Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

CH20150 > NMR Spectroscopy > Flashcards

NMR Spectroscopy Flashcards

(86 cards)

Start Studying
1
Q

What can NMR be used for?

A

Identifying molecules, MRI scans, cancer diagnosis and surgery.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the spin if there is an odd atomic number and odd atomic mass?

A

Spin half (1/2)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the spin if there is an even atomic number and odd atomic mass?

A

Spin half (1/2)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the spin if there is an odd atomic number and even atomic mass?

A

Spin integer.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the spin if both the atomic number and atomic mass is even?

A

Spin zero.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the charge distribution of a nucleus with spin half?

A

Spherical charge distribution.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the charge distribution of a nucleus with a spin greater than or equal to 1?

A

Non-spherical charge distribution.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is precession?

A

SLow movement of axis of a spinning body.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What does precession occur at?

A

The Larmor frequency, v.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the equation for the Larmor frequency?

A

v = γB0/2π

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is B0?

A

The applied magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the significance of the Larmor frequency?

A

It is the origin of chemical shift, and is the frequency at which the protons resonate at.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the equation of the number of energy states a nucleus has?

A

2I + 1, e.g. for hydrogen, (1/2 x 2) + 1 = 2.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the equation for the separation of the two energy states in HNMR?

A

ΔE = hv, so ΔE = h(γB0/2π)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens if an electromagnetic frequency equal to the Larmor frequency is applied?

A

The nucleus resonates.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What does it mean that a nucleus resonates?

A

Usually, most spins are in the lower energy state. When this EM frequency is applied, there is a transition between the spin up and spin down (low spin and high spin) energy state, which results in a signal on the NMR spectrum.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the equation for chemical shift, δ?

A

Shift, δ = v (Hz)/ Spectrometer frequency (MHz)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Why is chemical shift measured in parts per million?

A

The equation for chemical shift is given in Hz per million Hz (MHz), “Hz per million Hz”.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How can resolution be increased?

A

Increasing the spectrometer frequency.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is resolution?

A

Ability to distinguish two peaks.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What does having a stronger magnet result in?

A

Greater sensitivity and resolution - need less sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What happens if electrons are placed in a magnetic field?

A

They will circulate and generate an electric current.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What does electric current induce?

A

A magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

How does the induced magnetic field compare to the applied magnetic field?

A

It opposes it.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
What happens to the applied magnetic field the nucleus feels in an area of high electron density?
There is a greater induced magnetic field and the electrons are SHIELDED from the applied magnetic field.
26
What happens to the applied magnetic field the nucleus feels in an area of low electron density?
There is a reduced induced magnetic field and the electrons are less shielding. The nucleus feels more of the applied magnetic field and is DESHIELDED.
27
What happens if the applied magnetic field is decreased?
The Larmor frequency is decreased and the chemical shift decreases.
28
What are the intramolecular factors affecting chemical shift?
Inductive effects, anisotropy, resonance, Van der Waals forces and isotope effects.
29
What are the intermolecular factors affecting chemical shift?
Temperature, hydrogen bonds and solvent.
30
What are inductive effects?
When the nucleus is bound to something electron withdrawing - these groups remove electron density and result in DESHIELDING. The chemical shift moves to a higher frequency.
31
What is an isotropic effect?
Influence is exerted in all spatial directions.
32
What is an anisotropic effect?
Influence is exerted non-uniformly - it has direction.
33
What is the case of anisotropy in double bonds?
Above and below the pi bond is shielded, whereas to the side is deshielded.
34
What is the case of anisotropy in triple bonds?
Atoms directly attached to the triple bond are in the plane of shielding - the induced magnetic field opposes the applied magnetic field.
35
When can anisotropy be seen in sigma bonds?
In locked conformations (e.g. rigid six membered rings) as the effect is cancelled out by the movement of alkyl groups usually.
36
What is seen with the shift of a proton attached to a double bond with an C=O group adjacent?
There is deshielding (increased chemical shift) due to the positive charge on the carbon that can be seen in the resonance form.
37
When are two protons shift equivalent?
If they are interchangable - by a rapid process or by a symmetry operation.
38
What is a symmetry operation?
The movement of an object such that the object is left indistinguishable from the original.
39
What is a C2 axis?
The molecule can be rotated 180 degrees around an axis and is indistinguishable.
40
What are homotopic protons?
Protons that have an identical chemical shift in achiral and chiral solvents.
41
What is inversion symmetry?
If the molecule is reflected in an internal plane of symmetry to give an indistinguishable molecule.
42
What are enantiotopic protons?
Protons that have identical chemical shifts in achiral solvents, but not chiral ones. Replacing each proton gives enantiomers.
43
What are diastereotopic protons?
Protons that are chemical shift inequivalent.
44
What is an example of a type of pair of protons that are always diastereotopic?
A pair of protons next to a chiral centre.
45
What determines whether one peak or two peaks is visible on a spectrum with two protons with different chemical shifts?
The rate of exchange - this question is worded weirdly lol. (page 25 of handout). If the interchange is rapid the the average of the two peaks will be obtained.
46
What is the equation for the rate of interconversion?
Rate of interconversion = 2πΔv
47
In what case are peaks resolved?
In the case where the rate of rotation is less than or equal to the rate of interconversion.
48
What is the basis of spin-spin coupling?
Nuclear spins on neighbouring atoms can couple through intervening bonds. Coupling arises from interaction of nuclear spins with electronic spins in the bond.
49
What is the magnitude of the coupling constant independent of?
The spectrometer.
50
What is a spin system?
A self-contained set of spin coupled nuclei.
51
How can spin systems be isolated from one another?
An insulator such as a heteroatom or quaternary carbon.
52
What is a geminal system?
Two protons attached to the same carbon.
53
What is a long range system?
4 or more bonds between protons.
54
What is a vicinal system?
Adjacent carbons.
55
What are the factors affecting the coupling constant?
Geometry, number of bonds, heteroatoms, angle strain, bond length and hybridisation.
56
What does a strong coupling between two nuclei equate to?
A large J value.
57
What has stronger coupling: cis or trans atoms?
Trans.
58
What does the Karplus equation describe?
How the coupling constant changes as the angle between the coupling varies.
59
At what angle between two atoms is there the strongest coupling?
180 degrees (10Hz)
60
At what angle between two atoms is there the weakest coupling?
90 degrees (0Hz)
61
What is the limitation of the Karplus equation?
It only works for unstrained hydrocarbons e.g. no ring/alkene, but the same relationship applies.
62
What happens as you increase the number of bonds between which nuclei couple?
The strength of the coupling decreases.
63
What is usually the point in which distance is too great for coupling to occur?
More than 3 bonds apart.
64
What is an exception to the point in which distance is usually too great for coupling to be present?
In benzene, H 4 bonds apart exhibit coupling (1-3 Hz).
65
What are some other examples of long range coupling?
Allylic (double bond included in distance of 4 bonds), propargylic (triple bond included in distance of 4 bonds), homoallylic (double bond in distance of 5 bonds), homopropargylic (triple bond in distance of 5 bonds).
66
What are first order splitting patterns?
When there is a large difference in shift compared to the value of the coupling constant. The peaks are far apart and follow Pascal's triangle in terms of the number of peaks and intensity.
67
What do peak intensities always add up to?
2^N where N = the number of protons the signal is coupling to.
68
What is the equation for multiplicity in first order splitting?
2nI + 1, where n = number of chemically equivalent protons and I = spin number. So for protons, the equation is n +1 (that is where the n+1 rule comes from)
69
How does drawing stick diagrams change if the coupling constants for two different sets of protons is the same?
They can be treated as equivalent as Pascal's triangle can be followed directly.
70
What are second order splitting patterns?
Patterns that occur when the shift is similar - e.g. a proton attached to the same carbon that is diastereotopic.
71
What is roofing?
When pairs of diastereotopic protons roof towards to each other. This is useful for more complicated spectrums.
72
What is significant about second order splitting patterns?
The peaks are shifted inwards - they are not symmetrical around the chemical shift.
73
Why does coupling to proton on oxygen not usually occur?
Rapid proton exchange - with H2O or acid impurities, the H on the OH group is exchanged with the H on the H2O.
74
When can coupling to proton on oxygen occur?
When the proton exchange is slow e.g. CH3-CH-OH in dry d6-DMSO.
75
What is a D2O shake?
If a sample is mixed with D2O, you can identify the OH/NH2/amide protons. The peak will disappear once the D2O has been added if the proton is on one of these groups.
76
Why do we not see coupling to c13 if the spin is a half?
As C13 is not very abundent, satellite peaks are observed.
77
Do we see coupling to F?
Yes - values are usually relatively large.
78
In what case are nuclei magnetically equivalent?
If they are chemical shift equivalent and are coupled equally to other nuclei in a spin system.
79
What are the features of 13carbon nmr?
It has a low natural abundance, gyromagnetic ratio is 1/4 of that of 1H, the coupling constants for 13C-1H are large and there are a large range of chemical shifts.
80
What is significant about the coupling of C13 to H1 in 13C NMR?
It is removed - broadband decoupling. This is when doublets/triplets are removed.
81
Does magnetic equivalence apply for CNMR?
No.
82
What are the two types of 2D NMR?
COSY and NOESY.
83
What is COSY?
Coupling through bonds. Correlations between protons = cross peaks. It shows coupling between protons in a spin system.
84
What is NOESY?
Coupling through space. Correlations between protons = cross peaks, and shows protons which are close in space (less than 4 angstroms away). They may or may not be coupled through bonds also.
85
What is HSQC?
Heteronuclear single quantun coherence. It shows directly coupled 1H and 13C. There are no cross peaks.
86
What is HMBC?
Heternuclear multiple bond correlation - it shows 1H and 13C nuclei coupled over longer ranges (2-4 bonds). There are no cross peaks and don't see the directly bonded 1H to 13C.

CH20150 (1 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions