Hydroxymethyl radical self-recombination in high-temperature water

Area of Science:

• Chemical Kinetics
• Physical Chemistry
• Radiation Chemistry

Background:

• The hydroxymethyl radical (•)CH2OH is a key intermediate in radiation chemistry and organic synthesis.
• Understanding its self-recombination kinetics is crucial for predicting reaction pathways in aqueous systems.

Purpose of the Study:

• To investigate the self-recombination kinetics of (•)CH2OH radicals in neutral aqueous solution.
• To determine the temperature and pressure dependence of the recombination rate constant.
• To elucidate the mechanisms of dimerization and disproportionation pathways.

Main Methods:

• Pulse radiolysis was employed to generate (•)CH2OH radicals.
• Transient absorption spectroscopy was used to monitor radical decay.
• Experiments were conducted at temperatures up to 300 °C and pressures up to 220 bar.
• Ab initio calculations were performed to support mechanistic interpretations.

Main Results:

• The self-recombination of (•)CH2OH follows second-order kinetics.
• The rate constant at 22 °C is 2k = 1.4 ± 0.1 × 10(9) M(-1) s(-1).
• The reaction exhibits Arrhenius behavior with an activation energy (Ea) of 12.7 ± 0.9 kJ/mol.
• Recombination is slower than diffusion-limited at elevated temperatures.
• Dimerization has a negative activation entropy barrier, while disproportionation likely involves water molecules.

Conclusions:

• Both dimerization and disproportionation pathways for (•)CH2OH self-recombination possess significant activation barriers.
• The negative activation entropy for dimerization suggests a highly ordered transition state.
• Water molecules play a role in lowering the activation energy for the disproportionation channel, leading to formaldehyde formation.