### Abstract

Previous experiments [J. Phys. Chem. A 116, 2833 (2012)] have studied the dissociation of 1,2-diiodoethane radical cation () and found a one-dimensional distribution of translational energy, an odd finding considering most product relative translational energy distributions are two-dimensional. The goal of this study is to obtain an accurate understanding of the potential energy surface (PES) topology for the unimolecular decomposition reaction C _{2}H _{4}I ^{+}+I. This is done through comparison of many single-reference electronic structure methods, coupled-cluster single-point (energy) calculations, and multi-reference energy calculations used to quantify spin-orbit (SO) coupling effects. We find that the structure of the reactant has a substantial effect on the role of the SO coupling on the reaction energy. Both the BHandH and MP2 theories with an ECP/6-31++G* basis set, and without SO coupling corrections, provide accurate models for the reaction energetics. MP2 theory gives an unsymmetric structure with different C-I bond lengths, resulting in a SO energy for similar to that for the product I-atom and a negligible SO correction to the reaction energy. In contrast, DFT gives a symmetric structure for, similar to that of the neutral C _{2}H _{4}I _{2} parent, resulting in a substantial SO correction and increasing the reaction energy by 6.0-6.5kcalmol ^{1}. Also, we find that, for this system, coupled-cluster single-point energy calculations are inaccurate, since a small change in geometry can lead to a large change in energy.

Original language | English |
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Pages (from-to) | 2599-2609 |

Number of pages | 11 |

Journal | Molecular Physics |

Volume | 110 |

Issue number | 19-20 |

DOIs | |

State | Published - Oct 1 2012 |

### Keywords

- chemical dynamics
- density functional theory
- electronic structure theory
- spin-orbit coupling
- unimolecular dissociation

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## Cite this

*Molecular Physics*,

*110*(19-20), 2599-2609. https://doi.org/10.1080/00268976.2012.725137