NMR Spectroscopy Flashcards
What is NMR (Nuclear Magnetic Resonance) and what is used for?
- An analytical technique.
- Used to work out structure of an organic molecule.
Name some isotopes with the property “spin”. What is spin?
Spin is a magnetic proporty that some nuclei contain such as H¹, C¹³
How does NMR spectroscopy.
- In an NMR spectrometer, nuclei with the magnetic property, spin, such as H¹ and C¹³ are subjected to a strong magnetic field.
- This causes the direction of spin of these nuclei to align with the direction of the magnetic field (a low-energy state) as this is the most stable alignment as opposed to if the direction of spin was in the opposite direction to the magnetic field (high-energy state)
- When these nuclei are then subjected to radio waves with a range of frequencies, each nucleus absorbs energy of the frequency which corresponds to the difference in energy between its low-energy state and its high-energy state (energy gap) and switches spin direction from one to the other. The absorption can be detected and converted into a spectrum.
What does the resonance in nuclear megnetic resonance mean?
When the spin of the nuclei later shifts direction (i.e. in line with the magntic field), they emit the same amount of energy they absorbed - this is known as relaxation.
Resonance is the cycle of excitation and relaxation as long as the frequency of the wave equals the energy gap.
What does the energy required to change the aligment depend on? i.e. what determines the value of the energy gap?
The energy required to change the alignment (from low-energy state to high-energy state) depends on the enivronment that the atom is in and can be compared to a standard.
Why does the environment that a nucleus is in affect the energy required to change the alignment?
This is due to nuclear shielding.
Nuclear shielding of a nucleus occurs when weak magnetic fields from the electrons surrounding that nucleus, and electrons of neigbouring nuclei, shield it from the effects of the external magnetic fields.
The density of these electrons give atoms different environments, each with different resonant frequencies (the frequency to change the nuclei spin direction).
What is a chemical shift?
This is the name given to the difference in energy absorbed by nuclei in different environments relative to the standard substance (TMS).
Measured in ppm, parts per million.
What was the reference substance used and why is a reference substance used?
TMS (Tetramethylsilane) is used as the reference chemical and produces the peak at zero ppm i.e. its chemical shift is zeroed. Also, its absroption peak is at a lower frequency than just about everything else, so any chemical shifts larger than that will always be to the left of it on the spectrum.
- A peak at a chemical shift of, say, 2.0 means that the hydrogen atoms which caused that peak need a magnetic field two millionths less than the field needed by TMS to produce resonance. A peak at a chemical shift of 2.0 is said to be downfield of TMS. The further to the left a peak is, the more downfield it is.
- Downfield means more left on the graph.
Why is TSM a good standard, reference chemical to use?
- Contains both carbon and hydrogen so canb e used for both carbon and proton NMR.
- Produces one sharp signal as hydrogen atoms are in a single environment. As there is only one carbon atom, will only produce one signal in carbon-13 NMR.
- Non toxic and inert so unlikely to react with chemicals that are being investigated
- Volatile and low BP, so easily removed from sample at the end.
Most NMR spectroscopic analysis is achieved by first dissolving the molecule in a solvent. Which solvent is used and why?`
- Used to investigate organic compounds, so organic solvent must be used.
- Organic solvents however have carbon and hydrogen that would produce signals all over the spectrum so to avoid this, we use H² (deuterium) instead of H¹ as the nuclei of the isotope deuterium, is not active in NMR so will not appear on the spectrum. The carbon in the solvent is likely to be carbon-12 and this has no spin, so would not appear in NMR spectrum either.
- Therefore a good deuterated solvent to use is deuterated trichloromethane (CDCl3)
*D becuase its H² (deuterium) instead of H¹.
What are the two types of NMR spectroscopy?
Carbon-13 NMR Spectroscopy
Proton NMR spectroscopy.
How does carbon-13 NMR spectroscopy work? (We have already looked at the method)
NMR spectroscopy where the nuclei being detected is that of carbon-13 (the only isotope of carbon that has the property spin)
*nuclei with the same number of neutrons and protons have no spin.
What does the number of peaks on the carbon-13 NMR spectrum indicate?
The number of carbon environments
What does the chemical shifts (ppm) on the carbon-13 NMR spectrum indicate?
The type of carbon environment (data sheet)
How do you calculate the number of carbon environments from just looking at the organic molecule?
If the molecule is not symmetrical just count the number of carbons to give you the carbon environment. If the molecule is symmetrical, draw the line of symmetry, and count the number of carbon atoms that are symmetrical on both sides as just one, and any carbon atoms, exactly on the line of symmetry as another.
How many carbon environments in:
1) Pentane?
2) Ethanol?
3) Propanone?
4) Propan-2-ol
5) 2-methylbutane
6) Benzene
7) 1,2-dichlorobenzene
8) 1,3-dichlorobenzene
9) 1,4-dichlorobenzene
1) 3 carbon environments
2) 2 carbon environments
3) 2 carbon environments
4) 2 carbon environemnts
5) 4 carbon environments
6) 3 carbon environments
7) 3 carbon environments
8) 4 carbon environments
9) 2 carbon environments
*Explain last three.
Why does C-O or C=O bond have a greater chemical shift than C-C?
The electrons around a carbon generate small magnetic fields that affect the net field experienced by each carbon nucleus. In general, electrons surrounding an atom move in such a way so as to create a field at the atom that tends to counteract the applied magnetic field. The electrons thus “shield” the carbon nucleus from the applied magnetic field and this means that less energy is necessary to excite the carbon nucleus from one spin state to another and therefore its chemical shift comes at a lower frequency than it would otherwise. For example, the carbon atom in a carbonyl group has a relatively low electron density around it, and thus is relatively “deshielded” and consequently has a higher chemical shift than most other types of carbons. The electronegative oxygen pulls electrons away from the carbon nucleus leaving it more exposed to any external magnetic field. That means to bring the nucleus into the resonance condition, a higher energy, higher frequency wave is needed. The greater the external magnetic field, the higher the chemical shift.
THIS IS IT BASICALLY - SHORTEN THIS DOWN WHEN MEMORISING.
How do you know which peak is related to which carbon environment?
You use a mixture of two methods:
1) The score value
2) The presence of electronegative atoms/the proximity effect.
