Reliable low-cost theoretical procedures for studying addition-fragmentation in RAFT polymerization

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Chemical Kinetics

Background:

• Beta-scission reactions are fundamental in chemical fragmentation processes.
• Accurate calculation of reaction enthalpies is essential for predicting reaction pathways and kinetics.
• Existing computational methods have limitations in accurately determining these enthalpies for diverse systems.

Purpose of the Study:

• To evaluate various computational methods for calculating beta-scission reaction enthalpies.
• To identify an accurate and practical computational approach for larger chemical systems.
• To assess the performance of DFT, MP2, and ONIOM-based methods.

Main Methods:

• Density Functional Theory (DFT) methods
• Second-order Møller–Plesset perturbation theory (MP2) and its variants (ROMP2, SCS-MP2, SOS-MP2)
• G3(MP2)-RAD composite method
• ONIOM (Our Own N-layered Integrated molecular Orbital and Molecular Mechanics) hybrid method

Main Results:

• DFT methods showed significant errors (over 20 kJ mol⁻¹) in calculating fragmentation enthalpies.
• ROMP2 methods performed better but failed for radicals with delocalized unpaired electrons.
• An ONIOM approach, combining MP2/G3(MP2)-RAD with a G3(MP2)-RAD core, accurately reproduced results within 5 kJ mol⁻¹ for larger systems.

Conclusions:

• Standard DFT and MP2 methods are insufficient for accurate beta-scission enthalpy calculations.
• The proposed ONIOM approach provides a practical and accurate method for studying larger systems.
• This work offers a reliable computational tool for theoretical chemists and researchers in related fields.