Topic 7

• Molecular ion


  The peak with the largest m/z, due to the complete molecule, equal to the Mr of the molecule
• Molecular ion formation


  Molecule loses an electron and becomes both an ion and a free radical
• Fragmentation


  Molecular ion fragments due to covalent bonds breaking, producing an ion and a free radical
• Stable ions


  • Carbocations R+ and acylium ions [R-C=O]+ are common, the more stable the ion the greater the peak intensity
• Fragmentation examples


  • [CH3CH2CHO]+. → [CHO]+ + .CH2CH3

  [CH3COCH3]+. → [CH3CO]+ + .CH3
• It is not possible for propanone to fragment to give a peak at 29, so the fragmentation patterns can distinguish between the structural isomers of propanone and propanal
• Infrared spectroscopy


  Certain bonds in a molecule absorb infrared radiation at characteristic frequencies causing the covalent bonds to vibrate
• Bonds that absorb infrared radiation

  • C-O

  C=O

  1. H
  2. H (carboxylic acids)
  3. H
  4. H (alcohols, phenols)
• Molecules which change their polarity as they vibrate can absorb infrared radiation, e.g. C-H, C=O, O-H
• Molecules such as H2, O2 and N2 cannot change their polarity as they vibrate so can't absorb infrared radiation
• The absorption of infrared radiation by bonds in this type of spectroscopy is the same absorption that bonds in CO2, methane and water vapour in the atmosphere do that cause them to be greenhouse gases
• 1. H absorptions tend to be broad
• Absorption or trough in between 1640-1750 cm-1 range


  Indicates presence of C=O bond
• Absorption or trough in between 2500-3300 cm-1 range


  Indicates presence of O-H bond in an acid
• What is infrared spectroscopy?

  Used infrared radiation to increase the vibrational energy of covalent bonds in a sample.
• Why do you think o2 and N2 are not greenhouse gases but co2, h20 and ch4 are?

  The polar compounds or polar bonds will absorb will absorb the IR radiation With right frequency. The covalent bonds in molecules of complex ions together are not static, they vibrate.
• How to calculate wave number?

  1/ wavelength
• What are the two regions in IR spectrum?

  Fingerprint region, functional group region
• What are wave numbers?

  Are used as a measure of the wavelength or frequency of the absorption.
• What is transmittance?

  Is shown on the vertical axis which is shown as a percentage of radiation transmitted.
• What is the fingerprint region?


  Region that is unique to the molecule. Less than 1500
• What is the functional group region?


  Region used to identify functional groups. More than 1500
• Mass spectrometry


  An analytical technique used to identify different isotopes and find the overall relative atomic mass of an element

  
• Time of Flight (TOF) Mass Spectrometry

  1. Vapourise sample

  2. Ionise atoms

  3. Accelerate ions

  4. Deflect ions by magnetic field

  5. Detect ions

  
• Time of Flight (TOF) Mass Spectrometry


  • Records the time it takes for ions of each isotope to reach a detector
  • Produces spectra showing each isotope present
• 2+ charged ion


  • Affected more by the magnetic field producing a curved path of smaller radius
  • Mass to charge ratio (m/z) is halved

  
• Calculating relative atomic mass (Ar)

  1. Multiply each m/z value by its abundance

  2. Add each of these together

  3. Divide by the total abundance of all species present

  
• Using the calculated Ar value, the element can be identified by referring to the Periodic Table

  
• Predicting Mass Spectra


  1. Know the abundances of isotopes

  2. Generate mass spectra including relative peak heights

  
• Chlorine mass spectra


  • Characteristic pattern in a 3:1 ratio for Cl+ ions
  • Characteristic pattern in a 3:6:9 ratio for Cl2+ ions

  
• Infrared (IR) spectrometry


  Analytical technique that uses infrared radiation to determine the functional groups present in organic compounds
• Infrared (IR) spectrometry


  • Different bonds absorb the radiation in different amounts
  • Varying amounts of absorbance are measured and recorded
  • Allows certain bonds, and thus functional groups, to be identified
• Functional group peaks in IR spectra

  • -O-H alcohol group: 3230 - 3550 cm-1
  • -O-H acid group: 2500 - 3300 cm-1
  • C=C unsaturated group: 1620 - 1680 cm-1
  • C=O carbonyl group: 1680 - 1750 cm-1

  
• Fingerprint region


  • Unique for each species, containing tiny differences between each species
  • Acts as a molecule's 'fingerprint', allowing it to be identified