NMR spectroscopy

Question 1

What does NMR stand for?

Answer 1

Nuclear magnetic resonance

Question 2

What is B₀?

Answer 2

The externally applied magnetic field

Question 3

What is alpha spin?

Answer 3

When the proton’s magnetic moment aligns with the external magnetic field

Question 4

What is beta spin?

Answer 4

When the proton’s magnetic moment aligns against the external magnetic field.

Question 5

How do alpha and beta spin compare to each other?

Answer 5

They are opposite spin directions, and beta is a higher energy state than alpha

Question 6

What effect does increasing B₀ have on the difference between alpha spin and beta spin?

Answer 6

It increases the difference in energy between them. Beta spin becomes a higher energy state, the stronger the external magnetic field.

Question 7

How would you get a proton to flip from an alpha spin state to a beta spin state?

Answer 7

Apply energy

Question 8

What happens when a proton flips from a beta spin state to an alpha spin state?

Answer 8

It releases energy

Question 9

What is detected by NMR?

Answer 9

The energy frequencies released by protons as they revert from beta spin state to alpha spin state

Question 10

What does the frequency of an NMR signal refer to?

Answer 10

The difference in energy between the alpha and beta spin states of the protons.

Question 11

What does the letter I (capital i) refer to in physics?

Answer 11

Current

Question 12

What does a moving charge create?

Answer 12

A magnetic field

Question 13

If a current is moving in a clockwise loop (parallel to the ground) which way does the magnetic field go?

Answer 13

Down

Question 14

How do electrons move in relation to a current?

Answer 14

In the opposite direction

Question 15

What does B_in refer to?

Answer 15

Induced magnetic field. The magnetic field induced by electrons circulating around the proton.

Question 16

Diamagnetic materials are _________ by a magnetic field

Answer 16

repelled

Question 17

What is the relationship between an applied magnetic field and the consequent induced magnetic field?

Answer 17

The induced magnetic field opposes the applied magnetic field.

Question 18

What effect does an induced magnetic field have on the applied magnetic field for the proton?

Answer 18

It reduces the overall strength of the applied magnetic field on the proton.

Question 19

What does B_eff refer to?

Answer 19

The effective magnetic field - the effective magnetic field that the proton experiences after the influence of the induced magnetic field is factored in.

Question 20

B₀ - B_in = ?

Answer 20

B_eff

Question 21

What is nuclear shielding?

Answer 21

When circulating electrons create an induced magnetic field in response to an applied magnetic field, and the induced magnetic field lowers the effective magnetic field experienced by the proton

Question 22

Important to NMR, what is the difference between a hydrogen proton alone, or in a molecule?

Answer 22

A hydrogen proton alone is unshielded by electrons, a hydrogen proton in a molecule is shielded.

Question 23

What effect does nuclear shielding have on NMR frequency?

Answer 23

Frequency is the difference in energy between alpha and beta spin states. If nuclear shielding lowers the effective magnetic field experienced by a proton, this means a reduced difference in energy between alpha and beta spin states - a reduced frequency signal.

Question 24

When interpreting an NMR spectrum, which direction refers to increasing frequency?

Answer 24

Left

Question 25

A deshielded proton is likely to return a ____ frequency NMR signal

Answer 25

higher

Question 26

What are the pulses applied in NMR?

Answer 26

Pulses containing radiofrequencies which correspond to potential energy differences between alpha and beta spin states for protons in different substances

Question 27

In FT NMR, the ________ is held constant, while the _________ vary

Answer 27

applied magnetic field, radiofrequency pulses

Question 28

What do upfield and downfield refer to in NMR?

Answer 28

On the NMR spectrum, upfield refers to the lower frequency (right) end of the spectrum, downfield refers to the higher frequency (left) end of the spectrum.

Question 29

What does chemically equivalent mean in NMR?

Answer 29

When protons are in the same environment (and therefore have the same level of shielding)

Question 30

What does the number of signals from NMR tell us?

Answer 30

The number of groups of hydrogen protons which are not chemically equivalent (i.e. are in different environments)

Question 31

What does chemical shift refer to in NMR?

Answer 31

The resonant frequency of a nucleus relative to the standard (TMS) in an applied magnetic field