Spectroscopy

• Electromagnetic spectrum:

  • Light is one form of electromagnetic radiation
  • Light is only a very small part of the electromagnetic spectrum
  • Electromagnetic waves consist of electric & magnetic fields which are perpendicular to each other & to the direction of travel of the wave
  • The electric & magnetic fields vibrate at the same frequency as each other
• • Atom, molecules & ions can absorb ( or emit) electromagnetic radiation of specific frequencies, and this can be used to identify them:
  • UV/ visible --> movement of electrons to higher energy levels --> UV/ visible spectroscopy
  • Infra-red --> to vibrate bonds --> Infra-red spectroscopy
  • Microwaves --> to rotate molecules --> microwave spectroscopy
  • Radio waves --> to change nuclear spin --> NMR spectroscopy
• Bond vibrations:

  • All bonds vibrate at a characteristic frequency
  • There are different types of vibration
  • The frequency depends on the mass of the atoms in the bond, the bond strength and the type of vibration
  • The frequencies at which they vibrate are in the infra-red region of the electromagnetic spectrum
• IR spectra:

  • If IR light is passed through the compound, it will absorb some or all of the light at the frequencies at which its bond vibrate
  • Wavenumbers (cm^-1) are used as a measure of the wavelength or frequency of the absorption
  • Wavenumber = 1/ wavelength(cm)
  • IR light absorbed is in the range 4000-40cm^-1
  • Above 1500cm^-1 is used to identify functional groups
  • Below 1500cm^-1 is used for fingerprinting
• Fingerprinting (below 1500cm^-1):

  • Complicated & contains many signals - picking out functional group signals difficult
  • This part of the spectrum is unique for every compound, and so can be used as a "fingerprint"
  • This region can also be used to check is a compound is pure
• • Organic compounds are bombarded by high energy electrons which create ions that can be deflected by a magnetic field
  • CH3 CH2 CH2 CH3 --> [CH3 CH2 CH2 CH3]+ + e-
  • The greater the mass of the ion the less deflection occurs
• • The ion of the whole compound will create the largest peak on the spectra ( molecular ion peak)
  • The m/z of this peak corresponds to the Mr of the compound
  • Butane has a Mr of 58 so the molecular ion peak of its spectra will be at 58
• Identifying fragmentary ions:

  • The molecular ion is unstable and can fragment to form an ion and a neutral species
  • [CH3CH2CH2CH3]+ --> [CH3]+ + CH2CH2CH3
  • The m/z of each fragmentary ion can then be used to identify parts of the compound and suggest the possible structure
• Key fragmentary ion peaks:

  • Methyl [CH3]+ =15
  • Each successive member increases by a CH2 so goes up by 14
  • Ethyl [C2H5]+ = 29
  • Propyl [C3H7]+ =43
  • Hydroxyl [OH]+ = 17
• Identifying fragmentary ions:

  • 2-methylpropane, CH3CH(CH3)CH3, would have the following:
  • An molecular ion peak of 58 - same as butane
  • A peak at 43 due to [CH3CHCH3]+ , i.e: loss of methyl
  • No peak at 29 as there is no way to split the isomer to produce an ethyl [C2H5]+ fragmentary ion
  • A peak at 15 due to [CH3]+ but at a higher abundance than butane as there are three possible places for it to fragment from
• Advantage of mass spectrometry:

  • Cheap
  • Small quantities of samples required
• Disadvantage of mass spectrometry:

  • The sample is completely destroyed