![]() This radiation gets absorbed and reflected back to earth by atmospheric green-house gases such as carbon dioxide, water vapor and methane, keeping the Earth warm. The Earth then emits them back to the atmosphere as longer wavelength infrared waves. The Earth absorbs some of the energy from these waves warming it up. Out of this radiation, short-wavelength ultraviolet, visible, and the shorter wavelengths of infrared radiation get transmitted to the Earth. The Sun emits different forms of radiation. Infrared Radiation and Global warming Greenhouse Effect The fingerprint region is unique to each molecule, and can be used to identify a molecule by comparing it with known samples in databases. It represents the wavenumbers absorbed by the C-C and C-O bonds as well as the vibrations of the whole molecule. The “fingerprint region” is unique for all molecules. There is no clear trough at 3300 to 3500 cm-1 so there is no N-H bond. Infrared (IR) SpectroscopyĪnswer: There is no trough at 2220-2260 cm-1, so it has no C≡N bond, so it is not a nitrile. Worked example: Identify whether this is the spectrum of a primary amine, tertiary amine or a nitrile. There are separate -OH and C=O bonds in the molecule, so it is not a carboxylic acid. (2 marks) Infrared (IR) SpectroscopyĪnswer: Signal at 3230-3350 shows an O-H (alcohol) (1) Worked example: Identify the functional groups in the molecule C6H12O2, using the infrared spectrum given below. Worked example: Which bond does this trough represent? Infrared (IR) SpectroscopyĪnswer: This transmittance is produced by an O-H bond in a carboxylic acid. C=O bond – this is a strong, narrow trough at 1680-1750 cm-1 in aldehydes, ketones, carboxylic acids and esters.O-H in acids – produce a strong, broad trough as a wavenumber 2500-3000 cm-1.O-H in alcohols – produce a strong, broad trough at a wavenumber 3230-3550 cm-1.To interpret the IR spectrum of a given molecule, you need to look carefully at the wavenumber of the troughs and their shapes. Infrared spectroscopy can also be used to identify impurities in a sample. This information can be used to identify the functional groups present in the molecule. It shows you what radiation frequencies the organic molecule under investigation absorbs and hence the type of bonds present. The IR spectrum is a measure of the % transmittance of IR against the wavenumber. The table shows the infrared wavenumbers for different bond types: This table will be given to you on your exam data sheet. These frequencies are measured in wavenumbers, where: ![]() When chemical bonds absorb infrared radiation their kinetic energy increases and they vibrate faster.ĭifferent chemical bonds absorb different frequencies of infrared radiation. When infrared radiation is passed through a compound, some of the radiation is absorbed by the bonds within the molecule, and some is transmitted. Infrared waves are part of the electromagnetic spectrum. Organic Analysis - Infrared (IR) Spectroscopy (A-Level Chemistry) Infrared Spectroscopy
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