6.3.2 - spectrometry

• NMR stands for nuclear magnetic resonance
• NMR is used in MRI scanners
• On a low resolution HNMR you would see one peak for each hydrogen environments
• The area under the peak in HNMR is proportional to the number of H atoms represented by the peak
• Intergration number is the number of hydrogens in the environment
• TMS stands for tetramethylsilane

  Silicon atom with four methyl groups
• TMS is used to calibrate the NMR as it provides a peak at exactly 0ppm. Reference point for which all are measured.
• TMS is advantageous because it is

  inert

  non-toxic

  All hydrogens are chemically equivalent
• Splitting occurs because neighbouring hydrogen atoms affect the magnetic field of H atoms and cause their peaks to split
• The rule for splitting is the n+1 rule
• The n+1 rule is that if there are n inequivalent H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks
• 1 neighbouring hydrogen splits into 2 peaks known as a doublet
• 2 neighbouring hydrogens splits into 3 peaks known as a triplet
• 3 neighbouring hydrogens splits into 4 peaks known as a quartet
• 4 or more neighbouring hydrogens splits into peaks known as multiplets
• Solvents used for HNMR must not contain any hydrogen atoms as signals from the solvent would swamp the signals from the sample as there is more solvent than sample
• In HNMR deuterated solvents can be used

  eg) CDCl3 , D2O , C6D6

  or compounds that contain no hydrogen atoms or isotopes

  eg) CCl4 - tetrachloromethane
• The O-H peak disappears if D2O is used as a solvent Because an O-D bond is formed in preference to O-H due to protons that swap from one molecule to another. H2O is also formed
• Chemical shift is larger for hydrogen atoms closer to electronegative atoms or C=C
• in carbon 13 NMR there is no splitting
• in carbon 13 NMR there is one signal peak for each carbon environment
• It is easier to get a spectrum of HNMR than C13NMR because H are more abundant so have spin and show up.