Yin and yang in chemistry education: the complementary nature of FTIR and NMR spectroscopies

Significance of the Topic

Understanding both functional group characterization and molecular structure elucidation is vital in analytical chemistry education and practice. Combining FT-IR and NMR spectroscopy showcases how vibrational and magnetic resonance techniques complement one another to confirm chemical transformations.

Objectives and Study Overview

This study demonstrates the synthesis of aspirin (acetylsalicylic acid) and wintergreen oil (methyl salicylate) via acid-catalyzed esterification of salicylic acid. FT-IR provides functional group identification while NMR elucidates the complete molecular framework, highlighting their cooperative analytical power.

Methodology

• Acid-catalyzed esterification of salicylic acid with acetic anhydride to produce aspirin, and with methanol to produce wintergreen oil.
• FT-IR spectra collected from 4000–600 cm⁻¹ (10 scans, 4 cm⁻¹ resolution) using attenuated total reflectance.
• NMR spectra acquired on 45 MHz and 80 MHz benchtop instruments with 90° excitation pulse, 750 ms acquisition, 8 s recycle delay; 16 scans for neat samples, 64 for dissolved samples.
• Data processed via Fourier transform, phase correction, peak picking and integration in MestReNova software.

Instrumentation

• Thermo Scientific Nicolet iS5 FT-IR spectrometer with iD5 single bounce ATR accessory (diamond crystal).
• Thermo Scientific picoSpin 45 and picoSpin 80 NMR spectrometers with capillary cartridge probes.

Main Results and Discussion

FT-IR Analysis:

• Aspirin synthesis showed disappearance of phenolic O–H and anhydride bands and appearance of an ester C=O stretch at 1749 cm⁻¹; minor acetic anhydride residues detected.
• Wintergreen oil synthesis exhibited loss of carboxylic acid bands and emergence of an ester C=O band at 1674 cm⁻¹; residual methanol and salicylic acid impurities observed.

NMR Analysis:

• Aspirin: carboxylic proton at ~11.7 ppm, aromatic protons between 7–8.5 ppm, residual methyl at ~2.25 ppm; picoSpin 80 provided enhanced resolution of coupling patterns.
• Wintergreen oil: integration confirmed one carboxylic H, four aromatic H, and three methyl H on the ester; impurity signals from methanol and salicylic acid present.

Benefits and Practical Applications

• FT-IR and NMR together validate molecular identity by linking functional group presence to full structural information.
• Benchtop spectrometers offer rapid, in-lab analysis for teaching laboratories, research groups, and quality control.
• Illustrates foundational principles of spectral interpretation and the complementary roles of vibrational and magnetic resonance techniques.

Future Trends and Opportunities

• Integration of automated benchtop spectroscopy modules for hands-on learning and real-time process monitoring.
• Expansion to two-dimensional NMR and advanced IR imaging for more comprehensive structural analysis.
• Application of machine learning algorithms for rapid spectral interpretation and remote instruction.
• Miniaturization and cost reduction promoting broader adoption across academic, industrial, and field environments.

Conclusion

FT-IR and NMR spectroscopies provide complementary insights into molecular structure, with FT-IR delivering rapid functional group identification and NMR offering detailed structural elucidation. The Thermo Scientific benchtop instruments effectively demonstrate these techniques in an educational setting and serve as valuable tools for routine analytical workflows.

References

1. Parker, F. S. Applications of Infrared Spectroscopy. Plenum Press, 1971.
2. Griffiths, P. R., & de Haseth, J. A. Fourier Transform Infrared Spectrometry. Wiley, 1986.
3. Thermo Scientific basic organic functional group reference chart, Part Number XX51346_E 02/15M.
4. Nelson, J. H. Nuclear Magnetic Resonance Spectroscopy. Pearson Education, 2003.
5. Hornak, J. P. The Basics of NMR. J. P. Hornak, 1997–1999.
6. Slayden, S. Synthesis of Aspirin (Acetylsalicylic Acid), Laboratory Manual, George Mason University, 1999.
7. Steehler, G. A. Synthesis of Aspirin and Oil of Wintergreen. Roanoke College, 2015.