Ch_322a_2.015

Ch_322a_2.015 - Infrared Spectroscopy: An Instrumental...

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Infrared Spectroscopy: An Instrumental Method for Detecting Functional Groups Almost all organic molecules are "infrared active," meaning they absorb radiation in the infrared region. These absorptions are associated with transitions from lower to higher vibrational levels within molecules. The energy increase due to absorption of an infrared photon is typically 1 - 10 kcal/mol. ! E = h " lower vibrational state higher vibrational state The energy gap between the states depends on the atoms present, the bond strength, and the details of the molecular change.
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Spectral Presentation: A Preview Infrared spectra are recorded by passing a broad spectral range of the radiation through a sample, and noting the wavelengths that are absorbed. The pattern of wavelengths absorbed provides structural information on the organic compound. μ m (10 -4 cm) wavelength 4000 3000 2000 1000 wavenumber, cm -1 ! 2.5 3 4 5 6 7 10 15 transmittance (%) 100 50 0 absorbance .1 .4 .8 8 bands indicate absorption of ir radiation In addition to the spectral wavelength presented in micrometer or micron units (10 -4 cm or 10 -6 m), the spectrum is displayed in wavenumber units ( ! ). This unit is linear with frequency with ! = 1/ " (with " in cm) = ! /c.
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A Mechanical Model: Hooke's Law Two atoms connected by a covalent bond behave like two weights on the ends of a spring . When the weights are displaced from their rest position and released, they vibrate only with
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Ch_322a_2.015 - Infrared Spectroscopy: An Instrumental...

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