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Study Notes: The Interpretation of Nuclear Magnetic Resonance Spectra


Nuclear Magnetic Resonance Spectrometry (NMR):
All nuclei carry positive charge. In some nuclei, this charge spins on an axis to generate a magnetic dipole along the spin axis. When an external magnetic field is applied, the spin of a 1H proton will be either arrow up (parallel) or arrow down (anti-parallel) to the applied field. This yields two possible states for the proton: low and high energy respectively. The application of electromagnetic radiation can flip the parallel spin to an anti-parallel spin with the absorption of some of the radiation.

12C, 14N and 16O all have total nuclear spin = 0 so give no NMR signals.
1H and 13C have total nuclear spin = ½ and produce an NMR signal:
The high energy state is aligned against the applied field. A radio frequency (RF) signal is used to flip the spin. The value of the RF energy absorbed will vary according to the electronic environment (i.e. the degree of shielding by surrounding protons) of each proton in an organic molecule, with the whole range being about 10 ppm.

The sample (1 - 30 µg in ½ - 1 mL CCl4 or CDCl3) is placed in a spinning tube in the narrow gap between the poles of a powerful magnet (to ensure a homogeneous field). It is possible to scan smaller samples and a computer average of the transients (CAT) is taken.

Tetramethyl silane (TMS: (CH3)4Si ) is used as a reference standard. Its 12 protons all have an equivalent environment so all absorb the same energy to give a single peak. Almost all protons in organic compounds absorb down field (higher frequency) than the TMS peak which is arbitrarily assigned a value of zero.

The chemical environment (i.e. structural arrangement) of the protons in an organic compound can be deduced from the joint considerations of peak position, multiplicity of the peak and the area under the peak.

NMR spectrum of ethanol


Peak positions are denoted by the chemical shift parameter (delta ):

nu - nuTMS
delta =
   
nufield

which ranges from 0 to 8.5 ppm where Hz/MHz = ppm.

The chemical shift depends on nearby electronegative atoms withdrawing electrons (i.e. a deshielding effect) from the protons.

Thus
CH3-O
delta ~ 3.3
CH3-N
delta ~ 2.3
CH3-C
delta ~ 0.9

Crowding by neighbouring groups also causes deshielding:H-bonding (eg. ROH and RCOOH) causes a large down field shift. Diagram of H-bonding

 

Chemical Shifts:

Table of chemical shifts between methyl protons, methylene protons and methyne protons

Table highlighting the properties of different functional group protons

Spin-spin Coupling:

1,1,2-trichloroethane:

The doublet at delta4.0 belongs to the CH2 while the triplet at delta5.8 is the CH signal (further down field due to the proximity of two Cl atoms).

A proton on C2 experiences two slightly different magnetic fields from the single proton on C1 which has two possible spin states (arrow up,arrow down). Hence, its absorption will be split into two peaks of equal intensity since each spin state from the neighbouring proton induces a magnetic field of equal intensity.

1,1,2-trichloroethane

Since both protons on C2 are chemically equivalent, they both experience identical fields from the C1 proton. This causes a more intense absorption for each peak. The single proton on C1 experiences three slightly different magnetic fields from the pair of protons on C2 due to their spin couple states: arrow uparrow up, arrow uparrow down, arrow downarrow up, arrow downarrow down. Since the arrow uparrow down and arrow downarrow up states are equivalent, there will be three distinct peaks with absorptions in the ratio 1:2:1.

1-chloroethane:

The field for the identical protons on C1 is split into four (ratio 1:3:3:1) by the three protons on C2:
arrow uparrow uparrow up
arrow uparrow uparrow down,arrow uparrow downarrow down,arrow uparrow downarrow up
arrow downarrow uparrow up, arrow downarrow uparrow down, arrow downarrow downarrow up
arrow downarrow downarrow down

The three protons on C2 are split into a triplet (1:2:1) by the two protons on C1.

The quadruplet appears down field due to the proximity of the chlorine atom.

1-chloroethane

1-chloropropane: CH3-CH2-CH2-Cl

Triplets (1:2:1) arise for the protons on C1 and C3 which both have the two protons on C2 as neighbours.

The two protons on C2 have 5 (=3+2) nearest neighbour protons which produces a 6-fold split (1:5:10:10:5:1). In fact, this is really a 4 x 3 = 12-fold split but there is considerable overlap it appears as only 6 peaks.

1-chloropropane

2,2-dichloropropane:

A single peak is observed for this compound with its six chemically equivalent protons.

Since they are not on adjacent carbons, the fields are not perturbed so no splitting is observed.

NB. Long range coupling can be found in some organics.

2,2-dichloropropane

methanol:      CH3-O-H

No splitting since only protons bonded to adjacent carbons appear to cause coupling.

Hence, there are two singlets.
Similarly, functional group protons in -COOH and -NH- cause no splitting but -CHO can produce a doublet.

methanol

RULES:
Protons on the same carbon are chemically equivalent.
n protons on a neighbouring carbon cause n+1 peaks with intensities in the ratio of the polynomial coefficients of the expanded form of (1+x)n which can be found in Pascal's triangle.

Neighbouring
Hydrogens
Peaks
Intensities
n = 0
1
1
n = 1
2
1:1
n = 2
3
1:2:1
n = 3
4
1:3:3:1
n = 4
5
1:4:6:4:1
n = 5
6
1:5:10:10:5:1

Peak Integration:

The number of protons associated with a particular peak or peak group is given by the total area under the peak or group. When this integration is carried out for all peaks (groups), the analyst can examine the ratio of the areas to determine the relative numbers of protons in each group.

The integrated NMR spectrum for ethanol shows the integral as a line superimposed on the peak groupings. The vertical drop at each group is measured with a ruler and the ratio 1:2:3 emerges

NMR spectrum of ethanol

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