Nuclei with I>1/2 L8

Question 1

What do nuclei with I>1/2 possess

Answer 1

Nuclear spin and a nuclear electric quadrupole (they are quadrupolar)

Question 2

What shape is the nucleus when I=1/2

Answer 2

Circular

Question 3

What shape is the nucleus when I>1/2

Answer 3

Oblate

Question 4

Why is the nucleus for I>1/2 oblate

Answer 4

There is an ellipsodial charge distribution within the nucleus, so the protons are not distributed evenly

Question 5

What does EFG stand for

Answer 5

Electric field gradient

Question 6

What happens to quadrupolar nuclei in the presence of EFG

Answer 6

In the presence of an EFG the nuclear electric quadrupole couples with nuclear spins. This quadrupole has a fast relaxation rate so the nuclear spins relax rapidly (short t2) so there are broad signals

Question 7

Define quadrupolar broadening

Answer 7

Line broadening caused by interactions between quadrupolar nuclei (I>1/2) and local electric field gradients.

Question 8

What happens to quadrupolar nuclei in the absence of an electric field gradient (EFG)?

Answer 8

Without an EFG, there is no quadrupolar broadening . Quadrupolar nuclei (I>1/2) behave similarly to spin 1/2 nuclei. Signals are sharper because there’s no rapid relaxation

Question 9

What is an Electric Field Gradient (EFG) in NMR?

Answer 9

A measure of how the electric field changes across a nucleus due to asymmetry in the surrounding electron cloud or environment.

Question 10

Which symmetry elements could and wouldn’t have quadrupolar broadening

Answer 10

quadrupolar broadening: Molecules or ions with low symmetry (e.g., no inversion center) have an EFG.
No quadrupolar broadening : Highly symmetric environments like cubic or tetrahedrally structured eg Td or Oh

Question 11

How can nuclear electric quadrupole affects coupling

Answer 11

Scalar coupling is only observed if spin orientations of coupled nuclei are maintained for a certain minimum time

Question 12

What does the degree of linebroadening/ effect on coupling depend on

Answer 12

1) quadrupole moment Q of nucleus
2) severity of EFG at quadrupolar nucleus

Question 13

What is the symbol Q mean

Answer 13

Quadrupole moment

Question 14

Explain why a signal much not be visible for I>1/2 nuclei

Answer 14

If line broadening is very extreme then signals may not be visible. If the quadrupole moment is very large then the relaxation of nuclear spins is very rapid and the line may be so broad that it is not detectable

Question 15

Explain why we do not couple to halogens

Answer 15

In general because of the large quadrupolar moment of Cl,Br,I nuclei coupling to the nuclei is not usually observed

Question 16

Define quadrupole moment

Answer 16

The quadrupole moment is a measure of the asymmetry of the charge distribution within a nucleus. It quantifies how much the nuclear charge deviates from being spherically symmetric