NMR [Greenland]

Question 1

Describe the theory behind nuclear magnetic resonance spectroscopy

Answer 1

• Nuclei of certain atoms behave as if they are spinning and possess a magnetic moment
• Placing the nuclei in a strong magnetic field (B₀) causes small differences in the energies of the spin states
• These can be investigated using radio frequency radiation
• The difference in energy between the spin states depends on the chemical environment of the proton - enabling structural information to be extracted from the data

Question 2

Explain the method of NMR (advanced theory)

Answer 2

1. Sample is placed in an applied magnetic field (B₀)
2. The magnetic spins of the nuclei precess in alignment with the applied field (ground state)
3. Sample is irradiated by a short pulse of radiofrequency (Rf) radiation across the whole Rf spectrum - pushing a proportion of the nuclei into their excited state
4. Over a couple of seconds, the Rf detector acquires the radiofrequency output as the excited nuclei in the molecule return to their ground states
5. The output gathered by the Rf detector is called Free Inductive Decay (FID)

Question 3

How is ¹H NMR typically acquired?

Answer 3

With 8 pulses of radiofrequency radiation to improve signal to noise ratio

Question 4

Define: Precession

Answer 4

A change in the orientation of the rotational axis

Question 5

What is the formula for calculating the difference in energy between the ground state and excited state?

Answer 5

ΔE = hγB₀/2π

Question 6

Define the terms in the (triangle)E formula

Answer 6

h = Planck’s constant

γ= magnetogyric ratio (a constant for each nucleus)

B₀= applied magnetic field strength

E = energy

Question 7

How is the radiation emitted from an atom returning to ground state used to yield structural information?

Answer 7

• Excitation and relaxation require a specific frequency of radiation = resonant frequency
• This frequency is characteristic of the chemical environment of the proton
• So yields information concerning the structure of the molecule

Question 8

What is the formula to calculate an atom’s resonant frequency?

Answer 8

Since E = hυ

υ = γB_o/2π

Question 9

What does a bigger B₀ imply?

Answer 9

A bigger B₀ = a bigger difference in energy between the spin states, therefore a more sensitive instrument

Question 10

Explain how samples are prepared for NMR

Answer 10

1. NMR analysis normally carried out on solutions
2. 5mm internal diameter filled with 0.6ml of deuterated NMR solvent and 1-2mg of sample
3. A reference (tetramethylsilane, TMS) is added to provide an internal calibration of 0 ppm
4. Sample tube placed in spectrometer’s probe and spun at 30 revolutions/second via a feed of compressed air

Question 11

Why are deuterated solvents used in NMR?

Answer 11

To minimise interference from proton signals in the solvent e.g. CDCl3, DMSO-d6

Question 12

Why is tetramethylsilane (TMS) used to calibrate an NMR spectroscoper?

Answer 12

Its methyl protons resonate at the frequency of 0 ppm

Question 13

Why is the sample tube spun at 30 revs/s?

Answer 13

To ensure uniformity of the magnetic field across the sample

Question 14

Why do different ¹H nuclei resonate at very slightly different values of the applied external field (B₀) depending on their precise chemical environment?

Answer 14

• Because there is an induced magnetic field from the orbit of the electron which opposes the external field (B₀) at the nucleus
• The effective magnetic field experienced by the nucleus is slightly less than that of the external magnetic field (B₀) due to this electronic shielding factor
• Anything that alters the density around a nucleus (e.g. the electronegativity of the atoms nearby) slightly affects the resonant frequency of the proton

Question 15

List the 3 pieces of information that NMR provides

Answer 15

1. Chemical shift = indicates the functional groups present in the molecule
2. Integrals = how many protons are present in each chemical environment (molecular symmetry)
3. Peak splitting = the relationships of the groups of protons to one another (also called spin-spin coupling)

Question 16

What does the splitting pattern of a peak indicate?

Answer 16

• Indicates the number of protons on the adjacent carbon atom
• Can be calculated using Pascal’s triangle

Question 17

What does the integral show?

Answer 17

• Gives the area under the signal = proportional to the number of protons in that environment
• Their relative heights = relative number of protons present in each chemical environment

Question 18

Where is the chemical shift found?

Answer 18

The x axis

Question 19

Where is the chemical shift of an aromatic?

Answer 19

Around 7ppm

Question 20

How does peak splitting occur?

Answer 20

• Occurs as a result of spin-spin coupling between adjacent nuclei = the magnetic spin on 1 nuclei affects that of an adjacent nucleus
• Chemically different protons have different chemical shifts - if they are connected to each other by a small number of bonds they can cause mutual splitting of each other’s peaks
• Chemically equivalent nuclei have the same chemical shift and do not cause splitting amongst themselves i.e. H’s in the same environments do not cause splitting

Question 21

How does Pascal’s triangle provide information of the peak splits/intensities?

Answer 21

• The splitting pattern is n+1 of the number of protons on the adjacent carbon (coupling) e.g. triplet = 2 H’s on adjacent carbon
• Also the intensities of the peak splits are according to Pascal’s triangle e.g. quartet = 1:3:3:1

Question 22

How are coupling constants calculated?

Answer 22

1. Take the chemical shift of each of the lines of the peak splits
2. Work out the spacing between each (e.g. 0.027 ppm)
3. Multiply by the frequency (Hz) the compound was recorded at (e.g. 270Hz) 0.027 x 270 = 7,29 Hz

Question 23

List 2 benefits of NMR spectroscopy

Answer 23

1. Provides more molecular structure information than UV or IR
2. Can be used to fingerprint components within mixtures and quantitatively analyse drugs in formulations without need for separation

Question 24

List 3 negatives of using NMR

Answer 24

1. Expensive spectrometers
2. Highly trained staff required to operate them
3. Analysis technique is insensitive so lots of sample often required