NMR Flashcards
NMR
Nuclear Magnetic Resonance is a technique that gives a very detailed structure
When a Magnetic nucleus is placed in field there are few orientations (low E aligned with field) of different energy with small energy differences
Most useful nuclei for organic chemists is 13C and 1H.
Also use 15N and 31P particularly for larger
biomolecules.
What is relationship between spin and mass
Even atomic mass and atomic number = NO SPIN
Even atomic mass, ODD atomic number = INTEGRAL SPIN
ODD atomic mass, even/odd atomic number = HALF INTEGRAL SPIN
Describe CHEMICAL SHIFTS
magnetic field exerts some force on a charge; Current makes an magnetic field.
The number and distribution of electrons is variable
depending on bonding and structure within a molecule.
Electrons are also charged spinning particles, therefore they produce their own magnetic field which opposes the applied field. This induced field means that the nucleus experiences a LOWER magnetic field. The nucleus is ‘shielded
The greater the shielding the lower the resonance
frequency of the nucleus.
Varying the electron density within a molecule will
produce variable shielding effects and so different
chemical shifts will be observed for the same type
of nucleus at different sites in the molecule.
What is the larmour equation?
this equation predicts all the protons should resonate at the same frequency.
But not all protons will resonate equally as we never deal with just nuclei alone
What is a chemical shift?
It is the difference in FREQUENCY of a nuclear spin flip of the target nucleus and the reference frequency spin flip of the reference molecule.
It is controlled by spin flip energy (CHANGE IN ENERGY STATES) which is in turn controlled by the size of the magnetic fields; low magnetic field, small energy difference, low chemical shifts.
Their chemical environment determines the amount of variability in resonance frequency by a few Hz; the shift position on the spectrum and its number determines structure.
measured in ppm
It is dimensionless
chemical shift and NMR
In a sample there are lots of spin systems all varying individually to B1 (applied magnetic field) A pulse will excite all the frequencies at once so graph produced has a combination of all electron frequencies. This is called Free Induction decay (FID) and leads to the proton NMR spectrum
chemical shifts give information about the proton environment; what chemical group is it attached to
regions of chemical shift values correlate with the environment and the kind of molecule protons are part of.
How do protons directly attached to oxygen/nitrogen or sulphur differ from being bonded to carbon?
They can take part in Hydrogen bonding
They are exchangeable
Protons on nitrogen may not experience coupling to
their neighbours
Alcohols – the shift of hydroxyl protons is not fixed,
it varies with temperature, concentration and
solvent used due to H-bonding.
There is rapid exchange of protons between molecules and with any water present which is why hydroxyl protons appear as a broad peak.
Describe chemical shift spectrum
increasing destabilisation will put the molecule high on the chemical shift spectrum (downfield); a
nucleus
whose
chemical
shift
is
increased ,
feels
stronger
magnetic
field
due
to
the
removal
of
electron
density
increasing shielding by having a higher density electron density orbiting. NMR FREQUENCY will be shifted UPFIELD. This will place the molecule at a LOWER CHEMICAL SHIFT.
What is integration?
In NMR spectroscopy, the process of measuring the area of an NMR signal. It is used to determine the relative number of hydrogens that correspond to each signal
What is magnetic induction
where an external magnetic field causes electrons in an electron cloud to circulate. As these move around, they generate their own charge and then their own magnetic field
What is Tetramethylsilane (TMS)
Used as a chemical shift reference as it will have a shift of 0.00ppm.
What is spin-spin splitting in proton NMR?
Interactions between non equivalent protons on adjacent carbons will give a corresponding number of peaks with the n+1 rule
2 peaks is called a doublet, 3 peaks is a triplet, and 4 peaks is a quartlet
Equivalent protons of the EXACTLY THE SAME ENVIRONMENT do not split each other.
The signal of a proton with n equivalent neighboring H’s is split into n + 1 peaks. 2 Hydrogens close by (2 bonds) will split the target Hydrogen into 3 peaks; 3 neighboring H would make a quartet
Protons that are farther than two carbon atoms apart do not split each other
Carbon spectra comparisons
1) observed intensity is NOT PROPORTIONAL to the number of carbons of that type.
2) spin-spin coupling with other nuclei like protons but decoupled spectras give singlets
3) chemical shifts divided into 4 main regions
4) The carbon13 spectrum is similar to proton spectra however the range of shifts is much greater
5) the chemical shifts are caused by type and proximity of other nuclei
chemical shifts divided into 4 main regions
1) 0-50 ppm – saturated carbon atoms : CH3, CH2, CH
2. 50-100 ppm – saturated carbons next to OXYGEN: CH3O, CH2O, etc.
3. 100-150 ppm – UNSATURATED carbon atoms C=C, aromatic C
4. 150-220 ppm – unsaturated carbon atoms next to oxygen e.g. C=O
What can you work out from carbon and proton spectras?
1) Number and type of protons
2) Functionality of compound
3) Neighbouring spin systems
4) Types of carbon
Usually enough to determine or confirm the structure of simple compounds along with IR and MS data
