Potential energy surface for dissociation including spin-orbit effects

Matthew R. Siebert, Adelia J.A. Aquino, Wibe A. De Jong, Giovanni Granucci, William L. Hase

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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 2H 4I ++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 2H 4I 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 languageEnglish
Pages (from-to)2599-2609
Number of pages11
JournalMolecular Physics
Issue number19-20
StatePublished - Oct 1 2012


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


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