32 - Structure Determination (NMR) Flashcards
What is Nuclear Magnetic Resonance (NMR)?
A powerful technique that helps find the structures of complex organic molecules.
What is the theory being how NMR works?
Nuclei with either an odd mass number or an odd atomic number posses spin, this spin generates a magnetic field.
When placed in a strong external magnetic field these nuclear magnets align with either parallel (lower energy) or anti-parallel (higher energy) to the field.
There is an energy difference between these two states which depends on its magnetic field strength and the strength of the field.
If a radio wave matching the difference is applied some nuclei absorb it and flip to the higher energy level (resonance).
Each nuclei requires different radio frequency to resonate, so by detecting the frequencies at which different nuclei resonate and the types of atoms present the molecular structure can be determined.
What is carbon-13 NMR?
NMR used to determine the structure and chemical environment of carbon atoms in organic molecules.
Why is carbon-13 used?
Carbon-13 has a spin, unlike carbon-12
Why does carbon-13 NMR work?
Not all carbon-13 atoms in a molecule resonate at the same magnetic field strength, as all nuclei are shielded form the external magnetic field by the electrons around them.
Greater electron density = smaller the magnetic field felt by nucleus = lower resonant frequency.
Describe the process of NMR spectromerty?
Liquid samples are placed in a tube which spins in a magnetic field.
Solids are dissolved in solvents which won’t affect the spectrum.
TMS is added to provide the reference signal.
NMR instrument produces a graph of energy absorbed against chemical shift.
What solvents are used to dissolve solid samples?
CCl4 for non-polar compounds
CHCl3 - chloroform - for polar compounds
What is the formula for TMS?
Si(CH3)4
What is chemical shift?
The difference in resonant frequency of an atomic nucleus relative to a reference signal.
What are the units of chemical shift?
Parts per million (ppm)
Why is TMS used as a reference signal?(6)
Non-toxic
Inert - doesn’t react with the compound being analysed.
Low b.p so it can be distilled off and used again.
All hydrogen atoms are chemically equivalent so only produce one peak.
Has 12 hydrogens, so produces an intense peak and small amounts can be used.
Signal produced is outside the range of most protons, so it doesn’t obstruct any signals.
What is the formula for the chemical shift?
Spectrometer frequency (Hz)
Where are carbons closer to electronegative atoms shifted and why?
Shifted downhill to higher chemical shift values.
As the electronegative atom draws electrons away from the carbon so the carbon is de-shielded, therefore it feels a greater magnetic field, so it resonates at a higher frequency.
On carbon-13 NMR what does the number of signals on the spectrum indicate?
The number of different carbon atoms.
On carbon-13 NMR what does the chemical shift values indicate?
What environment the carbon is in.
What is proton NMR?
The application of NMR spectroscopy with respect to hydrogen nuclei.
How can you calculate the area under a peak in proton NMR?
Measured using integration, where the distance between the top and bottom lines gives the height and the heights from each signal are compared and made into a simple ratio.
Why does spin-spin coupling occur in proton NMR?
Because the applied magnetic field felt by a hydrogen atom is affected by the magnetic field of the hydrogen atoms on neighbouring carbons atoms.
What does spin-spin coupling tell us in proton NMR?
Gives information about how many neighbouring hydrogen atoms there are.
What are the four different types of spin-spin coupling and their ratio?
Doublet - 2 peaks - 1:1 = 1 neighbouring H
Triplet - 3 peaks - 1:2:2 = 2 neighbouring H’s
Quartet - 4 peaks - 1:3:3:1 = 3 neighbouring H’s
Quintet - 5 peaks - 1:4:6:4:1 = 4 neighbouring H’s
Why do hydrogens attached to an oxygen molecule not affect spliting?
The hydrogen atom attached to the oxygen rapidly exchanges with other protons on other molecules, therefore it is not attached to any particular oxygen long enough to register a splitting signal.
What does the number of signal groups indicate in proton NMR?
The number of different proton environments
What does the chemical shift indicate in proton NMR?
The proton’s environment
What does the peak area (integration) indicate in proton NMR?
The number of protons in each environment.
