O + C2H4 potential energy surface: excited states and biradicals at the multireference level

Abstract

The focus of this study is to understand the multiconfigurational nature of the biradical species involved in the early reaction paths of the oxygen plus ethylene PES. In previous work (J Phys Chem A 113, 12663, 2009), the lowest-lying O( 3 P) ? C 2 H 4 PES was extensively explored at the MCSCF, MRMP2, and MR-AQCC levels of theory. In the current work, ground and excited, triplet-and singlet-state reaction paths for the initial addition of oxygen to ethylene were found at the MCSCF and MRMP2 levels along with five singlet pathways near the Á CH 2 CH 2 O Á biradical at the MCSCF, MRMP2, and CR-CC(2,3) levels. One of these five paths can lead to the CH 2 CO ? H 2 products from CH 3 CHO rather than from the Á CH 2 CH 2 O Á biradical, and this pathway was investigated with a variety of CAS sizes. To provide further comparison between the MRMP2 and CR-CC(2,3) levels, MR-AQCC single-point energies and optimizations were performed for select geometries. After the initial exploration of this region of the surface, the lowest singlettriplet surface crossings were explicitly determined at the MCSCF level. Additional MRMP2 calculations were performed to demonstrate the limitations of single-state perturbation theory in this biradical region of the PES, and SO-MCQDPT2 single-point energies using SA MCSCF were calculated on a grid of geometries around the primary surface crossing. In particular, these calculations were examined to determine a proper active space and a physically reasonable number of electronic states. The results of this examination show that at least four states must be considered to represent this very complex region of the PES.

O + C2H4 potential energy surface: lowest-lying singlet at the multireference level

“…The current study adds to the previous paper [2] by examining the barriers farther from the ÁCH 2 CH 2 OÁ biradical PES region: the lowest-lying singlet paths 14-19 and 21-25 as labeled in Nguyen et al [12] at several levels of theory. This study examines the following reactions at multiple levels of theory.…”

“…The multireference procedure from the previous work [1,2] was used in the current work and is only summarized here. Multireference calculations were carried out at the complete active space self-consistent field (CASSCF) [20][21][22][23][24][25][26][27][28][29] level of theory with the aug-cc-pVTZ [30] basis set.…”

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In previous studies (West et al in J Phys Chem A 113(45):12663, 2009; West et al in Theor Chem Acc 131:1123, 2012, the lowest-lying O( 3 P) ? C 2 H 4 and singlet PES near the ÁCH 2 CH 2 OÁ biradical were extensively explored at several levels of theory.

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“…ethylene system, both theoretical and experimental [1][2][3][4][5] studies strive toward more accurate potential energy surface (PES) methods, which could subsequently serve as a model to advance combustion studies on larger hydrocarbon systems [6]. Our previous work [1,2] established that discerning transition states (TSs) and minimum energy paths (MEPs) for this system can sometimes be elusive at both the single reference and multireference levels.…”

“…Our first study [1] on the lowest-lying triplet examined pathways 1-9 from Nguyen's [12] work with an additional triplet path 10, and our second study [2] analyzed excited states, surface crossings, and reactions near the ÁCH 2 CH 2 OÁ and ÁCH 2 ÁCHOH biradicals. The current study adds to the previous paper [2] by examining the barriers farther from the ÁCH 2 CH 2 OÁ biradical PES region: the lowest-lying singlet paths 14-19 and 21-25 as labeled in Nguyen et al [12] at several levels of theory.…”