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Chapters 13 & 14 - CHAPTER 13 14 Mass Spectrometry...

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CHAPTER 13 & 14 Mass Spectrometry, Infrared Spectroscopy, and NMR Spectroscopy Chapter 13: Mass Spectrometry & Infrared Spectroscopy A. Mass Spectrometry 13.1 Introduction Provides information about the molecular formula (MF) and structure of a compound. A mass spectrometer is designed to do three things. i. Convert neutral atoms or molecules to positive ions. ii. Separate the ions on the basis of their mass-to-charge (m/z) ratio. iii. Measure the relative abundance of each type of ion. From the above information, the molecular weight and the MF of an unknown compound can be determined Low-resolution mass spectrometry: distinguishes between ions that differ by 1 amu. High-resolution mass spectrometry: distinguishes between ions that differ by 0.0001 amu. This gives the MF of a compound. 13.2 Mass Spectrum A mass spectrum is a plot of the relative abundance of each ion versus mass-to- charge ratio. Base Peak: This is the most abundant peak that is assigned an arbitrary intensity of 100. It is the most intense peak. The relative abundance of all other ions is reported as a % of abundance of the base peak. Molecular Ion (M): The species formed by the loss of a single electron from a molecule. Done by Dr. Felix N. Ngassa for CHM 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 1
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With the exception of isotope peaks at M + 1 and M + 2, the molecular ion has the highest mass in a mass spectrum. The molecular formula of the parent molecule is written in brackets with a plus sign to show that it is a cation and a dot to show that it has an odd number of electrons. A full interpretation of a mass spectrum is difficult and time consuming. A. Organic Halides (M + 2 Peaks): The only elements to give significant M + 2 peaks are Cl and Br. If no significant M + 2 peak is present, these elements are absent. Organic chlorides show two peaks for the molecular ion (M and M + 2) in a 3:1 ratio. Organic bromides show two peaks for the molecular ion (M and M + 2) in a 1:1 ratio. B. Nitrogen Rule: If a compound has an odd number of nitrogen atoms, its molecular ion will appear at an odd m/z value. If a compound has zero or an even number of nitrogen atoms, its molecular ion will appear at an even m/z value. C. Index of Hydrogen Deficiency (Unsaturation Number): From the MF of a compound, the number of rings and/or π bonds can be determined. The unsaturation number is calculated by the formula: U = C + 1 -1/2X + 1/2Y.
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