How to Get Started with Infrared Spectrum Analysis for Beginners?

In a previous introduction to infrared spectroscopy, especially spectrum analysis, many of you showed great interest. To address any remaining questions, today I'm adding an extra session on the essence of infrared spectrum analysis! This content is full of valuable information, so be sure to save and like it. Combined with the previous introduction to infrared spectrum analysis, today's content should clear up most of your doubts regarding infrared spectrum analysis.

Steps for Infrared Spectrum Analysis

Determining the Type of Compound

Organic vs. Inorganic Compounds

Saturated vs. Unsaturated Compounds

Alkenes or Aromatics

Inferring Possible Functional Groups

1.First look at the characteristic frequency region (3600-1350 cm-1), then at the fingerprint region (1350-400 cm-1).

2.First observe strong peaks, then weak peaks.

3.First identify characteristic absorption peaks, then find related peaks for confirmation.

4.Calculate the degree of unsaturation of the molecule (as previously explained). Based on the unsaturation result, infer possible functional groups in the molecule.

Analysis Based on Absorption Peaks

1680-1880 cm-1: Check for strong peaks.

If strong peaks are present:

3300-2500 cm-1: Broad strong peak indicates carboxylic acids.

1300-1100 cm-1: Broad peak indicates esters.

1740-1720 cm-1: Strong peak, 2850-2720 cm-1: Weak peak indicates aldehydes.

1725-1705 cm-1: Strong peak indicates ketones.

1680 cm-1: Strong peak indicates amides.

1800 cm-1: Strong peak indicates acid halides.

1800-1700 cm-1: Double peaks indicate anhydrides.

If no strong peaks:

3600-3300 cm-1: Strong peak indicates alcohols.

1275-1060 cm-1: Strong peak indicates ethers.

1650 cm-1: Weak peak indicates alkenes.

1650-1450 cm-1: Series of peaks indicate aromatics.

No other peaks: 2860-2960 cm-1: Strong peak indicates alkanes, 667 cm-1: Indicates brominated hydrocarbons.

Fingerprint Region vs. Functional Group Region

Functional Group Region (4000-1300 cm-1): Identify functional groups in the compound.

Fingerprint Region (below 1300 cm-1): While some absorptions correspond to certain functional groups, many peaks in this region are indicative of the infrared characteristics of the compound, much like a human fingerprint. The functional group (characteristic) region helps identify existing functional groups, whereas the fingerprint region is useful for comparing with standard spectra (or known substances) to determine if an unknown substance matches a known one.

Summary of Infrared Absorption Spectra

Given a compound with the molecular formula C6H14, and its infrared spectrum below, try to deduce the structure of this compound.

Figure 1: The molecular formula C6H14