Alkali Cation Chelation in Cold β-O-4 Tetralignol Complexes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Polymer Science

Background:

• Understanding lignin structure is crucial for biomass conversion.
• Alkali metal cationization is a method to study lignin model compounds.
• Previous studies explored neutral and cationized β-O-4 dimers.

Purpose of the Study:

• To investigate the structural elements of alkali metal cationized tetralignol complexes.
• To identify the binding sites of alkali metal ions (Li+, Na+, K+) in these complexes.
• To understand the influence of alkali metal ion size on the conformation.

Main Methods:

• Cold ion spectroscopy techniques, including UV action and IR-UV double resonance.
• Gas-phase conformational analysis.
• Infrared (IR) spectral analysis of the OH stretching region.

Main Results:

• Multiple conformers were identified for Li+, Na+, and K+ cationized tetralignol complexes.
• Alkali metal ions bind in penta-coordinate pockets involving ether and OH groups across β-O-4 linkages.
• Distinct IR transitions below 3550 cm(-1) characterize binding sites based on M(+)···OH···O interactions.
• The size of the K+ ion mitigates these interactions in the major conformer.

Conclusions:

• Spectroscopic evidence elucidates the binding modes of alkali metal ions in tetralignol complexes.
• The number of M(+)···OH···O interactions dictates specific IR spectral features.
• Alkali metal ion size significantly influences the three-dimensional structure of cationized tetralignols.