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1、13.21Mass Spectrometry,Atom or molecule is hit by high-energy electron,Principles of Electron-Impact Mass Spectrometry,e,Atom or molecule is hit by high-energy electron,electron is deflected but transfers much of its energy to the molecule,e,Principles of Electron-Impact Mass Spectrometry,Atom or mo
2、lecule is hit by high-energy electron,electron is deflected but transfers much of its energy to the molecule,e,Principles of Electron-Impact Mass Spectrometry,This energy-rich species ejects an electron.,Principles of Electron-Impact Mass Spectrometry,This energy-rich species ejects an electron.,Pri
3、nciples of Electron-Impact Mass Spectrometry,forming a positively charged, odd-electron species called the molecular ion,e,+,Molecular ion passes between poles of a magnet and is deflected by magnetic field,amount of deflection depends on mass-to-charge ratio highest m/z deflected least lowest m/z d
4、eflected most,Principles of Electron-Impact Mass Spectrometry,Principles of Electron-Impact Mass Spectrometry,If 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 molecula
5、r ion often fragments to 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
6、pathways are available and a mixture of ions is produced.,mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass in mixture separation of peaks depends on relative mass,Principles of Electron-Impact Mass Spectrome
7、try,+,+,+,+,+,+,mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each atom of different mass in mixture separation of peaks depends on relative mass,+,+,+,+,+,+,Principles of Electron-Impact Mass Spectrometry,20406080100 120,m/z,m/z =
8、 78,Relative intensity,Some molecules undergo very little fragmentation,Benzene is an example. The major peak corresponds to the molecular ion.,all H are 1H and all C are 12C,one C is 13C,one H is 2H,Isotopic Clusters,78,79,79,93.4%,6.5%,0.1%,20406080100 120,m/z,Relative intensity,112,114,Isotopic C
9、lustersin Chlorobenzene,visible in peaks for molecular ion,20406080100 120,m/z,Relative intensity,77,Isotopic Clustersin Chlorobenzene,no m/z 77, 79 pair; therefore ion responsible form/z 77 peak does not contain Cl,Alkanes undergo extensive fragmentation,m/z,Decane,142,43,57,71,85,99,CH3CH2CH2CH2CH
10、2CH2CH2CH2CH2CH3,Relative intensity,20406080100 120,Propylbenzene fragments mostlyat the benzylic position,20406080100 120,m/z,Relative intensity,120,91,13.22Molecular Formulaas aClue to Structure,Molecular Weights,CH3(CH2)5CH3,Heptane,Cyclopropyl acetate,Molecular formula,Molecular weight,C7H16,C5H
11、8O2,100,100,Exact mass,100.1253,100.0524,Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas.,Molecular Formulas,Knowing that the molecular formula of a substance is C7H16 tells us immediately that is an alkane because it corresponds to CnH2n+2 C7H14
12、lacks two hydrogens of an alkane, therefore contains either a ring or a double bond,Index of Hydrogen Deficiency,relates molecular formulas to multiple bonds and rings,index of hydrogen deficiency =,1,2,(molecular formula of alkane molecular formula of compound),Example 1,index of hydrogen deficienc
13、y,(molecular formula of alkane molecular formula of compound),C7H14,(C7H16 C7H14),=,=,(2) = 1,=,Therefore, one ring or one double bond.,Example 2,C7H12,(C7H16 C7H12),=,(4) = 2,=,Therefore, two rings, one triple bond,two double bonds, or one double bond + one ring.,Oxygen has no effect,CH3(CH2)5CH2OH
14、 (1-heptanol, C7H16O) has same number of H atoms as heptane,index of hydrogen deficiency =,1,2,(C7H16 C7H16O),= 0,no rings or double bonds,Oxygen has no effect,index of hydrogen deficiency =,1,2,(C5H12 C5H8O2),= 2,one ring plus one double bond,Cyclopropyl acetate,If halogen is present,Treat a halogen as if it were hydrogen.,CH3,Cl,H,H,C3H5Cl,same index of hydrogendeficiency as for C3H6,Rings versus Multiple Bonds,Index of hydrogen deficiency tells us the sum ofrings plus multiple bonds. Catalytic hydrogenation tells us how many multiple bonds there are.,
