Please note the direction of the horizontal axis (wavenumber) in IR spectra decrease from left to right. The wavenumber is defined as the reciprocal of wavelength ( Formula 6.3), and the wavenumbers of infrared radiation are normally in the range of 4000 cm -1 to 600 cm -1 (approximate corresponds the wavelength range of 2.5 μm to 17 μm of IR radiation). But instead of using frequency to show the absorbed radiation, wavenumbers (, in unit of cm -1) are used as a conventional way in IR spectra. The horizontal axis indicates the position of an absorption band.A molecule have a variety of covalent bonds, and each bond have different vibration modes, so the IR spectrum of a compound usually show multiple absorption bands. The spikes are called absorption bands in an IR spectrum. Lower values of % transmittance mean that some of the energy is absorbed by the compound, and gives the downward spikes. At the high end of the axis, 100% transmittance means no absorption occurred at that frequency. The solid line traces the values of % transmittance for every wavelength passed through the sample. The vertical axis is ‘% transmittance’, which tells how strongly light was absorbed at each frequency.With the basic understanding of the IR theory, we will take a look at the actual output from IR spectroscopy experiments, and learn how to get structural information from IR spectrum. Below is the IR spectrum for 2-hexanone. 6.3 IR Spectrum and Characteristic Absorption Bands

Ir functional groups table