B - Spectroscopy - NMR Flashcards
What happens to nuclei in an external magnetic field, B0?
Nuclei align either parallel or anti-parallel. The parallel position is slightly lower in energy and so is favoured.
What is the Larmor frequency, VL?
The nuclear spin axis is tilted with respect to the external field direction, so it precesses around B0.
The frequency at which this occurs is the Larmor frequency.
What is VL determined by?
The gyromagnetic ratio, γ.
This is constant for each nucleus, but different depending on the nucleus in question and B0.
How can spin-flips occur?
If the nucleus is irradiated with EM radiation in the radio wave region of the correct frequency, the nuclei will absorb the energy. The lower energy state spin-flips to the higher energy state
How is the total nuclear spin quantum number, I, determined?
It is determined by the number and relative proportions of protons and neutrons.
Usually only nuclei with an odd mass number are NMR active, and I can equal = 0, 1/2, 1, 3/2, 2, 5/2 etc
How is the number of quantised spin states worked out?
2I + 1
therefore spin 1/2 -> 2 x 1/2 + 1 = 2
so spin 1/2 nuclei can have 2 possible spin states, spin up or spin down
What is the equation for the difference in energy between two spin states?
E = γ x B0 x h / 2π
What is the equation for Lamor frequency, VL?
As E = h x VL
VL = γ x B0 / 2π
What is the solvent used in NMR?
Deuterated chloroform, CDCl3 (trichloromethane)
What is the standard used in NMR, and why is it used?
Trimethyl silane, TMS, Si(CH3)4
- unreactive
- liquid and mixes with common solvents
- gives a single NMR peak, which occurs at a lower frequency than most others
- highly volatile, so it can be evaporated off to recover the sample
What effect do electron clouds have on absorbing radio waves?
Electrons cause a small magnetic field that opposes B0.
This is felt by the nucleus, meaning the NMR frequency is slightly lowered.
This is known as shielding.
What is the equation for shielding?
Shielding = σ x B0, where σ = shielding constant
What is the equation for the effective field felt by a shielded nucleus?
What rules allow us to calculate this?
Beff = B0 ( 1 - σ )
Slater’s rules
What is the equation for the Larmor frequency of a shielded nucleus?
VL = γ x Beff / 2π
= γ x B0 ( 1 - σ ) / 2π
What is the effect of shielding on the NMR frequency?
Shielding causes a chemical shift of the NMR frequency.
Nuclei in different bonding environments experience different levels of shielding and, therefore, experience different magnetic fields, so they will absorb different frequencies.
The frequency is usually smaller than expected for an unshielded nucleus in the same magnetic field.
What is the inductive effect?
Protons bound in molecules will experience more or less shielding depending on the electron donating/withdrawing effect of neighbouring groups.
Electron donating = increases electron shielding around the proton -> NMR frequency goes down
Electron withdrawing = decreases electron shielding around the proton -> NMR frequency goes up
Shielding and NMR frequency are inversely proportional; low shielding means a high frequency / greater chemical shift
What is the equation to work out chemical shift?
Chemical shift, δ = ( νsample - νref) / νref x 106
Where do protons attached to -CH2 or -CH3 groups appear?
These protons are highly shielded and so lie close to the TMS peak.
They appear δ 1 - 4 ppm.
They may be affected by neighbouring atoms eg. O
Where do protons attached to O and N appear?
O -> protons are less shielded due to the electronegativity of O
2 - 5 ppm.
N -> appear in a similar range to -OH. but the peak is often broadened due to magnetic interactions with 14N, 15N nuclear spins
Where do protons attached to -CHO and -COOH appear?
These protons are highly deshielded due to the neighbouring O atoms, so appear δ 9 - 10 ppm
Where do protons in aromatic systems appear?
Electrons travelling around the ring in π orbitals generate a ring current; this usually adds to B0.
They will appear δ 8 - 9 ppm.
In some highly conjugated or aromatic molecules, H atoms may be inside the ring; the ring current then opposes B0, causing negative δ values
What is the NMR fine structure?
Absorption peaks consist of multiple very narrow lines that are best observed in high-resolution NMR.
What is spin-spin coupling?
Adjacent protons are in different shielding environments, so their magnetic environments are non-equivalent.
When one proton flips its spin, the other proton can be in its up or down state; this causes the magnetic field felt by the original proton to be slightly different, causing two closely spaced lines to be observed.
What does the total integrated area under each peak correspond to?
It corresponds to the number of protons producing that signal.
What is the coupling constant, J?
J measures the gap between splitting peaks.
The amount of splitting depends upon the strength of the magnetic interaction between the protons, which corresponds to the magnitude of J.
What is 2D NMR and what is it used for?
It uses radio wave pulses and computer processing to produce contour maps.
This is used to show the correlation between 1H NMR and 13C NMR.
This is often used to identify large biomolecules.
What is MRI and how is it used?
Magnetic Resonance Imaging
This looks at protons in H20 molecules.
It uses pulsed NMR to observe the relaxation time of protons orientated by the magnetic field.
Different regions are distinguished by the mobility of H20 molecules, so it can be used to look at soft tissue rather than bone.
Multiple layers can be looked at to produce a 3D image.
What if fMRI?
Functional Magnetic Resonance Imaging
fMRI looks at the magnetic spin state of Fe in oxygenated Hb during increased O2 consumption.
This is able to map brain activity in real time.
What is a PET scan?
Positron emission tomography
Detects the decay of radioactive 15O -> 122s
Emits e+
How does a multiplet arise?
It arises due to magnetic spin-spin coupling between the spins of protons that are on adjacent carbons in a structure, and that are in magnetically non-equivalent environments.
Multiplet patterns follow the n+1 rule, where n is the number of 1H nuclei on the neighbouring carbon atom
Why do NMR signals from protons attached to O or N always appear as broad singlets?
The magnetic coupling from which the multiplet arises does not occur across heteroatoms such as O or N
Protons attached to heteroatoms may be easily exchanged and can H-bond.
The chemical exchange which is also responsible for the broadening of the signal is mediated and accelerated by traces of water, acid and/or base.
When the exchange is very rapid on the NMR timescale, the spin states of protons on the heteroatom are averaged out and the signal appears as a broad singlet.
The rate of chemical exchange can be slowed down by cooling and removing all traces of the aforementioned substances.
What causes the broadening of an OH NMR peak?
Intermolecular and intramolecular H-bonding and chemical exchange.
How does NMR work?
Nuclei like 1H and 13C have a property called ‘spin’.
The spin results in the nucleus behaving like a tiny magnet, with N and S poles.
If the molecule is placed in an external magnetic field, the spins line up with the field direction.
If the sample is irradiated with EM radiation, it can cause the spins to ‘flip’.
The spin flips have a particular energy that depends on: - the size of the external field - the type of nucleus - the local environment of each atom.
The spin-flip energies occur in the radio wave region
We use radio transmitters and receivers to generate and detect the spin flip energies, forming an NMR spectrum
Different nuclei within a given molecule resonate at different positions
They can also interact with other magnetic nuclei in different ways, to produce characteristic patterns
