NMR Flashcards
What does NMR stand for?
Nuclear Magnetic Resonance
How does NMR work?
-analytical technique that allows the structure of a molecule to be determined
-analyses the energy of each bond environment
-different bond environments absorb different amounts of energy
-hence show up as different peaks on a spectra
What is used as the standard and why?
TMS - tetramethylsilane
-it has the same carbon and hydrogen environments, hence gives 1 peak
-inert
-non-toxic
-low BP = can be removed from sample easily
What peak does TMS give?
δ = 0ppm
What does δ symbol represent?
chemical shift
measure of the resonance frequency of a nucleus relative to a reference compound, typically tetramethylsilane (TMS)
What happens to carbon environments that are near an oxygen atom and why?
-O is very electronegative
-changes the bond environment and how it absorbs energy
-pulls electrons away from the carbon nucleus leaving it more exposed to any external magnetic field
-reduces shielding on the C atom
-chemical shift of carbon environment increases
examples of solvents
Deuterated Solvents in Proton NMR
When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent
Tetramethylsilane (TMS) is a commonly used solvent in NMR as it gives one sharp reference peak on NMR spectra
However, the proton atoms can still interfere with peaks of a sample compound
To avoid this interference, solvents containing deuterium can be used instead, e.g. CDCl3
Deuterium (2H) is an isotope of hydrogen (1H)
Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds
Therefore, the reference solvent peak will not interfere with those of the sample
Identifying the -OH or -NH signal in an NMR spectrum
In ¹H NMR, samples are dissolved in a solvent to help separate molecules and prevent them from interacting
The solvent must:
Be a good solvent for organic molecules
Not contain any hydrogen (¹H) atoms, so it does not interfere with the NMR signals
Deuterated and non-deuterated solvents
Carbon tetrachloride, CCl4:
This solvent does not contain hydrogen, so it does not produce 1H NMR signals.
It is suitable for 1H NMR but does not dissolve all molecules well.
Deuterated solvents are often used in 1H NMR spectroscopy because deuterium (²H) is an isotope of hydrogen with no nuclear spin, which does not affect NMR results
Deuterochloroform / Chloroform-d, CDCl3:
This is often preferred because it contains deuterium (2H) instead of hydrogen, so it does not interfere with the proton NMR spectrum
Deuterium oxide / Heavy water, D2O:
The deuterium atoms exchange reversibly with the protons in the -OH and -NH groups, allowing these signals to be identified in the NMR spectrum
Identifying -OH and -NH signals
Protons in -OH (hydroxyl) and -NH (amine) groups give singlet peaks in ¹H NMR, but these signals can be tricky:
They are broad or sometimes fall outside normal chemical shift ranges
The proton in these groups exchanges quickly with protons from water or acids, so only one peak appears
Their chemical shift ranges may overlap with other types of protons, making them difficult to interpret
To identify these groups more clearly, proton exchange with deuterium oxide (D2O) is used:
The deuterium atoms in D2O exchange reversibly with the protons in the -OH or -NH groups
-OH + D2O ⇌ -OD + HOD
-NH-CO- + D2O ⇌ -ND-CO + HOD
Since deuterium does not absorb in the same region as protons in the NMR spectrum, the signal for -OH or -NH disappears after D2O is added
This confirms the presence of -OH or -NH groups in the molecule
If a peak disappears after adding D2O, it must have been due to the exchange of a proton from an -OH or -NH group
This technique is particularly useful because -OH and -NH peaks can be broad and difficult to assign confidently without D2O exchange
