How to lnterpret an lnfrared Spectrum

Infrared spectroscopy is a technique for identifying chemical substances by analyzing changes in the vibrational frequency of chemical bonds in molecules. Analyzing IR spectra primarily involves identifying characteristic absorption peaks, which correspond to the vibrational frequencies of different functional groups in the molecule.

 

Analysis Workflow

1. Basic Concepts

(1) Wavenumber (usually represented as cm−1−1): Indicates the position of absorption peaks in the infrared spectrum, it is a measure of the absorption frequency, and is related to the difference in vibrational energy levels.

(2) Absorption peak: Appears as a peak on the spectrum, its position (wavenumber) and intensity can provide information about the chemical bonds and functional groups present in the molecule.

 

2. Key Wavenumber Areas and Corresponding Functional Groups

(1) 3200-3600 cm^-1: O-H bond stretching vibrations (broad band, visible in alcohols, phenols, and water); N-H bond stretching vibrations (generally around 3300-3500 cm−1−1, may appear as a pair of peaks).

(2) 2800-3000 cm^-1: C-H bond stretching vibrations (seen in alkanes).

(3) 2250-2100 cm^-1: Stretching vibrations of C≡C and C≡N triple bonds.

(4) 1680-1750 cm^-1: Stretching vibrations of C=O double bonds (carbonyl, such as ketones, esters, amides, etc.).

(5) 1600-1580 cm^-1 and 1500-1400 cm^-1: Correspond to the stretching vibrations of the C=C bond and the bending vibrations of the C-H bond in aromatic compounds, respectively.

(6) 1450-1370 cm^-1: C-H bond bending vibrations (seen in alkanes).

(7) 1300-1000 cm^-1: C-O bond stretching vibrations (alcohols, ethers, esters, etc.).

 

3. Analysis Steps

(1)  Identify broad bands and sharp peaks: Broad bands are usually associated with stretching vibrations of O-H or N-H bonds, while sharp peaks may be functional groups such as C=O.

(2) Determine the wavenumber: Check the wavenumber of the characteristic peak, and match it with the known absorption region of the functional group.

(3) Analyze peak shape and intensity: The shape and intensity of the peak can provide additional information about the type of bond and environment in the molecule.

(4) Consider the fingerprint region: The area below 1000 cm is called the fingerprint region. Although it is complex and difficult to directly analyze, it is very useful for the specific identification of compounds.−1−1

 

4. Comprehensive Analysis

Compare the observed absorption peaks with the possible structural features of the compound, and consider all the information to determine the specific structure of the compound. In practice, it is usually necessary to refer to standard spectra or databases for comparison to ensure the accuracy of the analysis.