Carbon NMR

Carbon NMR

1. Number of signals tells you the different number of environmentspresent

   • carbons do not sit in the same envorinments. it depends on what groups they are bonded to
   • electronegative atoms have effects on the nuclei spin of carbon, leading to different chemical shits.

2. the splitting of the signal tells you how many hydrogens are present in each carbob

3. chemical shit tells the the hybridisation of each carbon atom

   • chemical shift value depends on the structure. it depends on the environments the protons are in.
     • by adding electron withdrawing groups you change the chemical sshit

4. TMS is given as 0 on a chemical shift and is used as the reference value/ point

5. increasing the frequency only affects the resolution (makes it better) but it does not affect the chemical shift.

   • higher chemical shift allows for better resodution

6. height of peaks may be a rough guide but can not be intergrated.

7. NMR

   • JNJ
   • equipment
     • strong magnet
       • several miles of alloy wire wound to form a coil which is cooled to 4 kelvins in liquid helium. a current is introduced (50amps) and this runs forever. (liquid helium evaporates quickly.)
     • liquid nitrogen tank
       • surrounds the He tank ( nitrogen is cheaper than helium and it keeps the helium cold for longer inside the tank)
     • sample
       • sits in the centre in a 5mm glass tube. the tube spins on a jet of air
         • glass tube goes in at the same time as when the magnetic field is completely allighned.
     • the probe
       • sits around the sample and irradiates it with radio waves
     • computer
       • processes the absorption of radio waves by the sample (tells you the frequencies and absorption of energy)
   • nuclear spin
     • in many cases nuclei only have 2 spins, up or down
     • spinning of charged particles generates a magnetic field
     • in presence of external magnetic field spin staes become unequal in eanergy
       • if you put a magnet in the field, the nuclei will arrange themselves according to their spin.
         • the nuclei with lower energy will point in the same direction as the external magnetic field. the ones with higher energy will arrange themselves in the opposite direction
         • the difference in energy between two nuclear spin states varies with the strength of the external magnetic field
           • the bigger the magnetic field applied the bigger the energy difference between the high and low energy nuclei
             • the bigger the magnetic field applied the bigger the nuclear spin states. when the right amount of energy is reached (by changing the wavelength) you get a signal coming through
             • bigger magnetic field also means increased frequency required.
               • by tuning the frequency we are changing the energy to match the energy difference between two nuclear spins
     • nuclear spin transition
       • if you apply a source of radiation, equal in energy to the small difference between the two states, energy will be absorbed by the sample and a transition will take place.
         • some of the low energy nuclei will switch state and move to higher energy states.
         • not much energy required since the difference in energy needed to make the switch is very low.
   • Proton NMR
     • Exact position where a nucleus resonates depends on its exact magnetic environment (to some extent this is determined by electronic environment near by) so not all hydrogens are the same, they dont absorb the same amount of radio waves
       • if protons have several different hydrogen atoms, it will give several different signals
     • intergration
       • tells you how many protons are represented in that peak
       • the area under any proton NMR peak is proportional to the number of hydrogen atoms represented by the peak
     • NMR splitting
       • N+1 applies
         • The amount of splitting tells you about the number of hydrogens attached to the carbon atom or atoms next door to the one you are currently interested in. The number of sub-peaks in a cluster is one more than the number of hydrogens attached to the next door carbon(s). So - on the assumption that there is only one carbon atom with hydrogens on next door to the carbon we're interested in. singlet next door to carbon with no hydrogens attached, doublet next door to a CH group, triplet next door to a CH2 group, quartet next door to a CH3 group
       • caused by hydrogens on adjacent carbon atoms
       • aromatic hydrogens come at high delta (anisotropic) attached groups cause predictable upfield or downfield shifts
       • high resolution NMR splitting
         • 1 peak a singlet, 2 peaks in the cluster a doublet, 3 peaks in the cluster a triplet, 4 peaks in the cluster a quartet
   • anisotropic groups
     • aldehydes
     • arenes
     • alkenes
     • alkyne

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