C32 structure determination

• NMR spectroscopy is a technique used to find the structures of unknown molecules
• in NMR spectroscopy, a magnetic field is applied to a sample, surrounded by a source of radio waves and a radio receiver, which generates an energy change in the nuclei of atoms in the sample, this is detected
• C13 NMR spectroscopy is a form of NMR which determines the structure of a molecule by analysing the energy of each carbon environment
• a graph produced by an NMR instrument has chemical shift on the x-axis and energy absorbed on the y-axis, with zero on the right of the x-axis
• TMS is tetramethylsilane
• chemical shift is related to the difference in frequency between the resonating nucleus and TMS, it is measured in parts per million
• carbon environments which are close to an electronegative atom will have a chemical shift value which is shifted to the left because the atom draws the electrons away from the enviroment, giving it less shielding, so it has a greater chemical shift value
• number of peaks on the NMR spectrum represents the number of carbon environments
• proton NMR is a form of spectroscopy which analyses the different hydrogen environments in a molecule and displays them as peaks on a spectrum
• in proton NMR the peaks are measured against the TMS standard
• the TMS standard has a chemical shift of zero
• in proton NMR, the sample needs to be dissolved in a solvent that does not contain hydrogen because the hydrogen would interfere and produce peaks on the spectrum
• in proton NMR, common solvents are CCl4 or deuterated solvents, which are made of deuterium, an isotope of hydrogen which does not give a signal as it has an even number
• the height of the peaks of a proton NMR spectrum represent the relative intensity which corresponds to the number of hydrogens in that environment, shown as a number above the peak
• peaks are split into smaller clusters of peaks which represent how many hydrogen atoms are attached to the adjacent carbon atom in the molecule
• small clusters of peaks in an NMR spectrum show:

  • one peak = singlet = no H on adjacent carbon, could be -OH
  • two peaks = doublet = one H on adjacent carbon
  • three peaks = triplet = two H on adjacent carbon
  • four peaks = quartet = three H on adjacent carbon
• -OH groups are not visible as part of a splitting pattern
• a common combination of peaks in a proton NMR spectrum is triplet + quartet, representing CH2CH3
• the integration trace is a line produced by an instrument which represents the areas under the peaks, this corresponds to the ratio of the hydrogen atoms in the different environments
• if you zoom in on proton NMR peaks, they are split into patterns, this is spin-spin coupling
• spin-spin coupling happens because the applied magnetic field felt by any hydrogen atom is affected by the magnetic field of the hydrogen atoms on adjacent carbons
• the (n+1) rule tells you how many hydrogens there are on the adjacent carbon
• the (n+1) rule:

  n hydrogens on an adjacent carbon will split a peak into (n+1) smaller peaks
• when interpreting proton NMR spectra:

  • count the number of hydrogen environments
  • use the integration trace to find the ratio of hydrogens
  • use the splitting pattern to work out how many hydrogens each carbon has
  • use the distribution of the peaks to work out which have the most shielding
  • compare chemical shift values with data book values to determine the structure
• number of hydrogen environments = number of peaks except from the TMS peak which is at zero
• the integration trace shows the ratio of how many hydrogens are in each environment
• the splitting pattern shows how many hydrogens each carbon has on the adjacent carbon
• the distribution of the peaks is how far they are from zero, this shows which have the most shielding as the atoms with most shielding are furthest away from electronegative atoms such as oxygen
• the formula of TMS is Si(CH3)4
• the properties of TMS which make it good for proton NMR are:

  • inert so unlikely to react with the sample
  • easy to remove
  • volatile
  • produces a peak which is far away from the peaks produced by other molecules
• CCl4 is non-polar so is a good solvent for non-polar organic molecules
• CDCl3 is polar so is a good solvent for polar organic molecules
• if you see lots of hydrogens in the same environment, try lots of branches of methyl groups on the same carbon