lecture 12 - 12. Structure Determination: Mass Spectrometry...

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12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy Based on McMurry’s Organic Chemistry , 7 th edition
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2 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 19 th and early 20 th 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)
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3 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
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4 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
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5 The Mass Spectrum Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (roughly corresponding to the number of ions) (y-axis) 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 + )
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6 12.2 Interpreting Mass Spectra Molecular weight from the mass of the molecular ion Double-focusing instruments provide high-resolution “exact mass” 0.0001 atomic mass units – distinguishing specific atoms Example MW “72” is ambiguous: C
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lecture 12 - 12. Structure Determination: Mass Spectrometry...

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