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 (we say there are quadrupolar)

Question 2

What does quadrupolar mean

Answer 2

These nuclei have a non-spherical charge distribution, creating an electric quadrupole moment.

Question 3

What shape is the nucleus when I=1/2

Answer 3

Circular

Question 4

What shape is the nucleus when I>1/2

Answer 4

Oblate

Question 5

Why is the nucleus for I>1/2 oblate

Answer 5

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

Question 6

What does EFG stand for

Answer 6

Electric field gradient

Question 7

Why do I>1/2 nuclei have broad signals

Answer 7

The nuclear electric quadrupole found in these nuclei interact with EFG. 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 producing broad signals

Question 8

Define quadrupolar broadening

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

Which symmetry elements create an Electric Field Gradient (EFG)?

Answer 11

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

Question 12

How can nuclear electric quadrupole affects coupling

Answer 12

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

Question 13

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

Answer 13

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

Question 14

What is the symbol given to quadrupole moment

Answer 14

Q

Question 15

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

Answer 15

If line broadening is very extreme then signals may not be visible. Is Q 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 16

Describe the general trend for coupling to halogen when I>1/2

Answer 16

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