NMR and IR.pdf

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Cecilia Kutas

July 34 2012 CHM247H1Identification of Organic Compounds NMR and IR SpectroscopyELECTROMAGNETIC RADIATIONBoth NMR Nuclear Magnetic Resonance and IR infrared spectroscopy rely on electromagnetic radiation IR spectroscopy stretching and bending energies of bonds in specific functional groups Used for NMR Use detectors to figure out wavelengthswhat the absorbance of those wavelengths are etcElectromagnetic radiation acts as both a particle and a wave propagates as a wave in discrete quantities of energy quanta o As a particle one unit is a photonA photon contains a discrete amount of energy called a quantum o As a wave characteristic lengthmeasured cresttocrest or troughtotrough and frequencyreported in 1cycles per second s also called hertz HzFrequencyhow many waves will pass a given point per secondBoth the wavelength and frequencyandof a wave tell us its energy Ehhc8o c30x10ms speed of light 3434o hPlancks constant 158x10cals or 6626x10Js You arent responsible for these constants in this course o wavelength of radiationMOLECULAR VIBRATIONS Stretching energiesbending energies 15 Roughly 10 vibrationssecCX bonds usually absorb in the fingerprint region so we dont usually look at them for identifyingdetermining the structures of moleculesCHM247H1TYPICAL IR SPECTRUMEhhc Different functional groups vibrate at different energies resonate allowing identification of functional groups in a molecule using infrared irradiation We generally care most about the 15003500 range to the right of 1500cm1 is the fingerprint region which has many different absorbances making it difficult to determine exactly what is absorbing and how much so we do not generally analyze itDifferent signals are possiblewe can get broad deep wide intense medium slight etcFOUR DISTINCT REGIONS OF AN IR SPECTRUM Usually weak1600 not too intense not often the first thing1700 very distinct we look for CARBONYL GROUPSArguably the most important functional group in organic chemistry though in biochemistry it definitely isOO Need at least one Hattached to the COHRRRKetoneAldehydeOOO Amides are the most stableAlkoxide group Amine bonds in our physiology goodthing because thats what holds OROHRNHRRR our proteins together literallykeeping us from falling apart AcidEsterAmide
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