19. Analytical techniques II (NMR)

How many different carbon environments there are.

The difference in the energy absorbed by nuclei in different environments This difference is called ‘CHEMICAL SHIFT’

Radio waves - nuclei in a sample absorb radio waves which can cause them to flip Therefore resonance can be detected

How close the carbon atoms are to electronegative atoms like oxygen. The closer to electronegative atoms = less shielded therefore more chemical shift.

Solvent is CDCl3 - Solvent does not contain any hydrogens, so wont add an additional peak

Tetra-methyl-silane (TMS) –> TMS Has 4 carbon atoms all in identical environments -> So in 13C NMR and 1H NMR they produce a single strong absorption peak -> this peak is far away from the rest and so is set at 0 ppm (parts per million) TMS is chemically inert - will not react with the sample TMS is non toxic TMS is volatile so can be removed from the sample easily.

Low resolution nmr High resolution nmr - what was a single peak is now a cluster of smaller peaks: can be a singlet, doublet, triplet, quartet or multiplet of x

Number of hydrogens on adjacent carbon atoms = n Then do n+1 eg: if a CH3 is bonded to a CH2.. and you want to find the splitting pattern for the CH3, do 2+ 1 = 3 so it would be a triplet

No - any hydrogen bonded to a carbon that has a hydrogen bonded to an oxygen/ nitrogen will form a singlet

First carry out a H nmr of sample Then shake sample with D2O and place back in nmr spectroscopy and any suspected OH or NH peaks would disappear in the presence of D2O

When there is no hydrogens on the adjacent carbon atom - so the hydrogens are more than 3 bonds apart

The number of hydrogen atoms in that environment