# Infrared Spectroscopy Essay

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Infrared spectroscopy is a test used to determine the structural identities of unknown chemical compounds. It can determine the identity and relative purity of compounds by measuring how much the bonds between the atoms in the compound “bend” or “stretch”. Infrared spectroscopy works based on the idea that the bonds between atoms can absorb energy and that when they do, they act similarly to springs in that they can be stretched or compressed, and can be made to bend or “wag”. IR spectroscopy can give the identity of functional groups and atoms in a compound by analyzing the frequency of these bond movements. The pictures below give an example of these motions. The infrared spectrometer can cause these motions/vibrations to occur in the bonds by hitting the compound in question with light in the infrared range of the electromagnetic spectrum. Vibrations in bonds occur between 8 x 10-5 cm and 1 x 10-2 cm, and the typical IR spectrometer operates in frequencies between 5 x 10-4 cm and 25 x 10-4 cm or 4000 cm-1 to 400cm-1. Frequency is reported as either wavenumber (denoted by &#957;&#771;) or cm-1. &#957;&#771;= 1/ &#955;, or wavenumber is equal to one over wavelength. The energy absorbed by the molecule from the spectrometer can be calculated by the equation E=hc/ &#955; or, because of the relationship between wavenumber and wavelength mentioned above, E=hc&#957;&#771;. Using this information we can use Hooke’s law to calculate where a bond stretch (vibration) might occur. v= 1/2&#960;c &#8730;((f(m_1+m_2))/(m_1 m_2 )) &#957;&#771; is the vibrational frequency in cm-1 (wave number), c is the velocity of light in cm sec-1, m1 is the mass of atom 1 in g, m2 is the mass of atom 2 in g, and f is the force constant of the bond in dyne cm-1 (g sec -1). The force constant is a necessary addition to the equation because