3.3.15 NMR spectroscopy

¹H NMR

¹³C NMR

information about position of ¹H atoms in a molecule

information about position of ¹³C atoms in a molecule

NMR

scientists have developed a range of analytical techniques which together enable the structures of new compounds to be confirmed

NMR = nuclear magnetic resonance

a magnetic field is applied to a sample which is surrounded by a source of radio waves and a radio receiver; generated an energy change in nuclei of atoms; can be detected; electromagnetic energy is emitted which can be interpreted using a computer

carbon-12 does not have any nuclear spin, but carbon-13 does; only 1% of carbon atoms are carbon-13

carbon atoms in different functional groups resonate differently; all nuclei are shielded from the external magnetic field by electrons surrounding them; more electrons = more shielding; greater electron density around carbon-13 atom, the lower the frequency it resonates

chemical shift δ:

measured in parts per million, ppm

difference in frequency between resonating nucleus and TMS

no splitting of peaks

related the type of environment carbon-13 or proton is in

nearly all hydrogen atoms are ¹H so is easier to get NMR spectrum that for carbon-13

hydrogen atoms in different functional groups resonate differently; all nuclei are shielded from the external magnetic field by electrons surrounding them; more electrons = more shielding; greater electron density around carbon-13 atom, the lower the frequency it resonates

the further away a hydrogen atom is from an electronegative atom, the smaller its chemical shift

area of each peak is related to number of hydrogen atoms producing it

Tetramethylsilane

standard by which chemical shifts of other atoms are measured

chemical shift value of hydrogen atoms is zero in TMS

used to calibrate the spectrum at 0ppm

inert, non-toxic, easy to remove from sample

all 12 hydrogens are in the same environment

n+1 rule: n hydrogens on an adjacent carbon atom in a different environment will split a peak into n+1 smaller peaks
e.g. if a hydrogen has an adjacent carbon attached to three hydrogens in a different environment to itself, the peak will be split into 4

s - singlet
d - doublet
t - triplet
q - quartet
m - multiplet

run in solvent:

solvent does not contain any hydrogen atoms or signals from sample will be shadowed

e.g. tetrachloromethane, CCl₄

e.g. deuterium based solvents: CDCl₃, D₂O, C₆D₆

Sammer Sheikh