Infrared Spectroscopy: Fingerprint region The functional groups of the molecule can be found in the region between 4000 cm- cm-1 of the infrared spectra. The infrared spectroscopy data table is used to match the peaks in the spectrum with the functional groups that could have caused them to occur. The peaks in the IR spectra point downwards.īy analyzing the peaks in the IR spectrum, we can identify the functional groups in the molecule. Transmittance measures the percentage of radiation that passes through a sample, while the wavenumber is a measure of frequency that is inversely proportional to the wavelength. These peaks represent the vibrations caused when the molecule absorbs infrared radiation. The spectrum consists of dips in transmittance at certain wavelengths, which are referred to as peaks. In the infrared spectrum, transmittance is plotted along the y-axis, while the wavenumber is plotted on the x-axis. We can see an example below.Įxample of an infrared spectrum of a molecule Infrared Spectroscopy: Identifying organic moleculesĪn infrared spectrum of a molecule is a graph that is produced once the process of infrared spectroscopy has been performed. The amount of vibration caused depends on three primary factors: bond strength (stronger bonds vibrate at a higher frequency), mass of the atom (heavier atoms vibrate at a lower frequency), and bond length. Every molecule vibrates at a specific frequency, and each bond within the molecule has its own unique natural vibration frequency. This causes the covalent bonds in the molecule to stretch, bend, or twist. When organic molecules absorb infrared radiation, the bonds between the atoms vibrate even more. Think of a pair of atoms as constantly vibrating. Infrared Spectroscopy: Vibration of organic compounds This infrared radiation is absorbed by bonds between the molecules at different wavelengths. All organic compounds absorb infrared radiation. Infrared spectroscopy of organic compounds. This is the basis to distinguish functional groups with infrared spectroscopy. From the table, we also know that bonds in different functional groups absorb different frequencies of infrared radiation. 'Wavenumber' represents the number of waves in a given wavelength or distance. 'Bond' represents the functional groups of different organic compounds. Infrared spectroscopy tableĪs we can see in the image below, the table has two columns. This spectrum shows how much radiation is absorbed by the chemical at different frequencies. The sample absorbs the radiation, and the absorptions are then analyzed and printed or displayed on a computer as an absorption spectrum. The process of IR spectroscopy involves passing a beam of radiation through a sample in a spectrometer. There are two types of spectrometers used: dispersive and Fourier transform. Infrared spectroscopy is a tool used to identify functional groups in organic molecules. In this blog, we will discuss IR and how it works, show you the IR table, explore IR of organic compounds, and highlight its pros and cons. This technique works because each bond has a unique vibration frequency that we can differentiate. But how do we know which functional groups are in a molecule? We use infrared spectroscopy (IR) to identify them. Organic molecules have various functional groups such as alcohols, amines, alkanes, and alkenes.
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