Correlated Ab Initio and Density Functional Studies on H2 Activation by FeO(.)

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Biophysical Chemistry

Background:

• Hydrogen abstraction reactions are crucial in biological heme systems.
• Accurate theoretical modeling of these reactions is essential for understanding heme function.
• Previous studies have employed various computational methods with varying success.

Purpose of the Study:

• To evaluate and compare the accuracy of different computational methods for modeling the FeO(+) + H2 reaction, a model for heme hydrogen abstraction.
• To identify reliable and cost-effective computational approaches for studying biologically relevant heme systems.
• To establish a benchmark for future theoretical investigations of similar reactions.

Main Methods:

• Optimization of stationary points on sextet and quartet potential energy surfaces using density functionals and CASSCF.
• Calculation of energy profiles using various density functional theory (DFT) and coupled cluster (CC) methods.
• Application of correlated multireference ab initio methods including MRCI, MRAQCC, and SORCI, with corrections for relativistic effects, ZVE, CCVC, and size-consistency.

Main Results:

• Coupled cluster theory with single, double, triple, and perturbative quadruple excitations (CCSDT(Q)) was found to be the most accurate method, serving as a benchmark.
• Among density functionals, B3LYP demonstrated the best performance.
• The SORCI+Q method showed the lowest deviations from the CCSDT(Q) benchmark among correlated ab initio methods, with MRCI+Q and MRAQCC also providing qualitatively similar results.

Conclusions:

• The study validates the use of B3LYP for computational studies of heme systems.
• SORCI+Q is recommended as a highly accurate correlated ab initio method for such investigations.
• Several tested computational methods exhibit significant errors, highlighting the importance of careful method selection for studying electronically complex heme reactions.