Potential energy surface for dissociation including spin–orbit effects

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  → C₂H₄I⁺+ 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 C₂H₄I₂ parent, resulting in a substantial SO correction and increasing the reaction energy by 6.0–6.5 kcalmol⁻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.

Keywords:

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

Acknowledgements

This material is based upon work supported by the National Science Foundation under grant Nos. CHE-0957416, OISE-0730114, and TG-CHE-110010 (TeraGrid), and the Robert A. Welch Foundation under grant No. D-0005. Support was also provided by the High Performance Computing Center (HPCC) at Texas Tech University (TTU), under the direction of Philip W. Smith, the Texas Advanced Computing Center (TACC) of the University of Texas at Austin, and the TTU Department of Chemistry & Biochemistry cluster Robinson whose purchase was funded by the National Science Foundation under the CRIF-MU grant No. CHE-0840493.