Analytical Methods: NMR, IR, MS

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Index

• Mass Spectrometry (MS)
• Infrared Spectrometry (IR)
• Nuclear Magnetic Resonance (NMR)
• Problems

Mass Spectrometry (MS)

Mass spectrometry is one effective method of characterizing and beginning to identify compounds. By coupling gas chromatography and mass spectrometry, compounds can be separated and each fraction identified based on molecular mass.

• The principle of mass spectrometry depends on a compound being ionized and moving through a resolving magnetic field that separated compounds based on their charge and mass.

• Ionization of compounds usually occurs by the bombatdment of athe compounds with electrons. This may result in fragmentation of the sample such that the charged fragments are analyzed along with the partent compound.

• The charged fragments are then accelerated through an evaculated tube and then bent in a magnetic field until they reach a detector.

• By varying the strength of the magnetic field, different populations of charge/mass fragments will reach the detector. Such a magnetic scan p roduces a characteristic fragment pattern for each compound.

• With the help of computers and databases of compounds, samples can be identified.
• Problems in combining gas chromatography with mass spectrometer.
  • GC under pressure
  • MS under vacuum
  • Separation of carrier gas from sample
• Samples
  • Wide variety of samples
  • Need to be pure
  • Need to be volitile
  • Can derivitize problem compounds

Infrared Spectrometer (IR)

• Infra-red spectrometry is a non-destructuve analysis that can be applied to gasses, liquids and solids.
• The absoption pattern can be sued to identify and characterize pure compounds.
• The solvent in which the compound is dissolved should not absorb in the infra-red region.

• The movement of atoms in chemical bonds can be compared to balls on springs.
• When these bonds absorb certain wavelengths of energy an increase in stretching and bending of particular bonds can be observed.

Nuclear Magnetic Resonance (NMR)

• Molecules of about half of the known isotopes of chemical compounds have nuclei which behave like spinning magnets.
• When they are placed in a magnetic field, these nuclei absorb radio frequency energy.
• A Characteristic spectra based on the protons nucleus can be determined. Shifts in the nuclei spectra is dependent on the influence of neighboring nuclei.

Problems

Unknown A was found to have a minimal molecular weight of 100 amu.

IR spectroscopy gave the following scan:

The C13 NMR gave the following scan:

The proton NMR gave the following scan:

After you have determined the structure, send the answer to C.A. Rinehart

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