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Comprehensive Guide to Infrared Spectroscopy in Chemistry

Learn about the fundamentals of IR spectroscopy, bond vibrations, stretching frequencies, and unique regions in the IR spectrum. Understand absorption patterns in alkanes, alkenes, alkynes, alcohols, and carbonyl compounds.

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Comprehensive Guide to Infrared Spectroscopy in Chemistry

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  1. The Electromagnetic Spectrum

  2. Spectroscopy • Spectroscopy is a measurement of the wavelengths of electromagnetic radiation that are absorbed by a molecule. • For absorption to occur, the energy of the radiation must match the difference between two energy states in the molecule.

  3. Infrared Spectroscopy • Frequencies are reported in wavenumbers or cm–1 ~  = 1/ • IR absorptions in organic molecules occur from 4000 cm–1 – 400 cm–1. • When IR is absorbed vibrational excitation occurs. • Organic chemists prefer to view IR spectra as a plot of the percent of transmitted light versus wavenumber.

  4. An IR Spectrum

  5. Bond Vibrations • Absorption of IR light causes excitation of the vibrational motions available of a molecule. These include bond stretching vibrations and bond bending vibrations. • The vibrational modes of a molecule are quantized, so they occur only at specific frequencies which correspond to the frequency of IR light.

  6. Frequency of Stretching Vibrations • The frequency of a bond stretching vibrations depends on the bond strength and the masses of the bonded atoms. • Stronger bonds vibrate at a higher frequency. • Bonds with lighter atoms vibrate at higher frequency.

  7. Regions of the IR Spectrum

  8. Fingerprint Region • The IR spectrum has two regions: the functional group region (at  1500 cm-1), and the fingerprint region (at < 1500 cm-1).

  9. Alkanes Hexane has only C-C single bonds and sp3 hybridized C atoms. Therefore it has only one major absorption at 3000-2850 cm-1.

  10. Alkenes 1-Hexene has a C=C and Csp2-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: Csp2-H at 3150-3000 cm-1; Csp3-H at 3000-2850 cm-1; C=C at 1650 cm-1.

  11. Alkynes 1-Hexyne has a CC and Csp-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: Csp-H at 3300 cm-1; Csp3-H at 3000-2850 cm-1; CC at 2250 cm-1.

  12. Alcohols The OH group of the alcohol shows a strong absorption at 3600-3200 cm-1.

  13. Carbonyl Compounds The C=O group in the ketone shows a strong absorption at ~1700 cm-1.

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