Computational support for tunneling in thermal[1,7]-hydrogen shift reactions

Area of Science:

• Computational Chemistry
• Physical Organic Chemistry
• Quantum Mechanics

Background:

• The [1,7]-hydrogen shift is a fundamental reaction in organic chemistry.
• Understanding the mechanisms of thermal hydrogen shifts is crucial for predicting reaction outcomes.
• Kinetic isotope effects (KIEs) are powerful tools for probing reaction mechanisms, particularly for detecting quantum tunneling.

Purpose of the Study:

• To investigate the mechanism of the [1,7]-hydrogen shift in substituted 1,3,5-heptatrienes using computational methods.
• To compare computed kinetic isotope effects with experimental data for these specific substrates.
• To provide further evidence for the role of quantum tunneling in thermal antarafacial hydrogen shifts.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• The reaction pathways and transition states for the [1,7]-hydrogen shift were optimized.
• Kinetic isotope effects were computed based on the calculated vibrational frequencies.

Main Results:

• Calculated kinetic isotope effects were found to be significantly smaller than experimentally observed values for both substituted 1,3,5-heptatrienes studied.
• The discrepancies between computed and observed KIEs suggest that classical transition state theory may not fully capture the reaction dynamics.

Conclusions:

• The results strongly support the contribution of quantum mechanical tunneling to the observed rates of thermal, antarafacial [1,7]-hydrogen shift reactions.
• The study highlights the importance of considering non-classical effects like tunneling in the thermal hydrogen shifts.