of pages | 8 |

Journal | Journal of physical chemistry |

Volume | 100 |

Issue number | 13 |

DOIs | |

State | Published - Mar 28 1996 |