NMR

NMR

1. Carbon-13

   • Number of peaks indicates the number of carbon environemnts
   • Chemical shift of peak indicates the type of environment
   • Benzene has one peak
   • The size of peak tells nothing about the number of carbon atoms responsible

2. Proton

   • Requires a smaller sample than C13 NMR
   • Number of peaks gives the number of proton environments
   • Chemical shifts give the type of environment
   • Areas under the peak (integrals of the peaks) give the proportions of protons in each environment i.e. the numbers of protons responsible for that peak
   • Spin-spin coupling causes splitting of peaks. Protons that re close together, two bonds apart, interact called coupling which leads to the peak splitting. The splitting pattern can be used to find the number of protons on adjacent carbons atoms.
     • Rules of splitting
       • Equivalent protons do not split one another, neither do protons attached to a hydrogen of a nitrogen
       • The n+1 rule. n represents the number of protons on an adjacent carbon, the number of peak in the splitting pattern equals n+1
         • e.g.

3. NMR Spectra of -OH and -NH Protons

   • Can be filtered using deuterium oxide, D2O, known as heavy water with the isotope 2H (D), which doesn't produce a peak, and so any peak from an OH or NH disappears.
     • Occurs because a deuterium exchanges with the hydrogen causing the peak
       • e.g.
   • Chemical shifts can vary considerably depending on concentration, the solvent and other factors
   • Have singlet peak which may be broad, which occurs because of traces of water in the solvent which forms hydrogen bonds with the OH and NH protons

4. NMR in Medicine

   • Pharmaceutical companies use NMR as an analytical technique to check the composition of organic compounds during the synthesis of drugs
   • MRI Body Scans: Produce a 3-D image of the chemical structure of tissues. The patient is placed in a large cylindrical electromagnet and radio waves are sent through their body and protons resonate in response. It is less harmful than X-rays and rules out explorative surgery. Cannot be used for people with magnetic implants or heart pacemakers.
     • Most brain disorders result in changes to water content and this is reflected in the MRI image. Can visualise the position and size of cancer tumours and whether they have infiltrated the surrounding tissue. Used to identify the extent of injuries such as tears in muscles, tendons and ligaments.

5. Combined Techniques

   • Mass Spectrometry
     • #Molecular Ion, M+: The positive ion formed in mass spectrometry when a molecule loses an electron.
     • #Fragmentation: The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion.
     • Fragmentation patterns give clues about the carbon skeleton of the compound