Introduction, Chemical Shift and Integration

Question 1

What is the main type of NMR looked at in this course?

Answer 1

Solution phase NMR spectroscopy

Question 2

What property of atoms allow NMR to be performed

Answer 2

Nuclear spin
(many nuclei possess a magnetic moment - can be reguarded as behaving like small magnets)

Question 3

What happen when you put a nucleus wit a spin in a magentic field

Answer 3

The nucleus spins on its axis
In the presence of a magnetic field, this axis of rotation will precess around the magnetic field

Question 4

The frequency of the procession around the magnetic field by a nucleus is called…

Answer 4

Resonance OR Larmor Frequency

Question 5

How is nuclear spin calculated?

Answer 5

nuclear spin arises from the unpaired proton and neutron spins in the nucleus
(compared to electron spin from unpaired electrons)

Question 6

Fill out the following table

Answer 6

Question 7

Nuclear spin is characterised by which quantum number

Answer 7

Spin quantum number I

Question 8

Spin quantum number I , is proportional to

Answer 8

the nuclear magnetic moment μ
(related by this constant of proportionality, γ, which is different for each nucleus)

Question 9

Why is the magnetogyric (γ) different for each nucleus

Answer 9

due to each nucleus having a different ratio of its orbital angular momentum to its magnetic moment (due to each nucleus having different charge and mass)

Question 10

a spin of +1/2 is ……. to the magnetic field
a spin of -1/2 is ……. to the magnetic field

Answer 10

Question 11

The orientation of the nuclear magnetic momenet μ, is aligned to an arbitrary axis (z) = μz
μz is quantised due to the following equation
What values can m take?

Answer 11

m = +I, +(I-1), …0… , -(I-1), -I
there will be 2I + 1 allowed nuclear spin states of μz associated with nuclear spin I

Question 12

In a magnetic field, what values of m would there be aligned to z, when I =1/2

Answer 12

Question 13

In a magnetic field, what values of m would there be aligned to z, when I = 1

Answer 13

Question 14

In a magnetic field, what values of m would there be aligned to z, when I = 3/2

Answer 14

Question 15

What is the equation to would out the difference in energy between two spin states (ΔE)
this is also equal to resonance frequency (v)

Answer 15

B0 = applied magnetic field
γ = magnetogyric constant
v is the frequency which allows ΔE to occur

Question 16

Why do we deuterate chloroform as the solvent for NMR

Answer 16

• If you are running a 1H NMR, it means the protons observed within the spectrum are more from the compound and less from the solvent
• AND helps to avoid signal drift (due to slow rate of NMR) to allow sharp peaks

Question 17

Although a magnetic field of B0 is applied to the sample, the effective magnetic field (Beff ) experienced by each nucleus is not necessarily the same
Why?

Answer 17

σ is due to the motion of the electrons surrounding the nucleus, causing small induced magnetic fields opposing the applied field B0

Question 18

What happens when you apply an external magnetic field to benzene?

Answer 18

The external magnetic field (B0) is reinforced by the induced magnetic field
An example of this is in Benzene, where there is a circulating ring of pi electrons, which results in the deshielding on protons

Question 19

The induced magnetic field experienced by each nucleus is dependent on

Answer 19

Upon the chemical environment of the nucleus (Chemical shielding)

Question 20

How do we work out chemical shift?

Answer 20

x10⁶ make it into ppm
(Benefit of working it out this way is it’s independent of the applied magnetic field)

Question 21

What can be the issue with some intergration values?

Answer 21

Intergration values can sometimes be inaccurate as some nuclei relax slowly
(protons are only really good for intergration)