Gas-phase acidity of D-glucose. A density functional theory study.

Area of Science:

• Computational Chemistry
• Carbohydrate Chemistry
• Physical Organic Chemistry

Background:

• D-glucopyranose exists as alpha and beta anomers.
• Understanding the gas-phase acidity of carbohydrates is crucial for predicting their behavior in various chemical environments.
• Previous studies have explored the acidity of carbohydrates in solution, but gas-phase data is less common.

Purpose of the Study:

• To determine the gas-phase acidity of D-glucopyranose anomers.
• To investigate the effect of deprotonation on the stability and reactivity of D-glucopyranose.
• To evaluate the accuracy of different computational methods and basis sets for studying carbohydrate acidity.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP functional.
• Employing standard basis sets such as 6-31G(d,p) and 6-31+G(d,p).
• Calculations of gas-phase acidity (ΔacidG°(298)) using a higher-level basis set (B3LYP/6-311+G(2df,2p)).

Main Results:

• The anomeric hydroxyl group was identified as the most acidic for both alpha- and beta-D-glucopyranose, particularly when using the 6-31+G(d,p) basis set.
• Deprotonation of the anomeric hydroxyl group leads to C(1)-O bond elongation and a facile ring-opening process.
• Calculated activation barriers for ring-opening were very small, suggesting easy interconversion between deprotonated anomers.
• The absolute gas-phase acidity of alpha-D-glucose was calculated as 1398 kJ mol⁻¹, aligning well with experimental data.

Conclusions:

• The anomeric hydroxyl group's acidity in D-glucopyranose is significant in the gas phase.
• Computational methods, especially those including diffuse functions, are essential for accurately describing anionic carbohydrate systems.
• The findings provide valuable insights into the intrinsic reactivity and stability of carbohydrates, relevant for various chemical and biological processes.