Aromatic Charge Resonance Interaction Probed by Infrared Spectroscopy

Area of Science:

• Physical Chemistry
• Spectroscopy
• Chemical Physics

Background:

• Charge resonance is a key intermolecular force in aromatic dimer radical ions.
• High-resolution spectroscopic characterization of isolated dimers has been lacking.
• Understanding charge distribution is crucial for predicting molecular interactions.

Purpose of the Study:

• To precisely probe charge distribution in isolated aromatic dimer cations.
• To investigate the influence of symmetry and solvation on charge distribution.
• To establish a spectroscopic method for characterizing charge asymmetry.

Main Methods:

• Vibrational infrared spectroscopy of cold aromatic pyrrole dimer cations.
• Measurement of the isolated N-H stretch mode frequency (νNH).
• Systematic variation of dimer environment through functional group substitution and solvation.

Main Results:

• A linear correlation was observed between νNH and the partial charge (q) on pyrrole molecules.
• Symmetry reduction (e.g., N-methylpyrrole substitution) shifted charge distribution.
• Asymmetric solvation by N2 ligands also altered charge distribution.

Conclusions:

• The N-H stretch frequency serves as a precise probe of charge distribution asymmetry.
• Charge distribution is sensitive to subtle changes in molecular symmetry and solvation.
• This spectroscopic approach is applicable to various aromatic homo- and heterodimer cations.