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Medicine sem 1 > NMR > Flashcards

NMR Flashcards

1
Q

What is the frequency of NMR?

A

300-900 MHz

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2
Q

What magnetic properties do atomic nuclei have?

A

Nuclear spin and magnetic moment. This is because protons in the nucleus have a positive charge

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3
Q

What does the magnetic moment cause the nucleus to do?

A

Interact with an external magnetic field and re-align itself either with or against the field

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4
Q

What does magnetic moment depend on?

A

Depends on quantum properties of the atom.

Any atom with an odd number of neutrons and/or an odd number of protons have magnetic properties

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5
Q

What nuclei are not magnetically active and can never exhibit an NMR effect?

A

Nuclei with even mass numbers AND even atomic (proton) number (I=0)

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6
Q

What spin does 1H, 3H, 13C, 15N etc have?

A

1/2

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7
Q

What spin does 16O have?

A

0!!

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8
Q

What spin do nuclei with even mass numbers and odd atomic (proton) numbers have?

A

whole number: 1,2,3 etc

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9
Q

What spin do nuclei with odd mass numbers have?

A

1/2 , 1/3 etc

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10
Q

A nucleus with spin 1/2 can…

A

adopt 2 possible states, aligned with or against the field

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11
Q

What does alignment with the field require?

A

LESS energy, therefore the majority of spins align with the field

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12
Q

What will the number of nuclei in the low energy state (Nalpha) and the number in the high energy state (Nbeta) differ by?

A

An amount determined by the boltzmann distribution

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13
Q

What is the difference between protons in a medium field?

A

about 1 in 10^5

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14
Q

How can you increase magnetic resonance sensitivity?

A

by increasing magnetic field strength

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15
Q

What effect does irradiating the sample at the resonance frequency have?

A

disturbs the proportions of nuclei in each energy level, putting more into the higher energy level

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16
Q

How can transition from the lower energy level to the higher energy level occur?

A

By absorption of radio-frequency radiation of the correct frequency

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17
Q

What is the relationship between radio frequency and magnetic field strength?

A

Linear

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18
Q

What does NMR measure?

A

the absorption of radio-frequency radiation resulting in changes in nuclear spin orientation

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19
Q

When does NMR only occur?

A

when a sample is in a strong magnetic field

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20
Q

Describe a modern NMR instrument

A
  • The sample (~5-20mg) is dissolved in deuterated solvent (0.6 – 0.8 ml)
  • Placed in an NMR tube
  • Inserted into the NMR magnet
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21
Q

What is the Fourier Transformation (FT) experiment?

A
  • During a FT experiment, the sample is irradiated with a short pulse of a broad range of radiofrequency radiation
  • This results in simultaneous excitation of all nuclei of the sample
  • Nuclei relax back into a lower energy state releasing a pulse of electromagnetic radiation
  • The NMR spectrometer detects this radiation received from the sample
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22
Q

What is the net result of FT

A

Interfering wave patterns of differing frequencies –> Free Induction Decay (FID)

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23
Q

What is FID?

A

a plot of signal intensity against time?

24
Q

What does FT convert spectrum into?

A

converts from units of time to units of frequency

25
Q

In NMR spectra, what does the number of peaks correspond to?

A

The number of non-equivalent protons

26
Q

What does the position of the NMR signals indicate?

A

The electromagnetic environment of the protons which produced this signal

27
Q

What is the area under each NMR signal (the integral) proportional to?

A

the number of protons responsible for this signal. e.g. if AUC=3 , then there are 3 protons e.g. CH3

28
Q

What does the shape of each signal depend on?

A

Whether the proton is exchangeable or non-exchangeable

29
Q

What makes protons ‘equivalent’?

A

If they have the same electromagnetic environment, they are equivalent

30
Q

What is the quantum of energy required to promote nuclei to a higher energy level regarding protons?

A

the SAME for each proton

31
Q

What does resonance frequency depend on?

A
  • Depends on the chemical environment of the nucleus
  • Varies according to the structural factors of the molecule
  • Therefore, different nuclei of the same atom type (e.g. 1H) in different molecules exhibit different absorption frequencies
32
Q

What induces LOCAL magnetic field?

A

electrons surrounding the nucleolus

33
Q

What effect does LOCAL magnetic field have on magnetic field experienced by the nucleolus?

A

it is DECREASED - as the nucleolus is shielded

34
Q

What is the effect of electron density on local magnetic field?

A

The STRONGER electron density, the STRONGER the local magnetic field induced by electrons

35
Q

What do surrounding electrons generate?

A

A small induced field opposite to external field Bo

This results in a shielding effect. Therefore the nuclei are partially shielded from external magnetic field Bo

36
Q

What is chemical shift?

A

• The variation of resonance frequency V0 with chemical environment of the nucleus

37
Q

What are differences in resonance frequency much greater than?

A

resonance line widths –> therefore, distinct resonance lines are observed for chemically different nuclei

38
Q

What scale is used for chemical shift and why?

A

ppm (part per million)
• Resonance frequency of particular nucleus depends on the value of B0 in the instrument used:
ν = γB0/2π (Hz)
• Therefore frequency comparison for different NMR spectrometers is of no value
• The idea is to make frequencies INDEPENDENT of a strength of magnetic field

39
Q

What is used as a reference signal for chemical shift?

A

TMS (tetramethyl silane (CH3)4Si) as it is totally symmetrical, so a very intense signal

40
Q

What is the resonance frequency of the reference signal?

A

ZERO

41
Q

What chemical shift do nuclei LESS shielded than the reference have?

A

POSITIVE chemical shift

42
Q

What do nuclei MORE shielded than the reference have?

A

NEGATIVE chemical shift

43
Q

What effect will an electron-donating group attached to the atom have on shielding?

A

Will result in INCREASE of electron density surrounding the nucleus, having shielding effects, decrease in B’ and lower resonance energy

44
Q

What is B’

A

the reduced field experienced by the nucleus

45
Q

What effect will an electron-withdrawing effect attached to the atom have on shielding

A

Will result in DECREASE of electron density surrounding the nucleus, having de-shielding effects, increase in B’ and higher resonance frequency resulting in a larger chemical shift

46
Q

What will the H within an aromatic ring be effected by?

A

Will be highly de-shielded because aromatic ring is unsaturated

47
Q

What will the H on an aldehyde by effected by?

A

Will be highly de-shielded because its directly attached to carbonyl group which grabs electrons

48
Q

what do electronegative groups do to NMR signals?

A

They are “deshielding” and tend to move NMR signals from neighbouring protons further downfield (to higher ppm values)

49
Q

What do the π-system of alkenes, aromatic compounds and carbonyls do to NMR signals?

A

They strongly deshield attached protons and move them “downfield” to higher ppm values

50
Q

What are the key differences between Proton and carbon NMR?

A
  • Abundance: Natural abundance of 13C is 1.11% (Natural abundance of 1H is 99.98%)
  • Sensitivity: 13C NMR is much less sensitive than 1H NMR
  • Resonance frequency: At the same field strength 13C resonate at ¼ the frequency of 1H
  • Chemical shift: The chemical shift range for the 13C is ~20 times that of the equivalent 1H nuclei
51
Q

What does the multiplet structure of signals arise from?

A

An interaction between neighbouring nuclei (i.e. spin-spin coupling)
These interactions are transmitted by electrons through intermediate bonds

52
Q

What is the magnitude of the splitting called?

A

spin-spin coupling constant, J

Spin-spin coupling is usually restricted to a maximum of three bonds between the atoms which are coupled

53
Q

Coupling constant J is:

A

 independent of the magnetic field strength
 extremely variable (ranges from 0.05Hz up to kHz)
 dependent upon the number of bonds between the coupling nuclei and their molecular environment

54
Q

What are the types of coupling constants?

A

• Geminal coupling constant, 2J:
H-C-H  proton separated by 2 chemical bonds (etc)
• Vicinal coupling constant, 3J:
H-C-C-H
• Long-range coupling constants, 4J and 5J:
H-C-C-N-H H-C-X-Y-C-H

55
Q

Do chemically and magnetically equivalent protons couple to one another ?

A

NO

56
Q

What is the strongest coupling constant?

A

Geminal coupling constant
Interproton spin-spin coupling is largest between geminal protons
• It depends upon the angle between the two C-H bonds
• 2J is typically between 12 and 20 Hz and negative in sign

57
Q

What does vicinal coupling constant depend on?

A
  • It depends upon torsion angle between the two protons

* Magnitude of 3J ranges from 2Hz to 12 Hz and positive in sign

Medicine sem 1 (21 decks)

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