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 B ind larger. Nuclei are sheilded

Question 5

Describe nuclei in regions of low electron density

Answer 5

Nuclei in regions of low electron density experience lower B ind is smaller and 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 a reference compound and measure chemical difference

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 = 10^6(v-vref)/v measure
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

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
(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.

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. Also 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 as H atoms on edge of ring

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 B’. 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 nuclei not directly bonded fo 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

1H chemical shift for R-OH

Answer 31

0.5-5

Question 32

1H chemical shifts for R-NH2

Answer 32

0.5-5

Question 33

1H chemical shifts for R-CONH2

Answer 33

5-8

Question 34

1H chemical shifts for Ph-OH

Answer 34

4-8
Effect of ring current and H bonding

Question 35

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

Answer 35

Amount of deshielding depends on concentration and temperature

Question 36

Problems with OH hydrogen bonds

Answer 36

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