NMR Spectroscopy

Question 1

What is NMR spectroscopy?

Answer 1

Uses radio-frequency electromagnetic radiation to cause transitions between the Zeeman energy levels of nuclei in a magnetic field.

Question 2

What is spin?

Answer 2

A form of angular momentum possessed by all elementary particles, including protons + neutrons.

Question 3

What type of spin do protons + neutrons possess?

Answer 3

Nuclear spin (I).

Question 4

How do we expresses spin orientation?

Answer 4

mi

Question 5

What does I determine?

Answer 5

Whether a nucleus will give an NMR signal.

Question 6

What does I depend on?

Answer 6

The number of protons + neutrons. If I = 0 then the atom won’t give an NMR signal.

Question 7

In the absence of a magnetic field, the two spin levels will have the same energy. What causes spin levels to split energy?

Answer 7

Application of a magnetic field (Bo);
If spin lines up WITH the direction of the magnetic field, it will go to lower energy + be more stable.
If spin lines up AGAINST the direction of the magnetic field, it will go to higher energy + be more unstable due to repulsion.
This is the Zeeman effect.

Question 8

What would give bigger energy separations?

Answer 8

A bigger magnetic field.

Question 9

State the equation linking energy separation and magnetic field strength.

Answer 9

∆E = h§Bo/2pi = hv

Question 10

What does § indicate + what are its units?

Answer 10

§ = gamma
Represents magnetogyric ratio, indicates how magnetic a nucleus is.
Units = rad s^-1 T^-1

Question 11

What does v indicate?

Answer 11

v = resonance frequency
Differs between nuclei therefore we can only look at one at a time i.e. C13 or H1
Frequency v will induce transitions between the two states.

Question 12

How does NMR spectroscopy work?

Answer 12

A super conducting magnet maintained at liquid helium temperature by a surrounding vessel containing liquid helium.
This vessel is further surrounded by a vessel of liquid nitrogen to prevent helium evaporating.

Question 13

What is sigma and what does it measure?

Answer 13

Sigma is the shielding constant, it measures how different the frequency is for each nucleus.

Question 14

What causes the frequency of a nucleus to differ?

Answer 14

Nuclei resonate at different frequencies depending on their chemical environment i.e. what the surrounding atoms are.

Question 15

What is shielding?

Answer 15

Orbiting electrons around the nucleus set up a magnetic field which opposes the applied magnetic field, creating shielding.

Question 16

State the equation for shielding.

Answer 16

v = (§Bo)/2pi (1-sigma)

Question 17

How do we obtain data using NMR? (3 bullet points).

Answer 17

• use a magnetic field (Bo) to separate energy levels by the Zeeman effect
• use electromagnetic radiation to induce transitions between states
• a transition will occur when the oscillating frequency of the electromagnetic radiation matches v (resonance frequency)

Question 18

How are we able to compare spectra of different magnetic field strengths?

Answer 18

Vref (TMS), has very high shielding due to presence of many CH3 groups.

Question 19

State the equation for the scale of chemical shift (§).

Answer 19

§ (ppm) = 10^6 (Vresonance - Vref) / Vref.

Question 20

What happens when we increase the strength of Bo?

Answer 20

The separation energies and frequencies increase.

Question 21

What gives the signal in C13 NMR?

Answer 21

The C nuclei.

Question 22

What gives the signal in H1 NMR?

Answer 22

The protons.

Question 23

In C13 NMR, what does increasing electronegativity cause?

Answer 23

Increases the frequency of resonance of C13 nucleus.

Question 24

State the factors of shielding in C13 NMR (3).

Answer 24

1. The more H atoms on a carbon, the greater the shielding.
2. Adjacent electronegative atoms have a de-shielding effect.
3. Unsaturation (C=C) has a de-shielding effect.

Question 25

In H1 NMR, what has the lowest chemical shift?

Answer 25

CH3.

Question 26

State the factors of shielding in H1 NMR (1).

Answer 26

1. Adjacent electronegative atoms have a de-shielding effect.

Question 27

In H1 NMR, what does the peak intensity tell us?

Answer 27

The area under each resonance signal is proportional to the number of H1 nuclei giving rise to the signal.

Question 28

In H1 NMR, how can we use peak intensity?

Answer 28

To create an integer ratio of H atoms.

Question 29

In C13 NMR, what is the effect of symmetry?

Answer 29

C atoms in exactly the same environment will give peaks at exactly the same chemical shift.

Question 30

State what is mean by coupling (HINT: peak splitting).

Answer 30

When nuclear spin energy levels are perturbed by nearby nuclear spins, splitting NMR transitions into multiplets.

Question 31

How does coupling occur in C13 NMR?

Answer 31

Between the C13 and H1 nuclei of hydrogens attached to the same carbon.

Question 32

How does coupling occur in H1 NMR?

Answer 32

Between H1 nuclei on adjacent carbon atoms, NOT the same carbon atom.

Question 33

State what is meant by the “n+1” rule in C13 NMR. Give an example.

Answer 33

A C13 atom bonded to n H atoms will give rise to n+1 lines. For CH3, n = 3 and the signal will be split into n+1 fragments = quartet.

Question 34

State what is meant by the “n+1” rule in H1 NMR. Give an example.

Answer 34

The signal of a H1 atom adjacent to n equivalent hydrogen atoms will give n+1 lines. For CH - CH3, the CH signal will be split into n+1 fragments = quartet as the adjacent C atom has 3 hydrogens attached.