Experimental and computational bridgehead C-H bond dissociation enthalpies

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Bridgehead C-H bonds are sterically hindered and electronically unique.
• Accurate determination of their bond dissociation enthalpies (BDEs) is crucial for understanding reaction mechanisms.
• Gas-phase experimental data for these systems are scarce.

Purpose of the Study:

• To experimentally determine the gas-phase bridgehead C-H bond dissociation enthalpies for bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, and adamantane.
• To compare experimental BDEs with high-level G3 theory calculations.
• To investigate the relationship between BDEs and the flexibility of bicyclic ring systems.

Main Methods:

• Utilized a thermodynamic cycle involving acid enthalpies, hydrogen ionization energy, and halogen electron affinity to calculate BDEs.
• Employed high-level G3 theory for computational validation.
• Analyzed experimental and computational data for various bicyclic structures.

Main Results:

• Experimental gas-phase bridgehead C-H BDEs were determined: 105.7 ± 2.0 kcal mol⁻¹ for bicyclo[2.2.1]heptane, 102.9 ± 1.7 kcal mol⁻¹ for bicyclo[2.2.2]octane, and 102.4 ± 1.9 kcal mol⁻¹ for adamantane.
• Experimental results showed good agreement with G3 theory calculations.
• A correlation was observed between the determined BDEs and the flexibility of the bicyclic ring systems.
• Provided rare examples of gas-phase alkyl anions.

Conclusions:

• The thermodynamic cycle method provides accurate gas-phase bridgehead C-H BDEs.
• Ring flexibility significantly influences bridgehead C-H bond strength in bicyclic systems.
• Computational methods like G3 theory are reliable for predicting these BDEs.