Stone-Wales rearrangements in polycyclic aromatic hydrocarbons: a computational study

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Reaction Mechanisms

Background:

• Stone-Wales rearrangements (SWRs) are crucial in modifying polycyclic aromatic hydrocarbon structures.
• Understanding SWR mechanisms is key to designing novel carbon materials and predicting their stability.
• Pentafulvalene cores present unique challenges and opportunities for studying SWRs.

Purpose of the Study:

• To elucidate the mechanisms of Stone-Wales rearrangements in polycyclic unsaturated hydrocarbons with pentafulvalene cores.
• To determine the activation energies for various SWR pathways using advanced computational methods.
• To predict the feasibility of SWRs in specific, untested molecular systems.

Main Methods:

• Density functional theory (DFT) calculations.
• Coupled cluster (CC) methods for high-level electronic structure.
• Multiconfigurational (MC) approaches to capture complex electronic effects.
• Calculations performed at the BD(T)/cc-pVDZ//(U)M06-2X/cc-pVDZ level of theory.

Main Results:

• Free energies of activation (ΔG‡) at 1000 °C vary significantly, from ~70 kcal/mol for pentafulvalene to >110 kcal/mol for pyracyclene.
• Systems without pyracyclene subunits favor carbene-type mechanisms (ΔG‡ < 90 kcal/mol).
• Systems with pyracyclene subunits favor cyclobutyl mechanisms, with benzannelation reducing activation energy.

Conclusions:

• Computational predictions align well with existing experimental data for SWRs.
• SWRs in cyclopent[fg]aceanthrylene and dicyclopenta[fg,op]tetracene are predicted to be accessible via flash vacuum pyrolysis (ΔG‡ < 95 kcal/mol).
• The study provides a mechanistic framework for understanding and predicting SWRs in complex polycyclic hydrocarbons.