6.3: IR Spectrum and Characteristic Absorption Bands (2024)

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    6.3 IR Spectrum and Characteristic Absorption Bands

    Xin Liu

    With the basic understanding of the IR theory, we will take a look at the actual output from IR spectroscopy experiments, and learn how to get structural information from IR spectrum. Below is the IR spectrum for 2-hexanone.

    6.3: IR Spectrum and Characteristic Absorption Bands (2)

    Notes for interpreting IR spectra:

    • The vertical axis is ‘% transmittance’, which tells how strongly light was absorbed at each frequency. The solid line traces the values of % transmittance for every wavelength passed through the sample. At the high end of the axis, 100% transmittance means no absorption occurred at that frequency. Lower values of % transmittance mean that some of the energy is absorbed by the compound, and gives the downward spikes. The spikes are called absorption bands in an IR spectrum. A molecule have a variety of covalent bonds, and each bond have different vibration modes, so the IR spectrum of a compound usually show multiple absorption bands.
    • The horizontal axis indicates the position of an absorption band. But instead of using frequency to show the absorbed radiation, wavenumbers (6.3: IR Spectrum and Characteristic Absorption Bands (3), in unit of cm-1) are used as a conventional way in IR spectra. The wavenumber is defined as the reciprocal of wavelength (Formula 6.3), and the wavenumbers of infrared radiation are normally in the range of 4000 cm-1 to 600 cm-1 (approximate corresponds the wavelength range of 2.5 μm to 17 μm of IR radiation).
    6.3: IR Spectrum and Characteristic Absorption Bands (4)

    Please note the direction of the horizontal axis (wavenumber) in IR spectra decrease from left to right. The larger wavenumbers (shorter wavelengths) are associated with higher frequencies and higher energy.

    The power of infrared spectroscopy arises from the observation that the covalent bonds characterizing different functional groups have different characteristic absorption frequencies (in wavenumber, Table 6.1). The technique is therefore very useful as a means of identifying which functional groups are present in a molecule of interest.

    For example, the most characteristics absorption band in the spectrum of 2-hexanone (Figure 6.3a) is that from the stretching vibration of carbonyl double bond C=O, at 1716 cm-1. It is a very strong band comparing to the others on the spectrum. A strong absorbance band in the 1650-1750 cm-1 region indicate that a carbonyl group (C=O) is present. Within that range, carboxylic acids, esters, ketones and aldehydes tend to absorb in the higher wavenumber/frequency end (1700-1750 cm-1), while conjugated unsaturated ketones and amides tend to absorb on the lower wavenumber/frequency end (1650-1700 cm-1).

    Stretching Vibrations

    Generally, stretching vibrations the stretching vibrations require more energy and show absorption bands in the higher wavenumber/frequency region. The characteristics stretching vibration bands associated with the bonds in some common functional groups are summarized in Table 6.1.



    Characteristic IR Frequency range (cm-1)

    O-H stretching

    3200 – 3600 (broad)

    C=O stretching

    1650 – 1750 (strong) aldehyde

    C-H stretching

    ~ 2800 and ~ 2700 (medium) carboxylic acid

    C=O stretching

    1700 – 1725 (strong)

    O-H stretching

    2500 – 3300 (broad) alkene

    C=C stretching

    1620 – 1680 (weak)

    vinyl =C-H stretching

    3020 – 3080 benzene

    C=C stretching

    ~ 1600 and 1500 – 1430 (strong to weak) alkyne

    C≡C stretching

    2100 – 2250 (weak)

    terminal ≡C-H stretching

    3250 – 3350 alkane

    C-H stretching

    2850-2950 amine

    N-H stretching

    3300-3500 (medium)

    Table 6.1 Characteristic IR Frequencies of Stretching Vibrations

    The information in Table 6.1 can be summarized in the diagram that is easier to be identified (Figure 6.3b), in which the IR spectrum is divided in several regions, with the characteristic band of certain groups labelled.

    6.3: IR Spectrum and Characteristic Absorption Bands (5)

    The absorption bands in IR spectra have different intensity, that can usually be referred to as strong (s), medium (m), weak (w), broad and sharp. The intensity of a absorption band depends on the polarity of the bond, the bond with higher polarity will show more intense absorption band. The intensity also depends on the number of bonds responsible for the absorption, the absorption band with more bonds involved has higher intensity.

    The characteristic IR frequencies of stretching vibrations in Table 6.1 and Figure 6.3b provide very useful information to identify the presence of certain functional group, that can be generally summarized as:

    The polar O-H bond (in alcohol and carboxylic acid) usually shows strong and broad absorption bands that are easy to be identified. The broad shape of the absorption band results from the hydrogen bonding of the OH groups between molecules. The OH bond of alcohol group usually has absorption in the range of 3200-3600 cm-1, while the OH bond of carboxylic acid group occurs at about 2500-3300 cm-1 (Figure 6.4a and Figure 6.4c).

    The polarity of N-H bond (in amine and amide) is weaker than OH bond, so the absorption band of N-H is not as intense, nor that broad as O-H, and the position is in 3300-3500 cm-1 region.

    The C-H bond stretching of all hydrocarbons occur in the range of 2800-3300 cm-1, and the exact location can be used to distinguish between alkane, alkene and alkyne. Specifically:

    • ≡C-H (sp C-H) bond of terminal alkyne give absorption at about 3300 cm-1
    • =C-H (sp2 C-H) bond of alkene give absorption at about 3000-3100 cm-1
    • -C-H (sp3 C-H) bond of alkane give absorption at about ~2900 cm-1 (see the example of IR spectrum of 2-hexanone in Figure 6.3a, the C-H absorption band at about 2900 cm-1)

    A special note should be taken for the C-H bond stretching of an aldehyde group that shows two absorption bands, one at ~2800 cm-1 and the other at ~ 2700 cm-1. It is therefore relative easy to identify the aldehyde group (together with the C=O stretching at about 1700 cm-1) since essentially no other absorptions occur at these wavenumbers (see the example of IR spectrum of butanal in Figure 6.4d ).

    The stretching vibration of triple bonds C≡C and C≡N have absorption bands of about 2100~2200 cm-1. The band intensity are in medium to weak level. The alkynes can generally be identified with the characteristic weak but sharp IR absorbance bands in the range of 2100-2250 cm-1 due to stretching of the C≡C triple bond, and terminal alkynes can be identified by their absorbance at about 3300 cm-1, due to stretching of sp C-H.

    As mentioned earlier, the C=O stretching has strong absorption band in the 1650-1750 cm-1 region. Other double bonds like C=C and C=N have absorptions in bit lower frequency regions of about 1550-1650 cm-1. The C=C stretching of an alkene only shows one band at ~1600 cm-1 (Figure 6.4b), while a benzene ring is indicated by two sharp absorption bands, one at ~1600 cm-1 and one at 1500-1430 cm-1 (see the example of IR spectrum of ethyl benzene in Figure 6.4e).

    You will notice in Figure 6.3a and 6.3b that a region with the lower frequency 400-1400 cm-1 in the IR spectrum is called the fingerprint region. Kind of like a human fingerprint, the pattern of absorbance bands in the fingerprint region is characteristic of the compound as a whole. Even if two different molecules have the same functional groups, their IR spectra will not be identical and such difference will be reflected in the bands in the fingerprint region. Therefore the IR from an unknown sample can be compared to a database of IR spectra of known standards in order to confirm the identification of the unknown sample.

    6.3: IR Spectrum and Characteristic Absorption Bands (2024)


    What are the absorption bands in IR spectrum? ›

    The absorption bands in IR spectra have different intensity, that can usually be referred to as strong (s), medium (m), weak (w), broad and sharp. The intensity of a absorption band depends on the polarity of the bond, the bond with higher polarity will show more intense absorption band.

    What are the characteristic bands of IR spectra? ›

    6.3 IR Spectrum and Characteristic Absorption Bands
    FormulaBondCharacteristic IR Frequency range (cm-1)
    alcoholO-H stretching3200-3600 (broad)
    carbonylC=O stretching1650-1750 (strong)
    aldehydeC-H stretching~ 2800 and ~ 2700 (medium)
    carboxylic acidC=O stretching1700-1725 (strong)
    8 more rows

    Which absorption bands would you expect to see in the IR spectrum? ›

    he absorption bands expected in the IR spectrum are: a sharp peak at around 3300 cm^-1 due to the O-H stretch, a strong peak around 2100-2260 cm^-1 due to the C≡C stretch, and a sharp peak around 1710-1750 cm^-1 due to the C=O. stretch. The IR spectrum is used to identify functional groups present in a molecule.

    What are the characteristics frequencies of absorption in IR spectroscopy? ›

    Typical Infrared Absorption Frequencies

    3300-3400 (dil. soln.) To illustrate the usefulness of infrared absorption spectra, examples for five C4H8O isomers are presented below their corresponding structural formulas.

    What is the absorption spectrum of a band? ›

    The absorption spectra are where the light and dark bands are created. The emission spectra are what create the coloured bands. The absorption of the atoms within the substance can be calculated from the absorption spectra. The structure of a chemical or element can be determined using the emission spectra.

    How can I interpret an IR spectrum? ›

    The interpretation of infrared spectra involves the correlation of absorption bands in the spectrum of an unknown compound with the known absorption frequencies for types of bonds. This table will help users become more familiar with the process.

    What do absorption bands tell us? ›

    Each of the absorption lines corresponds to a specific electron jump. The shortest wavelength/highest energy light (violet 410 nm) causes the electron to jump up four levels, while the longest wavelength/lowest energy light (red 656 nm) causes a jump of only one level.

    What are the characteristics of IR bands? ›

    IR bands can be classified as strong (s), medium (m), or weak (w), depending on their relative intensities in the infrared spectrum. A strong band covers most of the y-axis. A medium band falls to about half of the y-axis, and a weak band falls to about one third or less of the y-axis.

    What is the strongest IR absorption? ›

    The carbonyl stretching absorption is one of the strongest IR absorptions, and is very useful in structure determination as one can determine both the number of carbonyl groups (assuming peaks do not overlap) but also an estimation of which types.

    What is the strongest absorption band in the infrared absorption spectrum at low resolution? ›

    Question: At low resolution, the strongest absorption band in the infrared absorption spectrum of 12C16O is centered near 2143 cm−1. Upon closer examination at higher resolution, this band is observed to be split into two sets of closely spaced peaks, one on each side of the center of the spectrum at 2143.26 cm−1.

    What is the major requirements for IR absorption? ›

    What is necessary condition for IR spectroscopy? The change in the electric dipole moment of the functional group present in a molecule or a sample during the vibration based on the selection rule for IR transitions is a necessary requirement for a molecule or sample to show infrared spectrum.

    What affects IR absorption? ›

    In general, IR absorption is caused by the interaction between the IR electric field vector and the molecular dipole transition moments related to the molecular vibrations. Absorption is at a maximum when the electric field vector and the dipole transition moment are parallel to each other.

    What determines absorption spectrum? ›

    A material's absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies of electromagnetic radiation. The absorption spectrum is primarily determined by the atomic and molecular composition of the material.

    What do the bands seen in the spectrum represent? ›

    In nuclear physics, spectral bands refer to the electromagnetic emission of polyatomic systems, including condensed materials, large molecules, etc. Each spectral line corresponds to the difference in two energy levels of an atom.

    What are the factors affecting absorption bands? ›

    The temperature, concentration and pH of the sample solution affect the position and shape of UV-Vis absorption bands.

    How do you calculate absorption band? ›

    By plotting the graph between (ahv)^(1/2) versus photon energy (hv) where, a (alpha) is the absorbance calculated from UV . (hv) can be calculated form wavelength using: (hv = 1240/wavelength);Extrapolating the straight line portion of the curves to zero absorption coefficient value gives the energy band gap value.

    What is a good IR spectrum? ›

    Independent from the technique that is used, you should obtain the IR spectra in the range from 600-4000 cm-1 (for organic compounds). Make sure that you check the background before you obtain the spectrum of your actual compound (What does the background show?).

    What information can be determined from an IR spectrum? ›

    The main use of this technique is in organic and inorganic chemistry. It is used by chemists to determine functional groups in molecules. IR Spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups.

    What are the absorption bands of water in IR? ›

    In the near-infrared range liquid water has absorption bands around 1950 nm (5128 cm1), 1450 nm (6896 cm1), 1200 nm (8333 cm1) and 970 nm, (10300 cm1).

    What are the absorption bands in the EM spectrum? ›

    The areas of the EM spectrum that are absorbed by atmospheric gases such as water vapor, carbon dioxide, and ozone are known as absorption bands.

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