For example, one can distinguish O–H and N–H stretches from each other because normally O–H stretching peaks are more intense and broader than N–H stretching peaks. Different functional groups have different peak intensities and peak widths. In these cases, how can one distinguish the peaks of different functional groups from each other? The answer is that in addition to peak position information, infrared spectra contain peak heights and peak widths. For example, both O–H and N–H stretches have peaks around 3350 cm -1 (4). The peak positions in an infrared spectrum are used to distinguish different functional groups from each other.ĭifferent functional groups can have peaks at about the same position. Thus, infrared spectroscopy is a type of functional group spectroscopy, and infrared spectrometers are used to detect functional groups. The peak positions in an infrared spectrum thus disclose the vibrational energy levels of the functional groups in a molecule, and when infrared spectra are analyzed, the peaks are assigned to specific vibrations of specific functional groups. The corresponding decrease in light energy at the absorption wavenumber gives rise to a peak in the measured infrared spectrum of the molecule. When infrared light of the same energy as a vibrational transition impinges upon a molecule, the energy may be absorbed. The wavenumber of electromagnetic radiation is proportional to energy (1). The x-axes of most infrared spectra are plotted in wavenumber (cm -1) units. Examples of functional groups include the methyl group (CH 3–), the carbonyl group (C=O), and benzene rings (C 6H 6). In most cases, these vibrations are localized to specific portions of a molecule called functional groups. The absorbed infrared energy causes the molecule's bonds to stretch and bend.
When a molecule absorbs infrared light, it undergoes a spectroscopic transition from a lower to an upper vibrational energy level, as seen in Figure 1.įigure 1: A spectroscopic transition occurs when infrared light is absorbed by a molecule and it is excited from a lower to an upper vibrational energy level. Infrared (IR) spectroscopy is widely used to determine the structures of unknown molecules, give a fingerprint of a sample, and measure the concentrations of molecules in samples (1–4). This first installment will present why this type of column is important, discuss some basic IR theory, and lay out a blueprint for future installments.
IR PEAKS CHART HOW TO
This new column will provide practical advice about how to do this. There is a continuing need for Fourier transform infrared (FT-IR) users to receive training in how to interpret the infrared spectra they measure.
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