The chemical shift L3

Question 1

What does resonant frequency depend on

Answer 1

Resonant frequency of nucleus depends on chemical environment

Question 2

Explain diamagnetic current

Answer 2

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.

Question 3

What does Bind mean

Answer 3

Induced magnetic field

Question 4

Describe nuclei in regions of high electron density

Answer 4

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

Question 5

Describe nuclei in regions of low electron density

Answer 5

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

Question 6

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

Answer 6

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

Question 7

How do we size up the difference in chemical shift

Answer 7

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

Question 8

How do we calculate chemical shift

Answer 8

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

Question 9

Why is TMS chosen as a reference in 1H nmr

Answer 9

• 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

Question 10

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

Answer 10

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

Question 11

Define internal reference

Answer 11

Added to NMR sample itself

Question 12

What is an external reference

Answer 12

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

Question 13

What is the convention in terms of drawing spectrum

Answer 13

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

Question 14

Deshielded synonyms

Answer 14

High frequency
Low field
Down field

Question 15

Shielded synonyms

Answer 15

Low frequency
High field
Upfield

Question 16

Effect of charge on chemical shift

Answer 16

Positive charge is typically deshielding
Negative charge is typically shielding

Question 17

Effects of oxidation states on chemical shift

Answer 17

Higher oxidation states typically deshielded whereas lower oxidation states are shielded

Question 18

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

Answer 18

• 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

Question 19

What is chemical shift ainsotropy

Answer 19

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

Question 20

Define tumbling

Answer 20

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

Question 21

Describe the effects that electronegativity has on chemical shift

Answer 21

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

Question 22

Effect of hybridisation
1H on sp3 (alkyl) C

Answer 22

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

Question 23

Effects of hybridisation
1H on sp2 (alkenyl) C

Answer 23

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

Question 24

Effects of hybridisation
1H on sp2 carbonyl C

Answer 24

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

Question 25

Effects of hybridisation 1H on sp2 (aromatic) C

Answer 25

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

Question 26

Effects of hybridisation 1H on sp (acetylenic) C

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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).

Question 29

Describe what chemical shifts anisotropy also affects

Answer 29

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

Question 30

Effect of H bonding in terms of chemical shift

Answer 30

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

Question 31

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

Answer 31

Amount of deshielding depends on concentration and temperature

Question 32

Problems with OH hydrogen bonds

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What is B’ proportional to

Answer 34

B0

Question 35

What does the reduced magnetic field mean for the nucleus

Answer 35

nucleus processes at lower larmor frequency Nucleus resonances at lower frequency

Question 36

Give examples of electron withdrawing substituents

Answer 36

NO2 and CN

Question 37

Give examples of electron donating substituents

Answer 37

NH2 and OCH3

Question 38

What does the ppm value of a given proton depend on

Answer 38

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

Question 39

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

Answer 39

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

Question 40

what does upfield mean

Answer 40

upfield means lower energy, right side of the spectrum

Question 41

What does downfield mean

Answer 41

Downfield means higher energy, left hand side of the spectrum

Question 42

What are the benefits of using a more powerful NMR spectrometer

Answer 42

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

Question 43

Describe when the chemical shift would be low

Answer 43

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

Question 44

Describe when the chemical shift would be high

Answer 44

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