Carbon 13 NMR Flashcards
What does NMR stand for
Nuclear magnetic resonance spectroscopy
How does NMR work?
Applying a strong electromagnetic field at a range of different frequencies within radio wave spectrum
And the nuclei of atoms in the sample absorb energy from radio waves depending on environment it’s in, this absorbance gives information on position of atoms within molecule
What does carbon 13 NMR give Info on?
The carbon environment of all carbons within the molecule: eg what it is bonded to
What affects the magnetic field experienced by the carbons? (That allows us to differentiate the carbon environments)
The electro negativity of the atoms/ groups bonded to the carbon
So a greater shift value = more electronegative group attached to carbon in this environment
What does each peak in a carbon 13 nmr graph show?
The number of different carbon ENVIRONMENTS - not number of carbons.
Carbons in exact same environment represented by a single peak
Why will more electronegative elements affect carbon environments?
Pull e- towards itself away from carbon so this decreases shielding of carbons nucleus: increased absorbance this shift value
Standard peak name on carbon 13
tetramerhylsicilane aka TMS
What is the point of TMS?
Provides a reference peak, of 0 so all peaks are a shift value away from the peak of TMS
Structure of TMS?
Si bonded to 4 CH3 grouos
What is good about TMS?
Produces only 1 peak at 0
Produces a peak at rhs because carbons are most shielded: large magnetic field required to bring this back to resonance as it does not absorb
Structure of a carbon 13 graph
Shows peaks at different points on the x axis
Which are chemical shift values, that decreases from left to right (TMS has peak at 0 at rhs)
When looking at chemical shift values in a table for carbon environments, what is important to remember?
Consider what is bonded to the CARBON not the R group as this is the carbon environment we are trying to find
Why might a chemical shift value be slightly larger than the table value?
Because it is close to an electronegative element which pulls the electrons away and thus causes less shielding of the carbon
What solvent is used in carbon 13 NMR?
CCl4
What should the solvent be?
Inert so won’t react with the sample
Non Polar so will dissolve non polar samples
No hydrogen so causes no peaks in proton nmr
