Ch12 - Infrared Spectroscopy and Mass Spectrometry...

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Infrared Spectroscopy and Mass Spectrometry Introduction It is fundamental for an organic chemist to be able to identify , or characterize , the new compound that he/she has just made. Sometimes this can be achieved by a chemical means , such as determining the elemental composition and molecular weight. If the compound has been made previously, it is possible to compare physical properties (boiling points / melting points, etc ) with literature values. Chemical tests can be used to determine whether certain functionalities are present or absent. But , these are not sufficient for either complex molecules or new molecules that have never been made before. Tests can be either destructive or non-destructive. (Combustion which gives elemental analysis is destructive , whereas NMR is non-destructive - you can recover your sample). Ideally, chemists want techniques that use small amounts of compounds, are non-destructive, quick and give unambiguous results. Spectroscopic techniques generally meet almost all of these requirements. The four most common are: Infrared spectroscopy (IR spectroscopy observes the vibration of bonds, and gives information about which functionalities are present). Mass Spectrometry (MS provides information concerning the mass of the molecule, and sometimes about its structure). Nuclear Magnetic Resonance Spectroscopy (NMR spectroscopy provides information about the numbers and environments of all the hydrogens (and Carbons and Fluorines) in a molecule. Probably the most important technique). cb93f1fd35e1ff111b9bdcfab99e737a0578d1aa.doc Page1
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Ultra Violet Spectroscopy (UV Spectroscopy deals with electronic transitions, and gives information mainly about multiple bonds and conjugation). The Electromagnetic Spectrum Visible, IR and UV light, microwaves and radio waves are all examples of electromagnetic radiation. They all travel at the same speed (the speed of light, 3x10 8 m/s), but differ in their wavelength and frequency. The frequency, ν , is the number of complete wavecycles to pass a fixed point in one second. (Usually in Hz, which means cps). The wavelength, λ , is the distance between any two peaks of the wave. Diagram 12-2 Wavelength and frequency are inversely proportional. λ = c/ ν where c is the speed of light. Electromagnetic waves travel as photons , which are packets of energy of zero mass. The energy of a photon is the product of its frequency and Planck's constant. E = h ν H= Planck's constant 1.58x10 -37 kcal-sec cb93f1fd35e1ff111b9bdcfab99e737a0578d1aa.doc Page2
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Under certain conditions, when a photon of correct energy strikes a molecule, it can be absorbed and this leads to reaction. This is why we write photochemical reactions involving h ν . Figure 12-1 (SLIDE)
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Ch12 - Infrared Spectroscopy and Mass Spectrometry...

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