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Unformatted text preview: 12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy Based on McMurry's Organic Chemistry, 7th edition Determining the Structure of an Organic Compound The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants In the 19th and early 20th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other Physical methods now permit structures to be determined directly. We will examine: mass spectrometry (MS) infrared (IR) spectroscopy nuclear magnetic resonance spectroscopy (NMR) ultraviolet-visible spectroscopy (VIS) 2 Why this Chapter? Finding structures of new molecules synthesized is critical To get a good idea of the range of structural techniques available and how they should be used 3 12.1 Mass Spectrometry of Small Molecules:Magnetic-Sector Instruments Measures molecular weight Sample vaporized and subjected to bombardment by electrons that remove an electron Creates a cation radical Bonds in cation radicals begin to break (fragment) Charge to mass ratio is measured 4 Mass Spectrometer p. 409 5 MAGNETIC SECTOR INSTRUMENT Fig. 12-1, p. 410 6 Mass Spectrometers Time of Flight MS Triple Quadrapole MS ~$70,000 ~$250,000 7 The Mass Spectrum Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (roughly corresponding to the number of ions) (yaxis) Tallest peak is base peak (100%) Other peaks listed as the % of that peak Peak that corresponds to the unfragmented radical cation is parent peak or molecular ion (M+) BASE ION PEAK (B+) = tallest peak in spectrum FRAGMENT ION PEAK (F+) MOLECULAR ION (M) MOLECULAR ION +1 (M+1) 8 12.2 Interpreting Mass Spectra Molecular weight fr...
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