How To Read Infrared Spectrum? Easy Chart Lookup

Infrared (IR) spectroscopy is a powerful analytical technique used to identify and characterize the molecular structure of a substance. The IR spectrum is a plot of the intensity of absorbed radiation versus the wavelength or frequency of the radiation. To read an IR spectrum, it’s essential to understand the relationship between the absorption peaks and the molecular vibrations that cause them.

Understanding IR Spectrum Basics

When a molecule absorbs IR radiation, it causes the molecule’s bonds to vibrate. These vibrations occur at specific frequencies, which correspond to specific wavelengths. The IR spectrum is a chart that plots the intensity of absorbed radiation against the wavelength or frequency. The resulting spectrum shows a series of peaks, each corresponding to a specific molecular vibration.

Key Regions of the IR Spectrum

The IR spectrum can be divided into several key regions, each corresponding to specific types of molecular vibrations:

1. Hydrogen Stretching Region (3600-3000 cm-1): This region is characterized by strong, broad peaks corresponding to O-H, N-H, and C-H bond stretches.
2. Triple Bond Region (2500-2000 cm-1): This region is characterized by strong peaks corresponding to C≡C, C≡N, and C-O bond stretches.
3. Double Bond Region (2000-1500 cm-1): This region is characterized by strong peaks corresponding to C=C, C=O, and C=N bond stretches.
4. Fingerprint Region (1500-500 cm-1): This region is characterized by a complex pattern of peaks corresponding to various molecular vibrations, including C-C, C-O, and C-N bond stretches and bends.

Easy Chart Lookup

To read an IR spectrum, you can use a chart lookup approach:

1. Identify the Key Peaks: Look for the strongest peaks in the spectrum and identify the corresponding wavelength or frequency.
2. Use a Correlation Chart: Refer to a correlation chart, such as the one below, to determine the likely molecular vibration corresponding to each peak.

1. Consider the Peak Intensity: The intensity of the peak can provide information about the concentration of the sample and the strength of the molecular vibration.
2. Look for Peak Patterns: Certain peak patterns can be indicative of specific molecular structures. For example, a strong peak at 1750 cm-1 accompanied by a weaker peak at 1200 cm-1 may indicate the presence of a carbonyl group.

Example IR Spectrum Analysis

Suppose we have an IR spectrum with the following peaks:

• 3400 cm-1 (strong)
• 2250 cm-1 (medium)
• 1600 cm-1 (strong)
• 1300 cm-1 (weak)

Using the correlation chart, we can assign the following molecular vibrations:

• 3400 cm-1: N-H stretch
• 2250 cm-1: C≡C stretch
• 1600 cm-1: C=C stretch
• 1300 cm-1: C-O stretch

Based on these assignments, we can infer that the molecule contains an amine group (N-H stretch), a triple bond (C≡C stretch), and a double bond (C=C stretch). The weak peak at 1300 cm-1 may indicate the presence of a C-O bond.

By combining the information from the correlation chart with an understanding of the key regions of the IR spectrum, we can gain valuable insights into the molecular structure of the substance being analyzed.

Conclusion

Reading an IR spectrum requires a combination of understanding the basics of IR spectroscopy, identifying key peaks, and using a correlation chart to assign molecular vibrations. By following the easy chart lookup approach outlined above, you can develop the skills needed to interpret IR spectra and gain a deeper understanding of molecular structure and composition.

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