The Carbocation Rearrangement Mechanism, Clarified

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• The precise structure and role of protonated cyclopropane (PCP(+)) intermediates in carbocation rearrangements remain a complex and debated topic in organic chemistry.
• Decades of research have yielded conflicting interpretations regarding the nature of PCP(+) structures, hindering a complete understanding of carbocation behavior.

Purpose of the Study:

• To elucidate the exact structural characteristics of protonated cyclopropane (PCP(+)) intermediates.
• To resolve ambiguities surrounding the mechanism of carbocation rearrangement involving PCP(+) species.
• To provide accurate energetic data for elementary steps in these rearrangements.

Main Methods:

• Advanced quantum-chemical computations were employed, including PBE molecular dynamics, PBE and CCSD optimizations, and CCSD(T) energy calculations.
• The study utilized high-level computational methods to model the electronic and structural properties of intermediates.
• Isotope-labeling experimental data was re-evaluated in light of the computational findings.

Main Results:

• Quantum-chemical computations reveal that protonated cyclopropane (PCP(+)) intermediates adopt "closed" structures, which are mesomeric forms between edge- and corner-protonated states.
• An updated mechanism for hexyl ion rearrangement was proposed, successfully explaining previously puzzling isotope-labeling experimental results.
• A comprehensive table of elementary-step barrier heights for the proposed mechanism has been generated.

Conclusions:

• The study clarifies the nature of protonated cyclopropane (PCP(+)) intermediates, identifying them as "closed" mesomeric structures.
• The revised carbocation rearrangement mechanism provides a unified explanation for experimental observations, resolving long-standing chemical mysteries.
• The calculated barrier heights and mechanism are valuable for predicting product distributions in organic reactions, with implications for fields like petroleum modification.