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1、13.21Mass SpectrometryAtom or molecule is hit by high-energy electronPrinciples of Electron-Impact Mass SpectrometryeAtom or molecule is hit by high-energy electronelectron is deflected but transfers much of its energy to the moleculeePrinciples of Electron-Impact Mass SpectrometryAtom or molecule i

2、s hit by high-energy electronelectron is deflected but transfers much of its energy to the moleculeePrinciples of Electron-Impact Mass SpectrometryThis energy-rich species ejects an electron.Principles of Electron-Impact Mass SpectrometryThis energy-rich species ejects an electron.Principles of Elec

3、tron-Impact Mass Spectrometryforming a positively charged, odd-electron species called the molecular ione+Molecular ion passes between poles of a magnet and is deflected by magnetic fieldamount of deflection depends on mass-to-charge ratiohighest m/z deflected leastlowest m/z deflected mostPrinciple

4、s of Electron-Impact Mass Spectrometry+Principles of Electron-Impact Mass SpectrometryIf the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose.However, the molecular ion often fragments to

5、a mixture of species of lower m/z.The molecular ion dissociates to a cationand a radical.Principles of Electron-Impact Mass Spectrometry+The molecular ion dissociates to a cationand a radical.Principles of Electron-Impact Mass Spectrometry+Usually several fragmentation pathways are available and a m

6、ixture of ions is produced.mixture of ions of different mass gives separate peak for each m/zintensity of peak proportional to percentage of each ion of different mass in mixtureseparation of peaks depends on relative massPrinciples of Electron-Impact Mass Spectrometry+mixture of ions of different m

7、ass gives separate peak for each m/zintensity of peak proportional to percentage of each atom of different mass in mixtureseparation of peaks depends on relative mass+Principles of Electron-Impact Mass Spectrometry20406080100 120m/zm/z = 78100806040200Relative intensitySome molecules undergo very li

8、ttle fragmentationBenzene is an example. The major peak corresponds to the molecular ion.HHHHHHHHHHHHHHHHHHall H are 1H and all C are 12Cone C is 13Cone H is 2H Isotopic Clusters78797993.4%6.5%0.1%20406080100 120m/z100806040200Relative intensity112114Isotopic Clustersin Chlorobenzenevisible in peaks

9、 for molecular ion35Cl37Cl20406080100 120m/zRelative intensity77Isotopic Clustersin Chlorobenzeneno m/z 77, 79 pair; therefore ion responsible form/z 77 peak does not contain ClHHHHH+100806040200Alkanes undergo extensive fragmentationm/zDecane1424357718599CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3Relative inten

10、sity10080604020020406080100 120Propylbenzene fragments mostlyat the benzylic position20406080100 120m/zRelative intensity12091CH2CH2CH310080604020013.22Molecular Formulaas aClue to StructureMolecular WeightsCH3(CH2)5CH3HeptaneCH3COOCyclopropyl acetateMolecular formulaMolecular weightC7H16C5H8O210010

11、0Exact mass100.1253100.0524Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas.Molecular FormulasKnowing that the molecular formula of a substance is C7H16 tells us immediately that is an alkane because it corresponds to CnH2n+2C7H14 lacks two hydroge

12、ns of an alkane, therefore contains either a ring or a double bond Index of Hydrogen Deficiencyrelates molecular formulas to multiple bonds and ringsindex of hydrogen deficiency = 12(molecular formula of alkane molecular formula of compound)Example 1index of hydrogen deficiency 12(molecular formula

13、of alkane molecular formula of compound)C7H1412(C7H16 C7H14)=12(2) = 1=Therefore, one ring or one double bond.Example 2C7H1212(C7H16 C7H12)=12(4) = 2=Therefore, two rings, one triple bond,two double bonds, or one double bond + one ring.Oxygen has no effectCH3(CH2)5CH2OH (1-heptanol, C7H16O) has same number of H atoms as heptaneindex of hydrogen deficiency = 12(C7H16 C7H16O) = 0no rings or double bondsOxygen has no effectindex of hydrogen deficiency = 12(C5H12 C5H8O2) = 2one ring plus one double bondCH3COOCyclopropyl acetateIf halogen is presentTreat a halogen as if it were hydrogen.CCCH3Cl