It involves variety of techniques based on absorption spectroscopy.The prerequisite for a substance to be IR active depend on changes in the dipole moment. The asymmetric stretch (B) is infrared active as thereis a change in the molecular dipole moment during this vibration.Infraredradiation can not make enough energy to produceelectronic as the UV can. Absorption of IR is limited to compounds with small energy differences inthe possible vibrational and rotational states.If the molecule absorb IR it depends on the vibration and rotations with a molecula that will change a net in the dipole moment of the molecule. Once when the frequency of radiation is same as the vibrational frequency of the molecule the radiation will be absorbed and make a change in the amplitude of molecular vibration.The air absorption inside of the infrared spectrometer is measured. The measured value is later then subtracted from the result to determine value of the sample. Units used in the infrared spectroscopy are called wavenumbers. An IR spectrometer find out how the absorption of a sample differs with wavenumber, cm-1, which is the reciprocal of the wavelength in cm (1/wavelength). The wavenumber is proportional to theenergy or frequency of the vibration of the bonds in the molecule.
Each bond in molecules is different and each of them will vibrate different and absorb infrared radiation in different wavelengths. This helps determine chemical structure of the molecule as the different molecules with different structures vibrate different. Once when these molecules absorb infrared radiation their bonds start stretch, bend and contract. Also to be infrared radiation absorbed by molecule, molecule has to vibrate while when the change of dipole moment occur.
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