=C-H (sp 2 C-H) bond of alkene give absorption at about 3000-3100 cm -1.≡C-H (sp C-H) bond of terminal alkyne give absorption at about 3300 cm -1.The C-H bond stretching of all hydrocarbons occur in the range of 2800-3300 cm -1, and the exact location can be used to distinguish between alkane, alkene and alkyne. The polarity of N-H bond (in amine and amide) is weaker than OH bond, so the absorption band of N-H is not as intense, nor that broad as O-H, and the position is in 3300-3500 cm -1 region. The OH bond of alcohol group usually has absorption in the range of 3200-3600 cm -1, while the OH bond of carboxylic acid group occurs at about 2500-3300 cm -1 ( Figure 6.4a and Figure 6.4c). The broad shape of the absorption band results from the hydrogen bonding of the OH groups between molecules. The polar O-H bond (in alcohol and carboxylic acid) usually shows strong and broad absorption bands that are easy to be identified. The larger wavenumbers (shorter wavelengths) are associated with higher frequencies and higher energy. 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 spectra chart