NMR Flashcards
What is the frequency of NMR?
300-900 MHz
What magnetic properties do atomic nuclei have?
Nuclear spin and magnetic moment. This is because protons in the nucleus have a positive charge
What does the magnetic moment cause the nucleus to do?
Interact with an external magnetic field and re-align itself either with or against the field
What does magnetic moment depend on?
Depends on quantum properties of the atom.
Any atom with an odd number of neutrons and/or an odd number of protons have magnetic properties
What nuclei are not magnetically active and can never exhibit an NMR effect?
Nuclei with even mass numbers AND even atomic (proton) number (I=0)
What spin does 1H, 3H, 13C, 15N etc have?
1/2
What spin does 16O have?
0!!
What spin do nuclei with even mass numbers and odd atomic (proton) numbers have?
whole number: 1,2,3 etc
What spin do nuclei with odd mass numbers have?
1/2 , 1/3 etc
A nucleus with spin 1/2 can…
adopt 2 possible states, aligned with or against the field
What does alignment with the field require?
LESS energy, therefore the majority of spins align with the field
What will the number of nuclei in the low energy state (Nalpha) and the number in the high energy state (Nbeta) differ by?
An amount determined by the boltzmann distribution
What is the difference between protons in a medium field?
about 1 in 10^5
How can you increase magnetic resonance sensitivity?
by increasing magnetic field strength
What effect does irradiating the sample at the resonance frequency have?
disturbs the proportions of nuclei in each energy level, putting more into the higher energy level
How can transition from the lower energy level to the higher energy level occur?
By absorption of radio-frequency radiation of the correct frequency
What is the relationship between radio frequency and magnetic field strength?
Linear
What does NMR measure?
the absorption of radio-frequency radiation resulting in changes in nuclear spin orientation
When does NMR only occur?
when a sample is in a strong magnetic field
Describe a modern NMR instrument
- The sample (~5-20mg) is dissolved in deuterated solvent (0.6 – 0.8 ml)
- Placed in an NMR tube
- Inserted into the NMR magnet
What is the Fourier Transformation (FT) experiment?
- During a FT experiment, the sample is irradiated with a short pulse of a broad range of radiofrequency radiation
- This results in simultaneous excitation of all nuclei of the sample
- Nuclei relax back into a lower energy state releasing a pulse of electromagnetic radiation
- The NMR spectrometer detects this radiation received from the sample
What is the net result of FT
Interfering wave patterns of differing frequencies –> Free Induction Decay (FID)
What is FID?
a plot of signal intensity against time?
What does FT convert spectrum into?
converts from units of time to units of frequency
In NMR spectra, what does the number of peaks correspond to?
The number of non-equivalent protons
What does the position of the NMR signals indicate?
The electromagnetic environment of the protons which produced this signal
What is the area under each NMR signal (the integral) proportional to?
the number of protons responsible for this signal. e.g. if AUC=3 , then there are 3 protons e.g. CH3
What does the shape of each signal depend on?
Whether the proton is exchangeable or non-exchangeable
What makes protons ‘equivalent’?
If they have the same electromagnetic environment, they are equivalent
What is the quantum of energy required to promote nuclei to a higher energy level regarding protons?
the SAME for each proton
What does resonance frequency depend on?
- Depends on the chemical environment of the nucleus
- Varies according to the structural factors of the molecule
- Therefore, different nuclei of the same atom type (e.g. 1H) in different molecules exhibit different absorption frequencies
What induces LOCAL magnetic field?
electrons surrounding the nucleolus
What effect does LOCAL magnetic field have on magnetic field experienced by the nucleolus?
it is DECREASED - as the nucleolus is shielded
What is the effect of electron density on local magnetic field?
The STRONGER electron density, the STRONGER the local magnetic field induced by electrons
What do surrounding electrons generate?
A small induced field opposite to external field Bo
This results in a shielding effect. Therefore the nuclei are partially shielded from external magnetic field Bo
What is chemical shift?
• The variation of resonance frequency V0 with chemical environment of the nucleus
What are differences in resonance frequency much greater than?
resonance line widths –> therefore, distinct resonance lines are observed for chemically different nuclei
What scale is used for chemical shift and why?
ppm (part per million)
• Resonance frequency of particular nucleus depends on the value of B0 in the instrument used:
ν = γB0/2π (Hz)
• Therefore frequency comparison for different NMR spectrometers is of no value
• The idea is to make frequencies INDEPENDENT of a strength of magnetic field
What is used as a reference signal for chemical shift?
TMS (tetramethyl silane (CH3)4Si) as it is totally symmetrical, so a very intense signal
What is the resonance frequency of the reference signal?
ZERO
What chemical shift do nuclei LESS shielded than the reference have?
POSITIVE chemical shift
What do nuclei MORE shielded than the reference have?
NEGATIVE chemical shift
What effect will an electron-donating group attached to the atom have on shielding?
Will result in INCREASE of electron density surrounding the nucleus, having shielding effects, decrease in B’ and lower resonance energy
What is B’
the reduced field experienced by the nucleus
What effect will an electron-withdrawing effect attached to the atom have on shielding
Will result in DECREASE of electron density surrounding the nucleus, having de-shielding effects, increase in B’ and higher resonance frequency resulting in a larger chemical shift
What will the H within an aromatic ring be effected by?
Will be highly de-shielded because aromatic ring is unsaturated
What will the H on an aldehyde by effected by?
Will be highly de-shielded because its directly attached to carbonyl group which grabs electrons
what do electronegative groups do to NMR signals?
They are “deshielding” and tend to move NMR signals from neighbouring protons further downfield (to higher ppm values)
What do the π-system of alkenes, aromatic compounds and carbonyls do to NMR signals?
They strongly deshield attached protons and move them “downfield” to higher ppm values
What are the key differences between Proton and carbon NMR?
- Abundance: Natural abundance of 13C is 1.11% (Natural abundance of 1H is 99.98%)
- Sensitivity: 13C NMR is much less sensitive than 1H NMR
- Resonance frequency: At the same field strength 13C resonate at ¼ the frequency of 1H
- Chemical shift: The chemical shift range for the 13C is ~20 times that of the equivalent 1H nuclei
What does the multiplet structure of signals arise from?
An interaction between neighbouring nuclei (i.e. spin-spin coupling)
These interactions are transmitted by electrons through intermediate bonds
What is the magnitude of the splitting called?
spin-spin coupling constant, J
Spin-spin coupling is usually restricted to a maximum of three bonds between the atoms which are coupled
Coupling constant J is:
independent of the magnetic field strength
extremely variable (ranges from 0.05Hz up to kHz)
dependent upon the number of bonds between the coupling nuclei and their molecular environment
What are the types of coupling constants?
• Geminal coupling constant, 2J:
H-C-H proton separated by 2 chemical bonds (etc)
• Vicinal coupling constant, 3J:
H-C-C-H
• Long-range coupling constants, 4J and 5J:
H-C-C-N-H H-C-X-Y-C-H
Do chemically and magnetically equivalent protons couple to one another ?
NO
What is the strongest coupling constant?
Geminal coupling constant
Interproton spin-spin coupling is largest between geminal protons
• It depends upon the angle between the two C-H bonds
• 2J is typically between 12 and 20 Hz and negative in sign
What does vicinal coupling constant depend on?
- It depends upon torsion angle between the two protons
* Magnitude of 3J ranges from 2Hz to 12 Hz and positive in sign
