Mass Spectrometry and Infrared Spectrometry
Mass Spectrometry and Infrared Spectrometry Chem 372
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This 2 page Class Notes was uploaded by Rachel Taylor on Wednesday January 20, 2016. The Class Notes belongs to Chem 372 at Eastern Michigan University taught by Dr. Friebe in Winter 2016. Since its upload, it has received 23 views. For similar materials see Organic Chemistry in Chemistry at Eastern Michigan University.
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Date Created: 01/20/16
Mass Spectrometry and Infrared Spectrometry Mass spectrometry: Determines molecular weight and formula of a compound Ionizes molecules with high energy electrons forming radical cations. Carbocations are highly unstable so they decompose into smaller fragments. Mass spectrometry measures relative abundance of fragments. Base peak is the fragment that is in the highest abundance. Usually the most stable carbocation fragment. The M peak is the peak with the same mass as the molecular ion (and molecule since electrons have negligible mass). The M+1 peak is immediately to the right of the M peak, and it accounts for carbon 13. The M peaks exist at or to the right of the base peak. Identifying atoms on the mass spectrum Nitrogen: An odd number of nitrogen atoms lead to an odd number of hydrogen atoms, leading to an odd mass of the M peak. Chlorine: there are two common isotopes that occur in a 3:1 ratio. The M+2 peak should be ¼ the size of the M peak. Bromine: there are two common isotopes that occur in roughly a 1:1 ratio, so the M peak and the M+2 peak should be roughly the same size. Loss of CH3: the carbocation formed always has a mass 15 units less than the molecular ion. Carbonyls: cleave off one R group Alcohols: cleave one R group or dehydrate. Loss of H2O always has 18 units less than molecular ion High resolution: Uses mass numbers to the 4 decimal allowing you to distinguish between molecules with similar masses. Infrared Spectrometry: Uses electromagnetic radiation to identify functional groups by measuring their unique absorption wavenumbers Wavenumber is the inverse of wavelength. IR Spectrometry identifies bond wavenumbers between 4000 cm and 400 cm , however, the fingerprint region between 1500 and 400 measures the single bonds, and is difficult to obtain useful information. 4000-2500 cm -1 2500-2000 cm -1 2000-1500 cm -1 1500-400 cm -1 H-bonds Triple bonds Double bonds Single bonds C-H CΞC C=C C-C O-H CΞN C=O C-O N-H C=N C-N C-X Table 1: IR spectrum functional groups by region The table below shows the relative wavenumbers that the different functional groups appear. Most professors will provide this on an exam, however, it is important to be familiar with what each of the functional groups look like on an IR spectrum. OH 3600-3200 Strong, broad NH 3500-3200 Medium 3 CspH 3000-2850 Strong CspH 3150-3000 Medium CspH 3300 Medium CΞC 2250 Medium CΞN 2250 Medium C=O 1800-1650 Strong C=C 1650 Medium 1600,1500 Medium Table 2: Functional groups, their relative wavenumber, and their peak intensity
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