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

Significance of the Topic

The complementary use of FT-IR and NMR spectroscopy is central to modern analytical chemistry and pedagogy. FT-IR excels at identifying functional groups through characteristic vibrational frequencies, while NMR provides detailed structural information by probing nuclear environments. Integrating both techniques in a teaching laboratory deepens students’ understanding of molecular identity, purity assessment and reaction monitoring.

Objectives and Study Overview

This study demonstrates an undergraduate experiment in which aspirin (acetylsalicylic acid) and wintergreen oil (methyl salicylate) are synthesized via acid-catalyzed esterification of salicylic acid with acetic anhydride or methanol. The goal is to illustrate how FT-IR and NMR spectra complement each other in confirming product formation, assessing purity and identifying residual reactants.

Methodology and Instrumentation

Reagents: salicylic acid, acetic anhydride, methanol, sulfuric acid catalyst.
FT-IR conditions: Nicolet iS5 FT-IR with iD5 ATR accessory; spectral range 4000–600 cm⁻¹; 10 scans per spectrum; 4 cm⁻¹ resolution.
NMR conditions: picoSpin 45 and picoSpin 80 benchtop spectrometers at 45 MHz and 82 MHz; 90° pulse; 750 ms acquisition; 8 s recycle delay; 16 co-added scans for neat, 64 scans for dissolved samples; data processed in MestReNova with zero-filling, phase correction, peak picking and integration.

Main Results and Discussion

Aspirin synthesis:

• FT-IR: disappearance of phenolic O–H band (3230–3000 cm⁻¹) and anhydride carbonyl bands (1822, 1751 cm⁻¹); appearance of ester C=O peak at 1749 cm⁻¹.
• NMR: salicylic acid shows carboxylic proton at 11.75 ppm and aromatic multiplet at 7–8.5 ppm; acetic anhydride gives singlet at 2.25 ppm; product spectrum displays carboxylic H near 10.5 ppm, methyl ester at 2.25 ppm and aromatic region consistent with ortho-substitution.

Wintergreen oil synthesis:

• FT-IR: loss of carboxylic acid bands (3000–2500, 1652 cm⁻¹); emergence of ester C=O at 1674 cm⁻¹; methanol O–H band at 3327 cm⁻¹ in reactant spectrum.
• NMR: carboxylic proton, four aromatic H integrations and methyl ester signal; picoSpin 80 offers sharper splitting patterns in aromatic region and clearer coupling constants.

Some product spectra show residual methanol or salicylic acid impurities, highlighting the value of both techniques in purity assessment.

Benefits and Practical Applications

The combined approach reinforces fundamental principles of spectroscopy, enables rapid functional group identification and structural confirmation, and supports hands-on learning in synthesis labs. It also equips students with skills in interpreting real-world analytical data and troubleshooting reaction impurities.

Future Trends and Applications

Advances in benchtop NMR and portable FT-IR systems will further democratize access to spectroscopic analysis. Integration of automated data processing, cloud-based spectral libraries and AI-driven interpretation tools will enhance real-time decision-making in research, quality control and field applications. Educational curricula may adopt virtual interactive modules that pair instrument simulations with live data acquisition.

Conclusion

FT-IR and NMR spectroscopies offer synergistic insights into chemical synthesis by combining functional group analysis with detailed structural elucidation. Their integration in undergraduate experiments fosters a deeper comprehension of molecular characterization and analytical problem-solving.

References

• Parker FS. Applications of Infrared Spectroscopy. Plenum Press; 1971.
• Griffiths PR, de Haseth JA. Fourier Transform Infrared Spectrometry. John Wiley & Sons; 1986.
• Thermo Scientific. Basic Organic Functional Group Reference Chart; Part Number XX51346_E.
• Nelson JH. Nuclear Magnetic Resonance Spectroscopy. Pearson Education; 2003.
• Hornak JP. The Basics of NMR. J P Hornak; 1997–1999.
• Slayden S. Synthesis of Aspirin (Acetylsalicylic Acid). George Mason University Laboratory Manual; 1999.
• Steehler GA. Synthesis of Aspirin and Oil of Wintergreen. Roanoke College; accessed June 4, 2015.