Summary 1 - IR UV

Summary 1 - IR UV - C-H bond! 5) The INTENSITY (i.e. how...

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Summary of Important Concepts: IR and UV Spectroscopy IR Spectroscopy: 1) Infrared Spectroscopy detects TYPES OF BONDS and FUNCTIONAL GROUPS, e.g. carbonyl groups C=O, alcohols O-H and amines N-H. 2) Different bonds absorb different frequencies of light – the spectrum shows absorbances at different frequencies. Units are wavenumbers (cm -1 ). A usual spectrum has a scale of 4000 – 600 cm -1 . 3) There are FOUR main regions of the spectrum to remember: ~4000 – 2500 cm -1 : X-H bond stretches (X is NOT an element – it means O, C or N!) ~2500-2000 cm -1 : Triple bonds, e.g. C≡C, C≡N ~1800-1500 cm -1 : Double bonds, e.g. C=C, C=O, C=N <1500 cm -1 is the fingerprint region. Contains SINGLE bonds NOT to H (i.e. C-C, C-O, C-N). 4) The FREQUENCY is dependent on BOND STRENGTH – the stronger the bond, the higher the frequency (in each region). Double bonds are stronger than single bonds! An sp 2 C-H bond is stronger than an sp 3
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Unformatted text preview: C-H bond! 5) The INTENSITY (i.e. how far the peak points downwards) is proportional to the dipole moment of the bond. C-C has a small dipole so is a small peak; C=O is a strong peak due to the large dipole. Two molecules with weak IR stretches: 6) See page 425 for a more detailed frequencies table. 7) Broad peaks are due to hydrogen bonding and show the presence of O-H or N-H. Carboxylic acids (COOH) are very broad. UV Spectroscopy: 1) A Conjugated π system (multiple double bonds separated by a single bond) is more stable than isolated π bonds due to overlap of p orbitals throughout the system. There can be no sp 3 centers between the double bonds: 2) Conjugated π systems can be observed by Ultraviolet (UV) Spectroscopy. UV absorptions are described by their WAVELENGTH, not frequency. Simple rule of thumb: the more conjugated double bonds, the longer the wavelength of UV absorption....
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This note was uploaded on 02/02/2012 for the course CHEM 112b taught by Professor Marsella during the Spring '11 term at UC Irvine.

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Summary 1 - IR UV - C-H bond! 5) The INTENSITY (i.e. how...

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