Nuclear Magnetic Resonance - NMR Spectroscopy Flashcards
What are the characteristics of nuclei of elemental isotopes?
- Have a characteristic spin (l)
- Some are integral (1,2,3), some are fractional
(1/2, 3/2, 5/2), some have no spin, l = 0 (12C, 16O) - The most useful in organic chemistry are 1H, 13C,
19F and 31P, all l = ½
What are the characteristics of spinning nuclei?
- Create a magnetic moment (μ)
- Can spin in all directions until an external field is
introduced
Describe what is happening in the introduction of external magnetic field (B0)?
- The spinning nuclei can now be aligned with the field
(α) or against the field (β) - The energy difference between the two spin states is
small - The β spin state is higher energy than the α
Describe what is happening in resonance
- When a nucleus is subjected to the right combination
of magnetic field and electromagnetic radiation the
spin can flip back and forth - The nucleus is in resonance and this is the energy
detected by NMR
What is shielding?
- Magnetic forcefield exerted by electrons: electrons
are circulating and generating a small induced
magnetic field that opposes the external field (B0) - Decrease in the magnetic field felt by the nucleus: this
results in a stronger field being needed for resonance
to occur - Electronegative atoms such as oxygen pull electrons
away from neighbouring atoms, deshielding them - Shielded atoms need higher field energy to cause
resonance
What is the electronegativity deshielding effect?
With increasing electronegativity, there is more deshielding of neighbouring
atoms as the electrons are drawn towards the more electronegative atom.
Chemical shift: distance from electron - withdrawing group?
The further away an atom is from the electronegative group, the less deshielding
effect is felt by the protons of that atom.
Chemical shift: addition of more electron - withdrawing groups?
If more than one EWG is present, the deshielding
effects are nearly (but not quite) additive.
Chemical shift: other groups with deshielding effects?
Vinyl (δ 5-6) and Aromatic Protons (δ 6-9): Electron circulation causes deshielding so lower fields
observed.
Aldehydes (δ 9-10): Carbonyl group has deshielding effect
OH and NH: These depend on the concentrations widely
- Concentrated solutions, deshielding caused by H-bonding, NH (δ 3.5) and OH (δ 4.5)
- Diluted solutions, no H-Bonding, NH and OH (δ 2.0)
- Broad peaks are observed due to H-bonding and proton exchange
COOH (δ 10+): C=O and O present so even greater deshielding
What do the number of signals NMR mean?
The number of NMR Signals present represent the number of proton environments
Protons in identical chemical environments have the same chemical shift – Chemically
Equivalent
What does the area under the peak mean?
The area under the peak is proportional to the number of protons in that environment
The integral trace cannot specify the number of
protons but instead shows the ratio
What is spin spin splitting + how does this happen?
Protons from different environments can influence the magnetic field of each other if they are
significantly close enough to one another. This can result in the splitting of their peaks as seen in the
NMR spectrum – Chemically equivalent protons do not split each other
splitting won’t happen on a if more than 3 bonds exist between protons
What are some unusual splitting patterns?
- 1 strongly EWG or EDG
- Symmetrical ortho distribution
- Unsymmetrical para distribution
What is a coupling constant?
A coupling constant (J) is the distance between the peaks of multiplets measured in Hz
They do not vary with the field strength of the spectrometer
What is homotopic?
Homotopic = if any H are replaced by
deuterium (D) nothing changes as the other two groups are equivalents
What is enantiotopic?
Enantiotopic = if any of the H are replaced by D the molecule becomes chiral (so they are called prochiral)
What is diastereotopic?
Diastereotopic = If either of the prochiral H
are replaced by D we create a second chiral
centre and the two resulting molecules will
be diastereomers (1x S,R, 1x R,R).
What does 13C NMR do?
- Determines the magnetic environments of the carbon atoms instead of protons
- 12C (99%) has no magnetic spin
- 13C (1%) does have a magnetic spin – but sensitivity is decreased by x100
- The resonance frequency of 13C is only one fourth of that for 1H NMR
(gyromagnetic ratio) - Hundreds of spectra were taken and then averaged (old technique)
- Now we have Fourier-Transform
What is Fourier transform NMR spectroscopy?
- Mathematical technique used to compute a spectrum from the FID
- A pulse is used to introduce non-equilibrium nuclear spin magnetization
- The relaxation of spins to their resonance is time dependent and measured to create
the spectrum
What are the differences between 1H & 13C NMR?
Operating frequency:
* Gyromagnetic ratio (ratio of magnetic moment to angular momentum) and resonance
frequency of 13C is about ¼ of that for 1H NMR
Peak Areas:
* The areas of the peaks of 13C are not necessarily proportional to the number of carbons giving
rise to those peaks – therefore can be used to identify the type of chemical environment but
may not relate directly to the number of carbons present
* Carbon atoms with 2 or 3 protons attached usually give the strongest absorptions while
carbon atoms with no protons attached usually give the weakest absorptions
Spin-Spin Splitting:
* Since only 1% of Carbon atoms are the 13C isotope, carbon-carbon splitting is ignored
* Carbon-Hydrogen splitting patterns can be complicated
What is the difference in proton spin decoupling in H and C?
- H are continuously in resonance (rapidly flip spins) due to being irradiated by a
broadband proton transmitter - The C nuclei see an average of the possible proton spin states
- Each C signal appears as a single unsplit peak because any C-H splitting has
been eliminated
What are the spectrums is run twice in off-resonance decoupling?
1st: Broadband-Decoupled Spectrum (indicated number of non-equivalent environments of C and their chemical shifts)
2nd: Off-Resonance-Decoupled Spectrum (multiplicities of signals indicate the number of H bonded to each C
What is DEPT + What it entails?
DEPT: Distortionless Enhanced Polarization Transfer
* All peaks remain as decoupled singlets
* Has better sensitivity and avoids overlapping multiplets
* Each 13C nucleus is magnetically coupled to the protons bonded to it
* There is transfer of polarization from the protons to the carbon nucleus
* How the polarization transfer occurs is determined by the number of protons
bonded to the 13C nucleus
How is interpreting 13C NMR spectra?
- The number of different signals implies how many different types of carbons are
present - The chemical shifts of those signals suggest what types of functional groups those
carbon atoms belong to - The splitting of signals in the Off-Resonance-Decoupled, DEPT-90 and DEPT-135
spectra indicate how many protons are bonded to each carbon atom
