Intro to NMR Spectroscopy

Question 1

Number of signals

Answer 1

• The number of signals in a 1HNMR spectrum indicates the number of different sets of H atoms.
• Chemically equivalent protons are interchangeable through a symmetry operation and produce only one signal.

Question 2

The NMR spectrometer

Answer 2

• The sample cell is a small cylindrical glass tube suspended between the magnet’s north and south pole faces.
• In the magnet gap there is a coil attached to a radiofrequency generator and radiofrequency detector coil.

Question 3

Chemical equivalence

Answer 3

• All the protons in a chemically identical environment
• Same chemical shift

Question 4

Resonance

Answer 4

• Absorption of electromagnetic radiation by a spinning nucleus and the resulting flip of its nuclear spin state
• Energy absorbed must equal the energy difference between the two states

Question 5

Nuclear shielding

Answer 5

• Not all protons in a molecule have resonance at the same frequency because they are surrounded by electrons
• Valence electrons generate a local magnetic field which opposes the applied magnetic field
• Each proton in a molecule is shielded from the applied magnetic field

Question 6

Chemical shift

Answer 6

• A measure of the degree to which a nucleus is shielded
• Defined relative to the frequency of absorption of a reference compound TMS
• Scale from 0-12 ppm

Question 7

Deshielding

Answer 7

• Double bond or electron-withdrawing atoms can reduce electron density around hydrogen nearby
• The local magnetic field generated by the electron is lower compared to a shielded hydrogen
• Hydrogen experiences a higher magnetic field

Question 8

Area of signals - integration

Answer 8

Integration or the area under each signal gives the relative number of protons giving rise to the signals

Question 9

Shape of signals - multiplicity

Answer 9

• Nonequivalent hydrogens on adjacent carbons have magnetic fields that can either align with or oppose the external magnetic field.
• Magnetic coupling causes the hydrogen to absorb slightly downfield when the external field is reinforced and absorb slightly upfield when the external field is opposed.
• All possibilities of magnetic interaction exist, resulting in the splitting of the signal.
• Multiplicity (splitting of signals) occurs due to coupling with neighbouring nuclei.
• Depends on the number of neighbouring protons. Follows the n+1 rule.

Question 10

Magnetic coupling

Answer 10

• Tells us the number of neighbouring H atoms
• Equivalent H atoms do not couple to each other
• H atoms bonded to the same C will split each other only if they are not equivalent

Question 11

13C NMR spectroscopy

Answer 11

• Nuclei of 13C have nuclear spin and are detected in the same manner as H atoms
• Most common method using proton decoupling, the carbon peaks appear as singlets
• The 2 most important features are: the number of signals and chemical shifts

Question 12

13C NMR - number of signals

Answer 12

• Gives the number of different types of C atoms in a molecule
• Signals are not split so the number of signals equals the number of lines in the 13C spectrum
• Peak intensity is not proportional to the number of absorbing carbons, so signals are not integrated

Question 13

13C NMR - chemical shift

Answer 13

• Occurs over a much broader range (0-220 ppm)
• Depend on the same effects as the chemical shifts in 1H NMR

Question 14

NMR in falsified medicines

Answer 14

• Identify a drug or excipient
• Evaluate impurity level
• Evaluate residual solvents
• Determine the isomeric composition
• Characterise an entire formulation