Monosaccharide Isomer Interconversions Become Significant at High Temperatures.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Carbohydrate Chemistry

Background:

• Monosaccharide anomerization is well-studied in solution, primarily focusing on pyranose isomers.
• High-temperature processes like pyrolysis involve complex monosaccharide transformations.
• Understanding gas-phase anomerization is crucial for predicting behavior in thermal processes.

Purpose of the Study:

• To predict gas-phase anomerization processes and rates for common monosaccharides.
• To investigate the influence of temperature on monosaccharide isomer distribution.
• To elucidate the mechanisms and energetics of ring-opening and -closing reactions.

Main Methods:

• Quantum-chemical calculations using the CBS-QB3 composite method.
• Computation of structures and thermochemistry for stable species and transition states.
• Prediction and fitting of high-pressure-limit Arrhenius reaction parameters from 300 to 1000 K.

Main Results:

• Calculated activation energies for ring-opening range from 162-217 kJ/mol.
• Catalytic participation of hydroxyl groups (intra- or intermolecular) significantly lowers activation energies (down to 67 kJ/mol).
• Equilibrium constants indicate a temperature-dependent increase in furanose and linear aldehyde anomers.

Conclusions:

• Low anomerization barriers and shifting equilibria lead to diverse monosaccharide isomer populations at high temperatures.
• Gas-phase anomerization pathways are sensitive to catalytic effects, particularly from hydroxyl groups.
• Computational predictions provide valuable insights into monosaccharide behavior during pyrolysis and other thermal treatments.