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CHM247H1 (26)
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Chapter 12

Ch.12 Summary.docx

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University of Toronto St. George
Barb Morra

CHM 247 CH.12 Structure Determination: Mass Spectrometry and Infrared Spectroscopy 12.6 Infrared Spectroscopy • Infrared (IR) spectroscopy: A kind of optical spectroscopy that uses infrared energy. IR spectroscopy is particularly useful in organic chemistry for determining the kinds of functional groups present in a molecule. -7 -4 • Range of 7.8x10 m to approximately 10 m. Only a portion is used by chemists. • Wavenumber (ṽ): The reciprocal of the wavelength in centimetres. -1 • Wavenumber: ṽ(cm ) = 1 / λ(cm) • The useful IR region is 4000 – 400 cm . Corresponds to energies of 48.0 kJ/mol to 4.80 kJ/mol. • All molecules have a certain amount of energy and are in constant motion, with their bonds stretching and contracting constantly. • Amount of energy a molecule contains is quantized (non-variable).Amolecule can stretch or bend at specific frequencies corresponding to specific energy levels. When a molecule is irradiated with electromagnetic radiation, energy is absorbed if the frequency of the radiation matches the frequency of the vibration. This results in an increased amplitude for the vibration. • We can find what kind of motions a molecule has by examining an IR spectrum since each frequency absorbed by a molecule corresponds to a specific motion. We can then determine the kinds of bonds (functional groups) present by examining the IR spectrum. 12.7 Interpreting Infrared Spectra • IR spectrums can be difficult to interpret; therefore they are often used for pure samples of fairly small molecules. Complex molecules IR spectrum can used because an IR spectrum -1 a unique fingerpr-1t for that molecule. Complex region (fingerprint region) is 1500cm to around 400cm . • Most compounds have IR spectrums that don’t change from one compound to another. • Helps to divide the region on an -1 spectrum into 4 parts: - Region from 4000-2500cm which corresponds to absorptions caused by N – H, C – H, and O – H single bonds stretching motions. N/O – H absorb around 3300 - 3600cm ; C – H stretching occurs near 3000 cm . -1 -1 - 2500-2000 cm : T-1ple bond stretching occurs here (C≡N, C≡C). - 2000-1500 cm : Double bonds (C=O, C=N, and C=C). Carbonyl groups around 1680-1750 cm .Alkene stretching around 1640-1
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