The chemical shift L3 Flashcards
What does resonant frequency depend on
Resonant frequency of nucleus depends on chemical environment
Explain diamagnetic current
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.
What does Bind mean
Induced magnetic field
Describe nuclei in regions of high electron density
Nuclei in regions of high electron density experience greater induced magnetic field so there is a larger shielding constant. Nuclei are shielded
Describe nuclei in regions of low electron density
Nuclei in regions of low electron density experience lower induced magnetic field so there is smaller shielding constant so nuclei are deshielded.
What does resonant frequency depend on and why is this a problem
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
How do we size up the difference in chemical shift
Differences in chemical shift v small 100-1000 Hz on a 100MHz spectrometer therefore multiply difference by 10^6 to get reasonable values
How do we calculate chemical shift
Chemical shift = (Vsamp-Vref)/Vref all x10^6
v is frequency
Why is TMS chosen as a reference in 1H nmr
- 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
What sort of reference is TMS and how does this differ from other references
TMS is used as an internal reference whereas most other nuclei require external references
Define internal reference
Added to NMR sample itself
What is an external reference
Can be added to inner part of concentric nmr tube - sample and reference do not mix.
What is the convention in terms of drawing spectrum
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
Deshielded synonyms
High frequency
Low field
Down field
Shielded synonyms
Low frequency
High field
Upfield
Effect of charge on chemical shift
Positive charge is typically deshielding
Negative charge is typically shielding
Effects of oxidation states on chemical shift
Higher oxidation states typically deshielded whereas lower oxidation states are shielded
Effects of neighbouring groups on chemical shift and what does this depend on
(Local diamagnetic shielding)
- 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
What is chemical shift ainsotropy
variation in a nucleus’s chemical shift due to the directional dependence of its local electronic shielding in a magnetic field
Define tumbling
Tumbling is the random rotational motion of molecules in solution that averages out anisotropic interactions
Describe the effects that electronegativity has on chemical shift
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
Effect of hybridisation
1H on sp3 (alkyl) C
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
Effects of hybridisation
1H on sp2 (alkenyl) C
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
Effects of hybridisation
1H on sp2 carbonyl C
Typically 9-10 ppm electronegative O further deshielded CH in aldehydes
Effects of hybridisation
1H on sp2 (aromatic) C
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
Effects of hybridisation
1H on sp (acetylenic) C
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
Describe resonance (mesomeric) effects in terms of electron withdrawing substituents
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
Describe resonance (mesomeric) effects in terms of electron donating substitutes
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).
Describe what chemical shifts anisotropy also affects
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
Effect of H bonding in terms of chemical shift
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
In terms of H bonding what does the amount of deshielding depend on
Amount of deshielding depends on concentration and temperature
Problems with OH hydrogen bonds
H atoms relatively weakly bonded in OH compounds, can lead to chemical exchange of O-H causes complications
What is the equation to calculate the field experienced by the nucleus
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
What is B’ proportional to
B0
What does the reduced magnetic field mean for the nucleus
nucleus processes at lower larmor frequency
Nucleus resonances at lower frequency
Give examples of electron withdrawing substituents
NO2 and CN
Give examples of electron donating substituents
NH2 and OCH3
What does the ppm value of a given proton depend on
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
What does the strength of the magnetic field that the protons feels depends on
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
what does upfield mean
upfield means lower energy, right side of the spectrum
What does downfield mean
Downfield means higher energy, left hand side of the spectrum
What are the benefits of using a more powerful NMR spectrometer
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
Describe when the chemical shift would be low
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
Describe when the chemical shift would be high
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
