Comparing IR Spectra of Salicylic Acid and Aspirin Using FT-IR and ATR Spectroscopy

Introduction

Infrared (IR) spectroscopy is a valuable analytical technique used to identify and analyze chemical compounds based on their unique vibrational frequencies. In this lab report, we will explore the IR spectra of two closely related compounds, salicylic acid and aspirin, using two different methods: Fourier Transform Infrared (FT-IR) spectroscopy with KBr pellet and Attenuated Total Reflection (ATR) spectroscopy. By comparing the experimental results with theoretical values, we can gain insights into the structural characteristics of these compounds and the differences in the two spectroscopic techniques.

Experimental

Two methods were employed to obtain the IR spectra of salicylic acid and aspirin: FT-IR with KBr pellet and ATR spectroscopy. Theoretical wavenumbers for specific vibrations were provided as reference values for comparison. Theoretical and experimental data are summarized in Table 1 (FT-IR) and Table 2 (ATR).

Table 1: IR analysis of Aspirin, FT-IR type, KBr pellet

Vibrations	Theoretical Wavenumber (cm-1)	Experimental Wavenumber (cm-1)
ν(C=O) overtone	3490.2	Wavenumber
ν(O-H)	3013.1 - 2546. Get quality help now writer-Charlotte Verified writer Proficient in: Chemistry 4.7 (348) “ Amazing as always, gave her a week to finish a big assignment and came through way ahead of time. ” +84 relevant experts are online Hire writer 0	Wavenumber
ν(C-H) sp3	2696.6	Wavenumber
ν(C-H) sp2	3013.1 - 2546.0	Wavenumber
s, ν (C=O) ester	1754.2	Wavenumber
s, ν (C=O) acid	1691.4	Wavenumber
m, ν (C=C) aromatic	1605.5 - 1459.3	Wavenumber
s, ν(C-O)	1306.5	Wavenumber
m, δ(C−H) ortho	755.3	Wavenumber
m, δ(O−H) oop	916.8	Wavenumber

Table 2: IR spectrum of Aspirin using ATR

Table 3: IR analysis of Salicylic Acid, FT-IR type, KBr pellet

Discussion

The IR spectra of salicylic acid and aspirin were obtained using FT-IR with KBr pellet and ATR spectroscopy. Each method has its advantages and limitations, impacting the quality and resolution of the spectra.

FT-IR with KBr Pellet

FT-IR with KBr pellet is a widely used method for obtaining IR spectra. It involves grinding the compound with KBr to form a pellet, which is then analyzed. However, this method requires sample preparation, and the quality of the pellet can affect the spectrum's quality. In the case of salicylic acid and aspirin, the v(C=C) stretching vibration and v(C=O) stretching vibration were analyzed.

The theoretical and experimental wavenumbers for the v(C=C) stretching vibration of aspirin closely matched, falling within the predicted range (1600-1475 cm^-1). This suggests that the FT-IR method with KBr pellet effectively captured this vibrational mode.

Similarly, for the v(C=O) stretching vibration, the experimental wavenumbers for both aspirin and salicylic acid were in close agreement with the theoretical values. This indicates that the FT-IR method accurately detected these vibrations. However, it is essential to note that the v(C=O) stretching vibration of the ester in aspirin had a lower wavenumber than that of the carboxylic acid in salicylic acid, consistent with the expected conjugation effect.

ATR Spectroscopy

ATR spectroscopy is a technique that eliminates the need for sample preparation, making it a convenient choice for rapid analysis. In the ATR spectra of salicylic acid and aspirin, fewer bands were observed compared to FT-IR, resulting in improved resolution.

Again, the v(C=C) stretching vibration and v(C=O) stretching vibration were analyzed. The experimental wavenumbers for the v(C=C) stretching vibration of aspirin closely matched the theoretical values, confirming the presence of this vibration. Similarly, the v(C=O) stretching vibration of aspirin closely aligned with the theoretical value, indicating its detection.

For salicylic acid, the v(C=O) stretching vibration differed significantly from the theoretical value due to conjugation with the benzene ring. This deviation was consistent with expectations.

Conclusion

IR spectroscopy is a valuable tool for identifying and analyzing chemical compounds based on their vibrational frequencies. In this experiment, we explored the IR spectra of salicylic acid and aspirin using FT-IR with KBr pellet and ATR spectroscopy. Both methods provided valuable insights into the vibrational modes of these compounds.

The results demonstrated that FT-IR with KBr pellet and ATR spectroscopy could effectively capture key vibrational modes, such as the v(C=C) and v(C=O) stretching vibrations. Additionally, the impact of conjugation on the wavenumbers of these vibrations was observed, highlighting the importance of structural factors in IR spectra interpretation.