Unsaturated Vinyl-Type Carbocation [(CH₃)₂C=CH]⁺ in Its Carborane Salts

Area of Science:

• Physical Chemistry
• Inorganic Chemistry
• Computational Chemistry

Background:

• Isobutylene carbocations are key intermediates in organic chemistry.
• Understanding carbocation stabilization is crucial for reaction mechanisms.
• Carborane anions offer unique counterion properties.

Purpose of the Study:

• To synthesize and characterize isobutylene carbocations with carborane anions.
• To investigate the influence of amorphous versus crystalline environments on carbocation structure and stability.
• To compare experimental findings with computational predictions.

Main Methods:

• Synthesis of isobutylene carbocation salts with CHB11Cl11- anion.
• Characterization using X-ray crystallography and Infrared (IR) spectroscopy.
• Computational analysis using HF, MP2, and DFT methods.

Main Results:

• Isobutylene carbocation (CH3)2C=CH+ was successfully isolated in amorphous and crystalline salts.
• X-ray crystallography revealed uniform ionic interactions in crystalline salts and asymmetric ion pairing in amorphous salts.
• IR spectroscopy showed significant hyperconjugation in the amorphous phase (150 cm-1 red shift) and charge transfer in the crystalline phase (C=C stretch drop of ~160 cm-1).
• Computational results contradicted experimental data regarding C=C bond frequency, likely due to neglecting environmental effects.

Conclusions:

• Carbocation stabilization mechanisms differ significantly between amorphous and crystalline phases.
• Hyperconjugation in amorphous phases and charge transfer in crystalline phases are key stabilization pathways.
• Environmental effects, particularly ion pairing, play a critical role in carbocation behavior and must be considered in computational models.