Figure 12-3 However,if a bond is symmetrical,and has zero dipole moment,the electric field does not interact with this bond. If the symmetrical bond is stretched,the dipole moment still remains zero. the vibration produces no change in the dipole moment,there is no absorption of energy. This vibration is said to be IR inactive. To recap: In general,ifa bond has a dipole moment,its stretching frequency causes an absorption in the IR spectrum. If a bond is symmetrically substituted and has zero dipole moment,its stretching vibration will (essentially)be absent from the spectrum. Measurement of an IR spectrum An IR spectrometer measures the frequency of IR radiation which is absorbed by a compound. A simple IR spectrometer consists of two beams of light which pass through a reference and a sample cell. These beams alternately pass through a monochromatic and a detector and the information is converted to a print out on a chart recorder The output signal is equal to the sample signal minus the reference signal (i.e.it corrects for the "background"-air or solvent absorption). rotating segmented miro monochromato chart recorder Chl2 IR and MS Page7Figure 12-3 However, if a bond is symmetrical, and has zero dipole moment, the electric field does not interact with this bond. If the symmetrical bond is stretched, the dipole moment still remains zero, the vibration produces no change in the dipole moment, there is no absorption of energy. This vibration is said to be IR inactive. To recap: In general, if a bond has a dipole moment, its stretching frequency causes an absorption in the IR spectrum. If a bond is symmetrically substituted and has zero dipole moment, its stretching vibration will (essentially) be absent from the spectrum. Measurement of an IR spectrum An IR spectrometer measures the frequency of IR radiation which is absorbed by a compound. A simple IR spectrometer consists of two beams of light which pass through a reference and a sample cell. These beams alternately pass through a monochromatic and a detector and the information is converted to a print out on a chart recorder. The output signal is equal to the sample signal minus the reference signal (i.e. it corrects for the "background" - air or solvent absorption). Ch12 IR and MS Page7