Topic 32: NMR Flashcards
Spin
Nuclei with either odd mass no. or odd atomic no. possess spin.
Angular momentum
Information from C13 nmr
- No. different signals in spectrum
- Position of the signals (chemical shift)
PEAK HEIGHT MEANS NOTHING
Information from H1 NMR
- No. different signals in spectrum
- Position of the signals (chemical shift)
- Intensity of different signals
- Splitting pattern of signals
Solvents used to dissolve solids in NMR
- CCl4 non-polar so for non-polar samples
- CDCl3 polar so for polar samples - deuterated chloroform
neither affect the spectrum
TMS
Tetramethylsilane (CH3)4Si
Reference for NMR
- non toxic
- inert
- low bp - distilled and reused
- H+ ions chemically equivalent with large peak
- doesn’t obscure other signals as peak is far from others at 0
δ in NMR
Chemical shift which shows the environment - what is attached to and what is attached to that (not just adjacent)
Desheilded
Point of NMR
Determines structure of molecule by showing how many “unique” carbon/ hydrogens there are in it
Sheliding in NMR
By being bonded to certain atoms changes the bond environment and so cause a chemical shift causing a different peak (oversimplifying stuff to do with electronegativity and sheilding)
Downfield on NMR
Left of the specta cause by more deshielding
Splitting for Carbon NMR
NOPE
One peak per carbon
Area under peak of 1H NMR
Proportional to no. hydrogens
How many of each type there are
Chemical shift def
Difference between the field strength at which it absorbs and the field strength at which TMS protons absorb
Temp/solvent effect (NMR)
No difference to proton chemical shift - constant
Shifting H downfield
Nearer to electronegative species shift downfield to higher δ values
Low resolution NMR
1 peak for each environmentally different group of protons
High resolution NMR
Gives more complex signals including doublets, triplets…
Number of peaks on 1H NMR (high res)
Number of chemically different Hs on adjacent atoms + 1
Doublet
2 peaks, 1 neighbouring H
Triplet
3 peaks, 2 neighbouring H
Quartet
4 peaks, 3 neighbouring H
Quintet
5 peaks, 4 neighbouring H
OH signal on 1H NMR
Unaffected by hydrogens on adjacent atoms. Always a singlet
Integration on 1H NMR
Determines RATIO of number of Hs in each environment not actual number
How to use integration in 1H NMR
Continuous integration line
Measure integration line rise between signals
Compare values and work out simple ratio
Why H on OH different? (NMR)
The H on OH rapidly exchanges with protons on other molecules and is not attached to any oxygen long enough to register splitting signal
Equivalent H atoms (NMR)
H attached to same C are equivalent
Can also be neighbouring as just have to be the exact same environment
Cause no splitting to each other
Cl-CH2-CH2-Cl
4 equivalent H
One peak no spliting
