NMR 1- The basics of the NMR phenomenon

Question 1

What is the definition of NMR?

Answer 1

It is the observation of the behavior of nuclear spins in magnetic fields and how they resonate with electromagnetic radiation.

Question 2

What is the nuclear spin quantum number?

Answer 2

It describes the angular momentum (spin) of a nucleus, arising from the spins and motions of protons and neutron

Question 3

What is the nuclear spin quantum number of deuterium?

Answer 3

I=1

Question 4

What is said about the nuclear spin quantum number of compunds with even or odd mass?

Answer 4

• Compounds with even mass has I as a whole number
• Compounds with odd mass cant have I as a whole number
• Compounds with even mass and even charge has I=0

Question 5

Name some atoms who are NMR-inactive

Answer 5

C_6^12,O_8^16, S_16^32
They are NMR-inactive because they have an nuclear spin of 0.

Question 6

Which nuclear spin quantum numbers are best for NMR?

Answer 6

I=1/2 are good, I=0 are bad and I=1 or more are also not ideal

Question 7

Which atoms have a nuclear spin quantum number of I=1/2?

Answer 7

H^1, C^13;N^15, F^19, P^11, Ag^109

Question 8

Which isotopes have a nuclear spin quantum number of I=1 or larger?

Answer 8

N_7^14, O^17

Question 9

Which isotope is possible to observe even though their I=1

Answer 9

Deuterium (H^2)

Question 10

47Ti has a nuclear spin quantum number of:
0, I∈ {1⁄2,3⁄2,5⁄2….},
I ∈ {1,2,3,…}

Answer 10

I∈ {1⁄2,3⁄2,5⁄2….}

Question 11

Atomic nuclei contain…. (select all correct answers, multiple answers allowed):
protons, electrons or neutrons

Answer 11

Protons and neutrons

Question 12

Atoms with the same number of protons, but different number of neutrons in their nuclei are called:
isomers, isotopes, isologues, homologues, orthologs or analogs

Answer 12

isotopes

Question 13

Ytterbium (element no. 70) has several naturally occurring isotopes.
168Yb abundance: 0.1% I=0

170Yb abundance:3.0% I=0

171Yb abundance:14.0% I=1/2

172Yb abundance:21.7% I=0

173Yb abundance:16.1% I=5/2

174Yb abundance:31.9% I=0

176Yb abundance:12.89% I=0
Which of the isotopes is best suited for liquid-state-NMR (only one answer allowed)?
168Yb

170Yb

171Yb

172Yb

173Yb

174Yb

176Yb

Answer 13

171Yb

Question 14

What is the main purpose of NMR?

Answer 14

To determine structures of molecules

Question 15

Which axis in NMR describes the magnetic field?

Answer 15

The z-axis

Question 16

What is the formula of the magnetic quantum number?

Answer 16

P_z=m∗ℏ

Question 17

What is the nuclear angular momentum?

Answer 17

• It is the total intrinsic angular momentum (spin) of a nucleus, resulting from the combined spins and orbital motions of protons and neutrons. It is described by the nuclear spin quantum number III, with magnitude:
  |I->|=√(I*(I+1) )∗ℏ

Question 18

What is the formula for determining ℏ

Answer 18

h/(2∗π) h is placks constant

Question 19

What is the magnetic moment?

Answer 19

• The magnetic moment is a measure of how strongly a particle or object (like an electron, proton, or nucleus) interacts with a magnetic field. It’s essentially the strength and orientation of a tiny magnet associated with a particle’s spin and/or motion.
  It is given by μ->=γ∗I->
  γ=gyromagnetic ratio (a constant specific to eachparticle)
  I ->=nuclear spin angular momentum

Question 20

What is the formula of the z-component of the magnetic moment?

Answer 20

(μ_z ->)=m∗γ∗ℏ
Here m is the magnetic quantum number

Question 21

Which conditions/values are allowed for the magnetic moment for the nuclear spin quantum number of I=1/2 and I=1?

Answer 21

• At I=1/2 m=-1/2 to +1/2
  -At I=1 m=-1, 0, +1

Question 22

What happens to spin energy levels in a stronger magnetic field?

Answer 22

The energy difference between spin states increases, and the nucleus requires higher frequency radiation to transition between them (higher Larmor frequency).

Question 23

What is the SI-Unit for the magnetic flux density?
Henry, Tesla or Siemens

Answer 23

Tesla

Question 24

Find the nuclear spin quantum number of 10B

Answer 24

3

Question 25

How many different orientations can the magnetic moment of 10B adapt in an external magnetic field?

Answer 25

7

Question 26

What is the energy difference between two neighbouring orientations for one mole of 10B nuclei at B0=14.1 T?

Answer 26

0.0257 J/mol

Question 27

Choose the correct term: The distribution of nuclear spins on the different orientational states follows a .... Gaussian distribution Laplace distribution Student's distribution Boltzmann distribution Binomial distribution Poisson distribution

Answer 27

Boltzmann distribution

Question 28

How does FT-NMR work?

Answer 28

It contains of three steps: - The sample is placed in a magnetic field and excited by an RF pulse. - The emitted signal (FID) is recorded in the time domain. - A Fourier Transform is applied to convert it into a frequency-domain spectrum.

Question 29

What is necessary to do for NMR to work?

Answer 29

You must place the sample in a strong magnetic field, apply radiofrequency pulses to excite the nuclei, and detect the resulting signal to generate an NMR spectrum.

Question 30

What is a advatage of FT-NMR?

Answer 30

That you can measure all frequencies in a sample at the same time

Question 31

What is the formula of the Larmor-frequency or precession?

Answer 31

v=γ/2π∗B It describes the frequency at which nuclei precess and must be matched by an RF pulse to excite the spins in NMR.

Question 32

Describe T1-relaxation

Answer 32

T₁-relaxation (also called longitudinal relaxation) is the process by which the net magnetization vector returns to its equilibrium along the z-axis after being disturbed by an RF pulse. The exponent is e^(−(T/T_1 ))

Question 33

Describe T2-relaxation

Answer 33

T₂-relaxation is when the signal in the xy-plane gets weaker over time because the spins fall out of sync. This causes the detected NMR signal to fade. It follows the exponential decay e^(−(T/T_2 ) )

Question 34

When a sample containing a large number of nuclear spins is brought into a magnetic field, the z-components of their nuclear magnetic moments will point: parallel to the magnetic field anti-parallel to the magnetic field 50:50 parallel and anti-parallel to the magnetic field 50-51% parallel and 49-50% anti-parallel to the magnetic field 51-55% parallel and 45-50% anti-parallel to the magnetic field 55-65% parallel and 35-40% anti-parallel to the magnetic field 65-75% parallel and 25-35% anti-parallel to the magnetic field 75-99% parallel and 1-25% anti-parallel to the magnetic field

Answer 34

50-51% parallel and 49-50% anti-parallel to the magnetic field

Question 35

What is the resonance frequency of 13C at a magnetic field strength of 18.8 T?

Answer 35

201, use the formula ν= (γ/(2π))⋅B_0 ​

Question 36

Choose the correct continuation of the sentence: The radiofrequency used to excite the nuclear magnetic moments has a Magnetic Field Component... ...parallel to the external magnetic field ...perpendicular to the external magnetic field ...antiparallel to the external magnetic field

Answer 36

...perpendicular to the external magnetic field

Question 37

Choose the correct word: The external magnetic field is referred to as.. B0, B1, B2 it is... always on, switched on to excite the nuclear magnetic moments, switched on and off periodcally

Answer 37

B0 and always on

Question 38

Is this statement true or false: "All atoms of an element in a molecule (e.g. all 1H-atoms) will have the exact same resonance frequency." true or false

Answer 38

False

Question 39

Which of the following statements is true? (more/less than one possible): The integral value of a signal corresponds to the number of atoms that this signal comes from. The integral value of a signal is proportional to the number of atoms that this signal comes from. The ratio between integrals of different signals corresponds to the ratio of the number of atoms belonging to those signals.

Answer 39

The integral value of a signal is proportional to the number of atoms that this signal comes from. The ratio between integrals of different signals corresponds to the ratio of the number of atoms belonging to those signals.

Question 40

What is the chemical shift and why is it important for NMR?

Answer 40

Chemical shift is the difference in resonance frequency of a nucleus compared to a reference compound. It reflects how the local electron environment affects the magnetic field experienced by the nucleus. It is important because it gives information about where a nucleus is in a molecule and it allows us to identify functional groups, determine structure, and distinguish between compounds.

Question 41

Describe the inductive effect (I-effect)

Answer 41

It’s the transmission of electron-withdrawing or donating effects through sigma bonds caused by electronegativity differences, affecting electron density and molecular properties.

Question 42

What is the formula for the ratio of spin in the parallel and antiparallel state (the Boltzmann distribution)?

Answer 42

N_β/N_α =e^(−ΔE/kT) - N_α, number of nuclei in the lower energy state (parallel) - N_β, Number of nuclei in the higher energy state (antiparallel) - ΔE, energy difference between the 2 spin conditions - k, Boltsmans constant (1.380649×10−23J/K) - T, temperature

Question 43

What is the formula for determining the energy difference?

Answer 43

ΔE=h∗γ/2π B - h, plancks constant (6.626Ø10^-34 J*s) - γ,gyromagnetic ratio - B_0, the strength of the magnetic field (in Tesla)