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NMR Flashcards

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1
Q

What are the chemical applications for NMR

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2
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What are the medical applications for NMR

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3
Q

What is nuclear spin

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4
Q

What would the net nuclear spin be for 2 protons / neutrons in 1 energy level with spin up and down be

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5
Q

What would the net spin be if only one spin state of each particle exists in the same energy level (eg two spin up)

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6
Q

What would the net spin be if there is two spins that cancel out but one spin of another type

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7
Q

What are all the possibilities of spin

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8
Q

What two states could the nuclei be in in NMR

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9
Q

What would happen if EM radiation is supplied to a nuclei

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10
Q

What are the frequently used nuclei in NMR

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11
Q

What is larmor precession*

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Larmor precession refers to the precession of the magnetic moments of particles, such as electrons or nuclei, in a magnetic field. When a magnetic moment is placed in an external magnetic field, it experiences a torque that causes it to precess around the direction of the field.

The frequency of this precession, known as the Larmor frequency, depends on the strength of the magnetic field and the characteristics of the magnetic moment. This phenomenon is important in various fields, including magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), and quantum mechanics, as it describes how spins behave in a magnetic field.

Precession is a change in the orientation of the rotational axis of a rotating body

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12
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What does the new process of NMR involve

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13
Q

When would NMR signal (absorption) occur occur
What happens when a magnetic field is applied to the sample

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Bottom cone because only some molecules will have anti parallel spin
Top cone because most molecules will have parallel spin to external magnetic field

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14
Q

What would happen if the radio frequency is applied at a 90 degree angle to the net magnetisation

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15
Q

What two things would happen when the short pulse is applied to the net magnetisation

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16
Q

What is the free induction decay and what is it generated by

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refers to the observable signal generated by nuclear spins after they are excited by a radiofrequency (RF) pulse. The FID is the raw data collected in NMR experiments, representing how the nuclear magnetization decays over time after the excitation pulse.

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17
Q

What are the parameters of the NMR spectrum

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18
Q

Why won’t every proton give the same NMR peak

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19
Q

What factors would increase/decrease shielding of the nucleus from external magnetic field

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20
Q

How is chemical shift measured

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TMS is v electropositive

Less e- around nucleus = less shielding = higher frequency

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21
Q

When can there be chemical shift equivalence between protons

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22
Q

What can chemical shift ranges be used for

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23
Q

What are integrals

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24
Q

How can the spins of neighbouring atoms affect the chemical shift of a particular atom

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25
Q

What do the different patterns mean

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26
Q

What are the typical coupling values for different structures

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27
Q

How can you identify a molecule with NMR

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28
Q

How would groups with readily exchangeable protons show up on an NMR spectrum

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29
Q

What 2 things happen as the net magnetisation (M0) returns to its original position (in an oscillating manner)

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Spin-lattice relaxation
Spin-spin relaxation

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30
Q

What is spin-lattice relaxation

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31
Q

What is spin-spin relaxation

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32
Q

What is the relaxation time

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33
Q

Explain this graph

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34
Q

What are dipolar couplings

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35
Q

What is the nuclear overhauser effect

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36
Q

What are the problems with carbon 13 NMR

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37
Q

What are the advantages of carbon 13 NMR

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38
Q

What are the 3 types of carbon 13 spectra*

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Proton-Decoupled Spectrum: Simplified, showing all carbons as singlets.
Off-Resonance Decoupled Spectrum: Shows limited splitting, with carbon signals split based on the number of attached protons.
DEPT Spectrum: Provides specific information about how many hydrogens are attached to each carbon and can distinguish between different types of carbon-hydrogen environments (CH₃, CH₂, CH).

39
Q

What is basic C13 spectra

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40
Q

What is decoupling and what are the 2 types*

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.

41
Q

What is the problem with NMR of peptides and proteins and now is this problem overcome + solution

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42
Q

What happens in 1D NMR

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It focuses on the interactions of nuclear spins, typically protons (¹H) and carbons (¹³C), to provide information about the molecular structure

Chemical Shifts: The position of the peaks along the x-axis (chemical shift scale) indicates the electronic environment of the nuclei, providing insight into the type of chemical groups present. For instance, protons in different environments (like methyl, methylene, or aromatic protons) resonate at different frequencies.
Multiplicity: The splitting patterns (multiplicity) of the peaks provide information about the number of neighboring hydrogen atoms (n + 1 rule), helping to deduce the connectivity of atoms in the molecule.
Integration: The area under the peaks correlates with the number of protons contributing to each signal. This allows quantification of the relative number of hydrogen atoms in different environments

43
Q

What happens in 2D NMR

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It provides more comprehensive data compared to traditional one-dimensional (1D) NMR by allowing the observation of interactions between different nuclei (like hydrogen and carbon) in a molecule

In 2D NMR, the signals are spread out in two dimensions instead of just one. This allows for the simultaneous observation of multiple interactions, which can provide a more complete picture of the molecular structure.
The two axes of the spectrum typically represent different nuclear spins (e.g., protons on one axis and carbons on another) or different interactions (such as chemical shifts or coupling constants

44
Q

What is correlation spectroscopy

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45
Q

What is nuclear overhauser effect spectroscopy

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46
Q

What other types of 2D correlation NMR are there

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47
Q

Whst is HMQC NMR

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is a type of NMR (Nuclear Magnetic Resonance) experiment that allows researchers to identify correlations between hydrogen atoms (¹H) and carbon atoms (¹³C) that are directly bonded to each other, typically through one bond
One peak for each proton-carbon pair

48
Q

What is HMBC NMR

A

used to determine the connectivity between atoms in a molecule, specifically between hydrogen atoms (¹H) and carbon atoms (¹³C) that are separated by two to four bonds. It is particularly useful for figuring out which carbon atoms are connected to which hydrogen atoms when they are not directly bonded, helping to piece together the structure of complex organic molecules.

49
Q

How do you calculate difference in energy between energy levels

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50
Q

Summarise the different types of 2D NMR

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51
Q

How can dipolar couplings occur

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52
Q

What does the area under peaks on NMR mean

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53
Q

Why are more spins parallel at equilibrium

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54
Q

What does the strength of the external magnetic field determine in NMR

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55
Q

What is nuclear spin

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56
Q

What was the old method of recording NMR spectrum

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57
Q

How does shielding affect nuclear magnetic field

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58
Q

How does the nuclear magnetic field of aromatic compounds differ

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59
Q

What is the lamor precession of nuclear spins

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60
Q

What is net magnetisation

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61
Q

What is the effect of a radio frequency pulse on a sample

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62
Q

What are short radio frequency pulses made up of

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63
Q

What is pulsed Fourier transform NMR

A

In pulsed NMR, instead of sweeping the frequency of the radiofrequency (RF) signal continuously (as in continuous wave NMR), a short, intense burst of RF energy is applied to the sample. This burst, known as an RF pulse, excites all nuclear spins in the sample at once across a range of frequencies.

A common pulse in NMR is a 90-degree pulse, which flips the nuclear magnetization from its equilibrium position (along the z-axis in the laboratory frame) to the xy-plane. Other pulse angles, like 180-degree or 45-degree pulses, can also be used depending on the experiment.

The pulse width and power determine the angle by which the nuclear spins are rotated

After the RF pulse is applied, the nuclear spins start to relax back to their equilibrium positions. During this process, the nuclei emit a weak RF signal called the Free Induction Decay (FID). The FID contains information about all the frequencies at which the nuclei resonate, but it’s recorded in the time domain as a decaying oscillation.

The Fourier transform decomposes the FID into its constituent frequencies, yielding the familiar NMR spectrum where the x-axis corresponds to frequency (or chemical shift) and the y-axis shows signal intensity.
Peaks in the spectrum correspond to specific resonances of different nuclei in the sample.

64
Q

What are the scalar coupling patterns

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65
Q

What is meant by geminal coupling

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66
Q

What are the possible alignments for the magnetic moment of protons

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67
Q

What would happen to the signal for the purple proton if the magnetic moment if the red proton aligns with the external magnetic field

(Scenario: two non equivalent protons on neighbouring carbons )

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68
Q

What would happen to the signal for the purple proton if the magnetic moment if the red proton aligns against the external magnetic field (scenario: two non equivalent protons on the same carbon )

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69
Q

For a molecule with two equivalent protons and one adjacent proton:
What would happen to the purple protons signal if both of the equivalent ( red) protons magnetic fields aligned with the external magnetic field

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70
Q

For a molecule with two equivalent protons and one adjacent proton:
What would happen to the the purple protons signal if both of the equivalent ( red) protons magnetic fields aligned against the external magnetic field

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71
Q

For a molecule with two equivalent protons and one adjacent proton:
What would happen for the purple protons signal if one of the equivalent ( red) protons magnetic fields aligned with the external magnetic field and the other against the external magnetic field

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72
Q

What would happen to the signal for the red equivalent protons if the purple protons magnetic field aligns with the external magnetic field

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73
Q

Explain the N + 1 rule

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74
Q

What is meant by source spin

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75
Q

What is meant by interesting spin

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76
Q

What different states could the interesting and source spins be in
What is meant by the active and passive spin

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77
Q

What is meant by population difference

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78
Q

What is meant by saturation

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79
Q

What happens when the Rf pulse is switched off

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80
Q

What is double quantum transition (NOE)

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81
Q

What is meant by zero order quantum transition

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82
Q

What is NOESY

A

NOESY (Nuclear Overhauser Effect Spectroscopy) is a two-dimensional (2D) NMR technique that provides information about spatial proximity between nuclei in a molecule. It’s particularly useful for determining 3D structures of molecules, especially in large biomolecules like proteins and nucleic acids. In a NOESY spectrum, cross-peaks indicate through-space interactions between nuclei that are close to each other, typically within 5 Å (angstroms)

83
Q

What are the axis for a NOESY spectrum

A

A NOESY spectrum is a 2D plot with two frequency axes (usually in parts per million, ppm):

The x-axis corresponds to the chemical shifts of one set of nuclei.
The y-axis corresponds to the chemical shifts of the same or another set of nuclei.

84
Q

What do the diagonal and cross peaks mean in a NOESY spectrum

A

The diagonal of the spectrum contains peaks where the chemical shifts on the x-axis and y-axis are the same. These are called diagonal peaks and correspond to the 1D NMR signals of individual nuclei.

Diagonal Peaks: These are auto-correlation signals, representing nuclei interacting with themselves (i.e., no spatial interaction between different nuclei).
Cross-Peaks: These appear off the diagonal and represent NOE interactions between nuclei that are spatially close but not chemically bonded (represents through space interactions between two protons that are spatially close)

85
Q

How do you interpret a NOESY spectrum

A

Step 1: Identify Diagonal Peaks:

Locate the diagonal line, where the x-axis equals the y-axis. Each peak on this line represents the chemical shift of a proton in the molecule, corresponding to the 1D NMR spectrum.
Step 2: Look for Cross-Peaks:

Cross-peaks off the diagonal show interactions between protons that are close in space but not directly bonded (through-space NOE). These are the key to determining spatial relationships.
Step 3: Correlate Cross-Peaks with Protons:

Identify which protons the cross-peaks correspond to by comparing the chemical shifts of the peaks on both axes. For example, if a cross-peak appears at the intersection of chemical shifts of proton A on the x-axis and proton B on the y-axis, it indicates that protons A and B are spatially close.

86
Q

What is a COSY spectrum

A

COSY (Correlation Spectroscopy) is a two-dimensional (2D) NMR technique used to identify through-bond couplings between nuclei, typically protons. In a COSY spectrum, cross-peaks indicate scalar (J) couplings between nuclei, providing information about the connectivity between atoms.

87
Q

What do the x and y axis in a COSY spectrum correspond to

A

The x-axis and y-axis both correspond to the chemical shifts (usually in parts per million, ppm) of the same set of nuclei, typically protons.
The spectrum is symmetric along the diagonal line, where the chemical shift of a nucleus on the x-axis equals its shift on the y-axis. This diagonal contains auto-correlation peaks (diagonal peaks), representing each proton’s own signal.

88
Q

What do the cross peaks and diagonal peaks indicate in a COSY spectrum

A

Cross-peaks appear off the diagonal, indicating correlations (couplings) between different protons that are through-bond coupled. (Indicate scalar coupling)

The diagonal peaks in a COSY spectrum are the same as what you would see in a 1D NMR spectrum. Each proton shows up as a peak along the diagonal.
These diagonal peaks help you locate where each proton’s chemical shift is on both axes. They serve as reference points for finding cross-peaks.

89
Q

How do you interpret a COSY spectrum

A

Step 1: Identify the Diagonal Peaks:

Find the diagonal peaks that correspond to individual protons (chemical shifts) in the molecule. These diagonal peaks represent the same information you would get from a 1D NMR spectrum.
Step 2: Look for Cross-Peaks:

Cross-peaks indicate which protons are coupled to each other through bonds. If a cross-peak exists between proton A (at position δA on the x-axis) and proton B (at position delta B on the y-axis), it means A and B are coupled via bonds, typically 2 or 3 bonds apart.
Step 3: Determine Coupling Relationships:

The cross-peaks tell you which protons are connected by chemical bonds. For example, if proton A shows a cross-peak with proton B, it suggests that proton A is directly bonded (or two bonds away) from proton B. This is useful for piecing together the structure of the molecule.

Structural Biology (8 decks)

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