3.15 NMR Spectroscopy Flashcards
What is NMR spectroscopy?
A method used to determine the molecular structure of a compound using an external magnetic field.
How is a nucleus shielded from the external magnetic field?
Shielded by the electrons surrounding the nucleus.
Why do nuclei absorb different amounts of energy?
They all have different environments.
What affects electron shielding?
The atom/s adjacent to nucleus.
i.e. If an electronegative element is near the carbon atom it will pull electrons toward itself and reduce shielding.
How do different environments affect nucleus in NMR?
Nuclei feels magnetic field differently depending on environment. Different amounts of energy and frequencies picked up by NMR.
What is considered as same environment in NMR?
Bonded to atom/atom group which are identical.
Name the reference compound used in NMR. What is the structure?
Tetramethylsilane (TMS)
Si(CH3)4
Why is TMS used as reference?
It has identical environments which results in one singlet peak.
Advantages of using TMS.
Inert, non-toxic & volatile so easy to remove from sample.
How is a sample for NMR made?
- Add TMS to sample
- Put in NMR machine
- Measure the chemical shift against TMS (delta = 0 as reference)
- Remove TMS
Name the difference between TMS peak and peaks produced by sample being tested?
Chemical shift (Delta symbol)
Where is the reference point?
Delta = 0
How do you know the number of carbon environments on a 13C spectrum?
No. of different carbon environments = no. of peaks
What direction are the peaks which are less shielded ?
left
What direction are the peaks which are more shielded?
Right
In CH2ClCH3, which carbon atom would have a higher chemical shift?
The carbon that is directly bonded to the chlorine atom will have a higher chemical shift as it will be closer to the chlorine atom and therefore it’s less shielded.
In CH2ClCH3, which carbon atom will have a lower chemical shift?
The carbon atom in CH3 will have a lower chemical shift as it’s further away from chlorine atom and more shielded and thus has a lower chemical shift.
What does 1H NMR show?
No. of different hydrogen environments and how many hydrogen atoms are in each environment.
How can you identify the number of hydrogen environments from a 1H NMR Spectrum
No. of different hydrogen environments = no. of peaks
What do the numbers above each peak indicate?
The ratio of hydrogen atoms in each environment
How would you calculate the number of hydrogen neighbours on an adjacent carbon molecule from a 1H NMR spectrum?
-Look at the no. of smaller peaks on each peak
-n-1 = no. of hydrogens on neighbouring adjacent carbon atom
Names of different number of peaks.
Singlet = 1 peak
Doublet = 2 peaks
Triplet = 3 peaks
Quartet = 4 peaks
How many hydrogen neighbours does a singlet, doublet, triplet and quartet have?
Singlet = 0 neighbouring hydrogens
Doublet = 1 neighbouring hydrogens
Triplet = 2 neighbouring hydrogens
Quartet = 3 neighbouring hydrogens
What are samples for 1H NMR dissolved in and why?
Non-hydrogen based elements like deuterated solvents or CCl4.
Used to avoid confusing the spectra.
