Determining Quantum Mechanical Methods Suitable for Quantitative Modeling of Hydrogen Atom Transfer by Halogen Atoms

Area of Science:

• Computational chemistry
• Quantum mechanics
• Synthetic organic chemistry

Background:

• Hydrogen atom transfer (HAT) reactions are fundamental in chemistry.
• Accurate modeling of reaction mechanisms and selectivity is vital for synthetic chemists.
• Previous computational methods have limitations in predicting HAT reaction energetics and regioselectivity.

Purpose of the Study:

• To systematically evaluate quantum mechanical methods for modeling HAT reactions.
• To identify a computationally efficient and accurate method for predicting reaction energetics and regioselectivity.
• To assess the applicability of these methods to modern synthetic organic reactions, including photoredox catalysis.

Main Methods:

• Systematic testing of various quantum mechanical methods against experimental data.
• Focus on saddle point energetics for predicting activation energies and regioselectivity.
• Comparison of DLPNO-CCSD(T) energies on M06-2X geometries with other high-level methods.
• Evaluation of computational efficiency and predictive accuracy (mean absolute error in ΔΔG‡).

Main Results:

• Accurate saddle point energetics are critical for reliable predictions.
• DLPNO-CCSD(T)//M06-2X provides a practical predictive accuracy (MAE of 0.6 kcal/mol in ΔΔG‡).
• This protocol is sufficient for predicting regioselectivity in HAT reactions.
• The method's performance is comparable to higher-level ab initio and variational transition state theory methods.

Conclusions:

• The DLPNO-CCSD(T)//M06-2X protocol offers a computationally efficient and accurate approach for modeling HAT reactions.
• This method aids in elucidating reaction mechanisms and predicting regioselectivity in synthetic organic chemistry.
• It is suitable for assessing the role of HAT in photoredox catalysis involving halogen atoms.