The fundamentals L1

Question 1

What do protons, neutrons and electrons all possess

Answer 1

Protons neutrons and electrons have intrinsic spin, angular momentum of 1/2 and orbital angular momentum

Question 2

How does the nucleus behave

Answer 2

The nucleus behaves as a single particle with spin angular momentum

Question 3

Define total angular momentum of the nucleus

Answer 3

Total angular momentum of nucleus is the sum of spin and orbital angular momenta of all protons and neutrons in the nucleus

Question 4

How if at all are spin and orbital angular momenta quantised

Answer 4

Spin and orbital angular momenta are quantised in both magnitude and direction

Question 5

How do we calculate magnitude

Answer 5

Magnitude = square root of [I(I+1)ħ]
Where I is nuclear spin
h is (h/2pi) and h is planks constant

Question 6

How do we calculate direction

Answer 6

Iz = mIħ
Iz = Z component of angular moment (with respect to an arbitrary Z axis
Mi - magnetic quantum number

Question 7

For a given nucleus how many possible spin states are there

Answer 7

For a given nucleus with spin I there are 2I+1

Question 8

How do we know the value of I for a particular nucleus

Answer 8

Spins of protons or neutrons can pair up and spins cancel
Protons do not pair up with neutrons. It is not possible to predict the value of I only whether it is zero, half integer or integer

Question 9

Describe nuclei with odd number of protons and or neutrons

Answer 9

Nuclei with odd number of protons and neutrons have a total angular momentum that is not zero therefore they have spin
They are therefore nmr active

Question 10

Describe nuclei with even number of protons and or neutrons

Answer 10

Nuclei with even number of protons and neutrons do not possess spin and are nmr inactive therefore cannot be detected

Question 11

Define isotope specific

Answer 11

Different isotopes of the same elements have different spins

Question 12

What does spin of a nucleus produce

Answer 12

Spin of nucleus creates movement of electrical charge which produces a magnetic field

Question 13

What does a nucleus with non zero spin have

Answer 13

A nucleus with non zero spin has a magnetic moment (mu)
Magnetic moment is a vector quantity quantised in the same way as I

Question 14

How do you calculate magnetic moment (mu)

Answer 14

Magnetic moment = magnetogyric ratio * I
Where I is angular momentum

Question 15

What is the magnetogyric ratio

Answer 15

Proportionality constant specific to each nucleus

Question 16

Describe nuclear spins in absence of an applied field

Answer 16

In absence of an applied magnetic field, nuclear spins are randomised
2I+1 spin states are degenerate

Question 17

Describe nuclear spins in presence of an external magnetic field

Answer 17

In the presence of an external magnetic field the 2I+1 spin states have different energies. The spins either align or oppose withe the magnetic field

Question 18

What is B0

Answer 18

B0 is the applied external magnetic field

Question 19

What are quantum restrictions

Answer 19

Quantum restrictions prevent spins aligning exactly with B0

Question 20

What do energy difference between spin states depend on

Answer 20

External magnetic field and magnetogyric ratio

Question 21

How do we calculate the energy difference between spin states

Answer 21

E=hv=(h/2pi)magnetogyric ratioB0

Question 22

Define resonant frequency

Answer 22

Resonant frequency is the frequency corresponding to energy of transition

Question 23

How do you calculate resonant frequency

Answer 23

(Magnetogyric ratio*B0)/2pi

Question 24

What are the selection rules in terms of energy levels

Answer 24

For a given B0 the spacing of E-levels are equal.
Change in magnetic quantum number must be plus or minus 1

Question 25

What happens to the nucleus in an applied magnetic field

Answer 25

In an applied magnetic field the nucleus processes about it own axis of spin, the frequency of precession is equal to the resonant frequency

Question 26

What happens upon application of radio frequency radiation of the same frequency to the nucleus

Answer 26

Causes resonance in the nucleus and energy can be absorbed and transitions between the spin states can occur

Question 27

What leads to population of higher energy states

Answer 27

Thermal motion

Question 28

What does the Boltzmann distributions tell us

Answer 28

Population difference between energy levels

Question 29

What is the equations for Boltzmann distribution

Answer 29

N upper/ N lower =e^deltaE/kt Where k is the Boltzmann constant

Question 30

What induces transitions between spin states

Answer 30

Irradiation of sample in external magnetic field at larmor frequency induces transitions between spin states

Question 31

Describe the probability of excitation

Answer 31

The probability of excitation (lower to upper) is the same as the probability of emission (upper to lower)

Question 32

What does detection of transitions rely on

Answer 32

Detection of transitions relies on very small population difference between energy levels. Ie the intensity of NMR signals depends on difference between the number of absorptions and emissions

Question 33

What does sensitivity for NMR depend on

Answer 33

Sensitivity greatest for nuclei with large magnetogyric ration Sensitivity increases with increasing B0 and increases with decreasing temp

Question 34

What is relaxation in terms of NMR

Answer 34

After excitation spins relax to re establish Boltzmann distributions. In absence of relaxation populations of spin states equal so no NMR signals detected so spins become saturated

Question 35

What are the two distinct relaxation processes

Answer 35

Spin lattice relaxation and spin spin relaxation

Question 36

Outline the spin lattice relaxation process

Answer 36

Loss of energy to surrounding eg solvent molecules, wall of nmr tube via tumbling and vibrations Tumbling at rate similar to larmor frequency creates oscillating field with enables relaxation

Question 37

Outline spin spin relaxation

Answer 37

Energy exchange between nuclear spins leads to randomisation of spins but no net loss of energy

Question 38

Describe the effect of relaxation on linewidths

Answer 38

Relaxation of nuclear spins are generally slow so linewidths are narrow, but in some cases relaxation of nuclear spins are very fast and peaks so broad they are difficult to observe

Question 39

How do we measure linewidths

Answer 39

Measurement of linewidths. Distance between half height on peak

Question 40

Why is spectroscopy at higher frequencies much more sensitive

Answer 40

Higher energy photons are easier to detect

Question 41

How do we optimise signal strengths in nmr

Answer 41

By using strong magnetic field to maximise delta E