Force-Assisted Orbital Crossing in Mechanochemical Oxirane Ring Opening

Area of Science:

• Polymer mechanochemistry
• Computational chemistry
• Organic reaction mechanisms

Background:

• Polymer mechanochemistry utilizes applied force to trigger chemical reactivity, often revealing novel reaction pathways.
• Strained cyclic molecules, such as oxirane, are key in force-sensitive applications due to their susceptibility to ring-opening reactions.

Purpose of the Study:

• To computationally investigate the force-dependent ring-opening reactions of oxirane.
• To explore the electronic characteristics of symmetry-allowed and symmetry-forbidden reactions under force.
• To understand how external forces alter potential energy surface (PES) topology and reaction mechanisms.

Main Methods:

• Density functional theory (DFT) and multireference computational methods were employed.
• Broken-symmetry DFT was evaluated against high-accuracy methods like CASPT2, MRCI, and ic-MRCC.
• Analysis of potential energy surface (PES) topology was used to study force-induced changes.

Main Results:

• External force significantly reduces the activation energy barriers for oxirane ring-opening reactions.
• Symmetry-forbidden reactions proceed via a diradical pathway due to orbital crossings induced by force.
• Unsaturated linkers were found to further decrease barrier heights, aligning with experimental observations.
• Force application transforms critical points on the PES, altering reaction pathways.

Conclusions:

• External force is a powerful tool to control and enable chemical reactions, including Woodward-Hoffmann-forbidden ones.
• Computational methods, particularly broken-symmetry DFT, accurately predict force-dependent reaction behaviors.
• Understanding PES topology changes provides crucial insights into force-modulated reaction mechanisms.