Intermolecular Interactions at the Silica-Liquid Interface Modulate the Fermi Resonance Coupling in Surface Methanol

Area of Science:

• Surface science
• Spectroscopy
• Physical chemistry

Background:

• Understanding solid/liquid interfaces is crucial in various chemical and physical processes.
• Vibrational sum-frequency generation (vSFG) spectroscopy is a powerful technique for probing interfacial molecular structures and dynamics.

Purpose of the Study:

• To investigate the effect of binary solvent mixtures on the vibrational properties of molecules at a buried interface.
• To quantify the influence of acetonitrile on the Fermi resonance of methanol at the hydrophilic fused silica/liquid interface.

Main Methods:

• Utilized vibrational sum-frequency generation (vSFG) spectroscopy to study the fused silica/binary solvent interface.
• Analyzed the Fermi resonance peak suppression and intensity changes in vSFG spectra with varying acetonitrile (MeCN) and methanol (MeOH) concentrations.
• Quantified Fermi resonance coupling using the Fermi resonance coupling coefficient (W) and the intensity ratio (R).

Main Results:

• High methanol concentrations suppressed the Fermi resonance peak in vSFG spectra.
• As methanol was diluted with perdeuterated acetonitrile, the Fermi resonance intensity progressively increased.
• Calculated Fermi resonance coupling coefficient (W) increased from 10 ± 10 cm⁻¹ to 46 ± 4 cm⁻¹ as MeOH mole fraction decreased from 1.0 to 0.1.
• Intensity ratio (R) increased from 0.01 ± 0.02 to 0.43 ± 0.16 over the same concentration range.

Conclusions:

• Solvation by acetonitrile effectively tunes the Fermi coupling of methanol vibrations at the silica/liquid interface.
• The study demonstrates a method for controlling and quantifying interfacial molecular vibrational behavior through solvent composition.
• Findings provide insights into molecular interactions at buried interfaces, relevant for catalysis, materials science, and nanotechnology.