Spectroscopy

• Covalent bonds in molecules possess energy and vibrate naturally.
• Bonds can absorb IR, which makes them bend/stretch more.
• The amount of bond stretch depends on mass of atoms and the strength of the bond.
• Wavenumber is proportional to frequency.
• Infrared spectroscopy identifies which functional groups are present.
• The fingerprint region is unique to the molecule.
• A beam of infrared radiation is passed through a sample to a detector; covalent bonds within the molecule absorb the IR radiation causing the covalent bonds to vibrate more. Characteristic absorptions can be matched to specific bonds in molecules. This enables chemists to determine the functional groups present.
• Absorptions are identified by their wavenumber (cm-1).
• Different covalent bonds will have different absorptions - they will absorb the IR at different wavenumbers.
• C-H bond causes a sharp peak between 2850-3100cm-1.

  (It is rarely necessary to interpret this peak due to almost all molecules having it).
• The region under 1500cm-1 is known as the fingerprint region.
• C=C in alkenes gives an absorption around 1620-1680cm-1.
• C=O in aldehydes, ketones, carboxylic acids, esters, amides, acyl chlorides, and acid anhydrides gives an absorption around 1630-1820cm-1.
• Aromatic C=C in arenes gives several absorption peaks in range of 1450-1650cm-1.
• Nitrile group gives an absorption between 2220-2260cm-1.
• C-H in alkyl groups, alkenes and arenes give an absorption around 2850-3100cm-1.
• N-H in amines and amides gives an absorption around 3300-3500cm-1.
• O-H in alcohols and phenols gives an absorption peak at 3200-3600cm-1.
• O-H groups in carboxylic acids have a broad peak at 2500-3300cm-1.
• Infrared spectroscopy is used to identify pollutants in vehicle emissions in the air. Sensors detect and measure the amounts of pollutants such as carbon monoxide, carbon dioxide and unburnt hydrocarbons.
• Infrared spectroscopy can be used to measure alcohol levels using roadside breathalysers - a ray of IR is passed through the breath that is exhaled into the breathalyser chamber. The characteristic bonds of ethanol are detected and measured - the higher the absorbance of IR, the more ethanol in the person's breath.
• The gases in the atmosphere that absorb IR are known as greenhouse gases.
• IR spectroscopy can be used to monitor the atmospheric concentration of greenhouse gases.
• Explain why scientists are most concerned about the impact of CO2 on global warming compared to other greenhouse gases.

  They have the highest atmospheric concentration.
• State the possible impact of global warming.

  -Melting of polar ice caps/rising sea levels.

  -Droughts in other regions of the world.
• Explain why different atmospheric gases have different impacts on global warming.

  Factors which controls effect on gases on global warming are:

  -ability to absorb IR

  -the conc of the gas in the atmosphere

  -the residence time of the gas in the atmosphere
• When a compound is analysed in a mass spectrometer, vaporised molecules are bombarded with a beam of high-speed electrons. These knock off an electron from some of the molecules, creating molecular ions.
• The peak with the highest m/z value is the molecular mass of the compound.
• The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13.
• The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule: the more carbon atoms, the large the [M+1] peak is.
• Different compounds may have the same molecular mass, to further determine the structure of the unknown compound, fragmentation analysis is used.
• Fragmentation peaks may appear due to the formation of characteristic fragments or the loss of small molecules.
• Simple alkanes are fragmented in mass spectroscopy by breaking the C-C bonds.
• Fragment [CH3] has a m/z of 15.
• Fragment [C2H5]+ has a m/z of 29.
• Fragment [C4H9]+ has a m/z of 57.
• Fragment [C5H11]+ has a m/z of 71.
• Fragment [C6H13]+ has a m/z of 85.
• Elemental analysis is used to determine the empirical formula.
• Mass spectrometry is used to determine the molecular mass and fragment ions from the whole molecule.