Singlet and triplet potential surfaces for the O2+C2H4 reaction

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Reaction Mechanism Studies

Background:

• Understanding the reaction between molecular oxygen (O2) and ethylene (C2H4) is crucial for various chemical processes.
• Potential energy surfaces (PESs) govern reaction pathways and kinetics.
• Investigating both triplet and singlet states is necessary due to potential intersystem crossing.

Purpose of the Study:

• To characterize the stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs).
• To determine the energy barriers for the addition of O(2) to C(2)H(4) and subsequent isomerization reactions.
• To explore the role of spin-orbit couplings and intersystem crossing in the reaction mechanism.

Main Methods:

• Electronic structure calculations using CASSCF and UB3LYP methods with the aug-cc-pVDZ basis set.
• Calculation of spin-orbit couplings using state-averaged CASSCF wave functions.
• High-accuracy energy calculations with the MRMP2/aug-cc-pVDZ method.

Main Results:

• The O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical has lower energy barriers on the singlet surface (6.1 kcal/mol) compared to the triplet surface (33.8 kcal/mol).
• On the singlet surface, the C(2)H(4)O(2) biradical can isomerize to dioxetane (7.8 kcal/mol barrier) and ethane-peroxide (21.3 kcal/mol barrier).
• The triplet and singlet PESs cross, suggesting a mechanism for triplet-singlet intersystem crossing, potentially enhanced by trapping in the triplet biradical intermediate.

Conclusions:

• The reaction between O(2) and C(2)H(4) proceeds through distinct pathways on triplet and singlet potential energy surfaces.
• Lower activation energies on the singlet surface favor the formation of various isomers.
• The calculated crossing points and biradical structures highlight the importance of intersystem crossing in the overall reaction dynamics.