Structural Evidence of Interanionic Hydrogen Bonding in Phosphoric Acid Solutions

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Spectroscopy

Background:

• Interanionic hydrogen bonding (IAHB) involves interactions between like-charged ions, defying traditional electrostatic models.
• Understanding IAHB is crucial for explaining chemical behavior in concentrated ionic solutions.

Purpose of the Study:

• To provide direct structural evidence for IAHB in concentrated aqueous phosphoric acid (PA) solutions.
• To elucidate the role of solvent in facilitating anion-anion association.

Main Methods:

• Oxygen K-edge X-ray absorption fine structure (XAFS) spectroscopy.
• Electron affinity time-dependent density functional theory (TD-DFT) calculations.
• Extended X-ray absorption fine structure (EXAFS) and Near-edge X-ray absorption fine structure (NEXAFS) analyses.

Main Results:

• Direct structural evidence of stable, cyclic phosphate-phosphate IAHB dimers formed at PA concentrations ≥7 M.
• Long-range ordering consistent with dimer formation observed via EXAFS.
• Concentration-dependent transition from monomeric to dimeric species confirmed by NEXAFS.
• DFT calculations indicated energetically favorable IAHB formation driven by polarization and charge transfer, mediated by solvent.

Conclusions:

• Solvent plays a critical role in facilitating anion-anion association through IAHB.
• Findings offer crucial structural insights into hydrogen-bonded networks in concentrated PA solutions.
• IAHB contributes significantly to the behavior of concentrated ionic systems.