Moderately strong phenols dissociate by forming an ion-pair kinetic intermediate

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Dynamics

Background:

• Understanding the dissociation mechanisms of moderately strong acids like hydroxyarenes is crucial.
• Previous studies have explored acid-base reactions, but the transient intermediates and proton transfer dynamics in hydroxyarenes remain less understood.

Purpose of the Study:

• To provide computational evidence for the formation and characteristics of an ion-pair intermediate in the dissociation of ground-state hydroxyarenes (Ar-OH).
• To investigate the lifetime, concentration, structural properties, and proton fluctuation dynamics of this intermediate.

Main Methods:

• Utilized computational methods to simulate the dissociation process of hydroxyarenes.
• Analyzed the structure, concentration, and dynamics of the transient ion-pair intermediate and its surrounding solvation shells.

Main Results:

• Identified a transient ion-pair intermediate with a lifetime of 3-5 picoseconds during hydroxyarene dissociation.
• The intermediate's concentration is approximately half that of the un-ionized acid at pH ~0.6.
• Observed intermediate C-O bond lengths (1.30 Å) and subpicosecond proton fluctuations within solvation shells (100-300 fs).

Conclusions:

• Ground-state hydroxyarene dissociation proceeds through a distinct ion-pair intermediate.
• Proton transfer dynamics within the solvation shells are rapid and occur during the intermediate's lifetime.
• These findings elucidate the complex mechanism of acid dissociation in solution.