FTIR Talk Letter Vol. 44

Significance of the Topic

Understanding low‐energy vibrations in microporous catalysts, enhancing spatially resolved infrared analysis, and accurately interpreting carbonyl stretching are critical for advancing catalyst design, material characterization, and quality control in analytical chemistry.

Objectives and Study Overview

• Demonstrate far‐infrared Fourier transform infrared spectroscopy (Far‐IR FTIR) for direct observation of alkali and molecular cations in zeolite structures using a conventional FTIR system.
• Evaluate new infrared microscopy detectors—Type II superlattice (T2SL) and thermoelectrically cooled mercury cadmium telluride (TEC MCT)—for high‐sensitivity micro area measurements.
• Provide practical rules for identifying and assigning carbonyl (C=O) stretching bands in organic and polymeric materials based on adjacent structural and electronic effects.

Methodology and Instrumentation Used

• Far‐IR FTIR Setup

• Commercial FTIR equipped with a 5 μm Mylar beam splitter and PE‐TGS detector.
• Variable‐temperature cell (–100 to 400 °C) with polyethylene windows and capability for in‐situ gas dosing and evacuation.
• Sample as 20 mm disks (5–100 mg) of ion‐exchanged zeolites; spectra averaged over 256 scans at 4 cm⁻¹ resolution.

• Infrared Microscope Detectors

• T2SL detector cooled by liquid nitrogen, offering detection up to 14.5 μm and superior linearity.
• Optional TEC MCT detector requiring no liquid nitrogen, with Peltier cooling and integrated temperature control.
• DLATGS detector for room‐temperature measurements (4,600–400 cm⁻¹) down to 100 μm apertures.
• Instruments: IRXross/AIRsight and IRTracer‐100/AIMsight systems with typical aperture sizes from 10 × 10 to 100 × 100 μm and varying scan counts.

• Carbonyl Spectral Analysis

• Review of C=O stretching frequency ranges for ketones, aldehydes, esters, lactones, conjugated carbonyls, acids, salts, and amides.
• Analysis of peak shifts induced by mass changes, bond angle distortion, inductive (I) and mesomeric (M) electronic effects of adjacent groups.

Main Results and Discussion

• Far‐IR FTIR of Zeolites

• Alkali metal cations in various frameworks exhibit distinct Far‐IR bands whose frequencies scale with cation mass (1/√m) and pore topology.
• In‐situ monitoring of NH₄⁺ and pyridinium cations confirmed reversible adsorption–desorption behavior and supported delocalized negative charge stabilization in zeolite lattices.

• Infrared Microscopy Detectors

• T2SL detectors maintain linear absorbance to values above 2.5, avoiding saturation seen in MCT above Abs ~1.5 and allowing reliable quantitation in thin films.
• TEC MCT detectors deliver sufficient sensitivity for 25 × 25 μm apertures without liquid nitrogen; DLATGS extends low‐wavenumber coverage at reduced resolution.

• Carbonyl Stretching Assignments

• Heavier substituents adjacent to the carbonyl shift C=O bands to lower wavenumbers compared to lighter H atoms.
• Ring strain (smaller cyclo‐ketones or lactones) distorts sp² geometry and shifts bands to higher wavenumbers.
• Electron‐withdrawing groups increase coupling and upshift C=O peaks (inductive effect), while conjugation and lone pairs (mesomeric effect) downshift them.

Practical Benefits and Applications

• Far‐IR FTIR enables direct characterization of cation locations and acid site dynamics in solid catalysts without specialized light sources.
• Advanced IR detectors expand micro area analysis capabilities to sub‐10 μm scales with high sensitivity and dynamic range, facilitating studies of heterogenous samples and thin films.
• Clear guidelines for carbonyl band assignment improve accuracy in polymer analysis, material identification, and monitoring of chemical modifications.

Future Trends and Potential Applications

• Integration of Far‐IR spectroscopy into routine catalyst screening and monitoring of in‐service deactivation.
• Development of room‐temperature superlattice detectors and further miniaturization for sub‐micron imaging.
• Expansion of IR spectral libraries with structure‐based predictive models and machine learning tools for automated functional group analysis.
• Application of carbonyl assignment rules to complex biomaterials, composite systems, and reaction monitoring in flow cells.

Conclusion

This integrated overview highlights how Far‐IR FTIR, innovative infrared microscopy detectors, and rigorous carbonyl spectral interpretation collectively advance analytical chemistry. Together, these developments offer powerful tools for probing material structure and reactivity at unprecedented sensitivity and spatial resolution.