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Structural chemistry > The chemical shift L3 > Flashcards

The chemical shift L3 Flashcards

1
Q

What does resonant frequency depend on

A

Resonant frequency of nucleus depends on chemical environment

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2
Q

Explain diamagnetic current

A

Electrons are charged, and experience a force in a magnetic field causing them to move (circulating in their orbitals). This induces local magnetic field which opposes direction of applied field at the nucleus.

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3
Q

What does Bind mean

A

Induced magnetic field

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4
Q

Describe nuclei in regions of high electron density

A

Nuclei in regions of high electron density experience greater induced magnetic field so there is a larger shielding constant. Nuclei are shielded

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5
Q

Describe nuclei in regions of low electron density

A

Nuclei in regions of low electron density experience lower induced magnetic field so there is smaller shielding constant so nuclei are deshielded.

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6
Q

What does resonant frequency depend on and why is this a problem

A

Resonant frequency of nucleus depends on B0 this is specific to a particular NMR spectrometer which is not useful. Instead measure frequency with respect to resonant frequency of nuclei in a reference compound

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7
Q

How do we size up the difference in chemical shift

A

Differences in chemical shift v small 100-1000 Hz on a 100MHz spectrometer therefore multiply difference by 10^6 to get reasonable values

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8
Q

How do we calculate chemical shift

A

Chemical shift = (Vsamp-Vref)/Vref all x10^6
v is frequency

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9
Q

Why is TMS chosen as a reference in 1H nmr

A
  • soluble in most organic solvents
  • unreactive
  • contains 12 equivalent 1H nuclei therefore strong signal
  • volatile B0 28 degrees, easily removed from sample if compound needs to be recovered
  • 1H very shielded so most nuclei have a positive chemical shift
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10
Q

What sort of reference is TMS and how does this differ from other references

A

TMS is used as an internal reference whereas most other nuclei require external references

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11
Q

Define internal reference

A

Added to NMR sample itself

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12
Q

What is an external reference

A

Can be added to inner part of concentric nmr tube - sample and reference do not mix.

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13
Q

What is the convention in terms of drawing spectrum

A

Convention - spectra are plotted with chemical shift increasing from right to left. Strongly shielded nuclei appear to the right of the spectrum whereas strongly deshielded nuclei appear to the left of spectrum

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14
Q

Deshielded synonyms

A

High frequency
Low field
Down field

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15
Q

Shielded synonyms

A

Low frequency
High field
Upfield

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16
Q

Effect of charge on chemical shift

A

Positive charge is typically deshielding
Negative charge is typically shielding

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17
Q

Effects of oxidation states on chemical shift

A

Higher oxidation states typically deshielded whereas lower oxidation states are shielded

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18
Q

Effects of neighbouring groups on chemical shift and what does this depend on
(Local diamagnetic shielding)

A
  • Neighbouring groups have there own electrons so create there own induced field, this will affect magnetic field experienced by neighbouring nuclei which effects shielding and therefore chemical shift. this depends on relative orientations of the two groups
19
Q

What is chemical shift ainsotropy

A

variation in a nucleus’s chemical shift due to the directional dependence of its local electronic shielding in a magnetic field

20
Q

Define tumbling

A

Tumbling is the random rotational motion of molecules in solution that averages out anisotropic interactions

21
Q

Describe the effects that electronegativity has on chemical shift

A

Electronegative groups withdraw electron density, which reduces valence electron density at neighbouring groups
Shielding decreases with increasing electronegativity
The effect diminishes with increasing distance of the nucleus from X
Effect increases with increasing substitutions by X

22
Q

Effect of hybridisation
1H on sp3 (alkyl) C

A

Typically 0-2 ppm in absence of other effects.
- As C is more electronegative than H, so the more C substituents the greater the deshielding of 1H.
- circulation of electrons in C-C bonds deshielded H anisotropic effects

23
Q

Effects of hybridisation
1H on sp2 (alkenyl) C

A

Typically 4-7 ppm 1H significantly deshielded. Sp2 C has more s character than sp3 C. S electrons are held closer to nucleus, 1H on sp2 C more deshielded

24
Q

Effects of hybridisation
1H on sp2 carbonyl C

A

Typically 9-10 ppm electronegative O further deshielded CH in aldehydes

25
Q

Effects of hybridisation
1H on sp2 (aromatic) C

A

Typically 7-8 ppm. B0 induces circulation of pi electrons (ring current). Induces magnetic field which opposes B0 at ring centre. Induced field reinforces applied field at H atoms on the endogenous of the ring so 1H deshielded

26
Q

Effects of hybridisation
1H on sp (acetylenic) C

A

Typically 2-3 ppm
50% s character would expect H to be strongly deshielded but circulation of pi electron density causes induced field which reduces net field experience by nuclei. 1H nuclei shielded

27
Q

Describe resonance (mesomeric) effects in terms of electron withdrawing substituents

A

These groups pull electron density away from the conjugated system through resonance. When the electron density around a nucleus decreases, the nucleus is less shielded from the external magnetic field, resulting in a downfield shift

28
Q

Describe resonance (mesomeric) effects in terms of electron donating substitutes

A

These groups donate electron density into the conjugated system through resonance. The increased electron density around a nucleus shields it more from the external magnetic field, resulting in an upfield shift (lower ppm in NMR).

29
Q

Describe what chemical shifts anisotropy also affects

A

Chemical shift anisotropy also affects other nuclei and nuclei not directly bonded to it. The effect of chemical shifts anisotropy reduced with increasing distance of nucleus as the induced field is weaker with increasing distance

30
Q

Effect of H bonding in terms of chemical shift

A

Hydrogen bonding deshields protons involved in the bond, causing downfield shifts (higher ppm). The proton on the hydrogen bond donor is typically shifted more downfield than protons not involved in hydrogen bonding

31
Q

In terms of H bonding what does the amount of deshielding depend on

A

Amount of deshielding depends on concentration and temperature

32
Q

Problems with OH hydrogen bonds

A

H atoms relatively weakly bonded in OH compounds, can lead to chemical exchange of O-H causes complications

33
Q

What is the equation to calculate the field experienced by the nucleus

A

B’ =B0 - Bind
Where B’ is the reduced magnetic field
B0 is the external magnetic field
Bind induced magnetic field opposite B0 which has been generated

34
Q

What is B’ proportional to

A

B0

35
Q

What does the reduced magnetic field mean for the nucleus

A

nucleus processes at lower larmor frequency
Nucleus resonances at lower frequency

36
Q

Give examples of electron withdrawing substituents

A

NO2 and CN

37
Q

Give examples of electron donating substituents

A

NH2 and OCH3

38
Q

What does the ppm value of a given proton depend on

A

depends on the energy difference of its upper and lower energy states which also depends on how much energy it received from the magnetic field

39
Q

What does the strength of the magnetic field that the protons feels depends on

A

how exposed it is to the field, the thing that protects the protons from being exposed to the field is the electron cloud exposing them

40
Q

what does upfield mean

A

upfield means lower energy, right side of the spectrum

41
Q

What does downfield mean

A

Downfield means higher energy, left hand side of the spectrum

42
Q

What are the benefits of using a more powerful NMR spectrometer

A

The frequency gap on the stronger field spectrometer is higher and the peaks are more separated. More powerful spectrometers the peaks are more easily resolved and do not overlap

43
Q

Describe when the chemical shift would be low

A

The chemical shift would be low if the applied magnetic field is in the opposite direction to the induced magnetic field as this gives a weak net magnetic field

44
Q

Describe when the chemical shift would be high

A

The chemical shift will be high if the applied magnetic field and induced magnetic field are in the same direction. Giving a stronger net magnetic field

Structural chemistry (11 decks)

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