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OH- NH-AWWRR
- OH and NH groups are very weird. Depending on different situations they produce different chemical shifts.
- To accurately determine which chemical shift belongs to these groups we use D20
- We measureme an NMR spectrum then add drops of D2O then re-measure, the peak DISAPPEARS!!!!
PROTON NMR SPECTROMETRY
AT THE END OF THIS PRESENTATION YOU SHOULD BE ABLE TO:
Deuterated solvents
-Samples in H-NMR spectrometry are dissolved in solvents.
- TMS is the one commonly used, however sometimes the protons interefere with the sample.
- To avoid this interference solutions containing Deuterium are used.
vs
low resolution NMR
high resolution NMR
now take textbooks and open page 441 and 443
1 analyse and interpret a proton (1
H) NMR spectrum of a simple molecule to deduce:
(a) the different environments of proton present using chemical shift values
(b) the relative numbers of each type of proton present from relative peak areas
(c) the number of equivalent protons on the carbon atom adjacent to the one to which the given proton is
attached from the splitting pattern, using the n + 1 rule (limited to singlet, doublet, triplet, quartet and
multiplet)
(d) the possible structures for the molecule
the area under the graph will tell you the number of hydrogens in that enviroment and the chemical shift values will tell you what group the carbon bonded to the hydrogen is in
It has 12 hydrogen atoms all of which are in exactly the same environment. They are joined to exactly the same things in exactly the same way. That produces a single peak, but it's also a strong peak (because there are lots of hydrogen atoms).
the peaks are split to show the the number of hydrogens on the adjacent molecule so if a cluster has been split 2 times (doublet) is has 2 hydrogen on the adjacent carbon
The electrons in the C-H bonds are closer to the hydrogens in this compound than in almost any other one. That means that these hydrogen nuclei are the most shielded from the external magnetic field, and so you would have to increase the magnetic field by the greatest amount to bring the hydrogens back into resonance.
TMS
non toxic and unreactive, it doesnt interfere with sample
Topic
recap...
chemical shift
enviroment
the area an atom with respects to its molecule
frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm
Topic
PREDICT THE CHEMICAL SHIFTS AND SPLITTING PATERNS OF THE PROTONS GIVEN IN A MOLECULE.
Topic
Topic
Topic
DESCRIBE THE USE OF TETRAMETHYLSILANE, TMS, AS THE STANDARD FOR CHEMICAL SHIFT MEASURMENTS
37.4.4 State the need for deuterated solvents, e.g CDCl3, when obtaining a proton NMR-spectrum
37.4.5 Describe the identification of O-H and N-H proton exchange using D20
Topic
WHAT REALLY HAPPENS?
Alcohols are very slightly acidic, The H on alcohol is attracted to the lone pair on the oxygen on the "heavy" water molecule
The negative ion bumps into another D20 reforming the alcohol but now with O-D
FUN FACT
-The peak of OH- and NH- groups is almost always a singlet. This is because it is not affected by hydrogens in neighbouring carbons
-Similarly, they have no effect on the peaks of the hydrogens in neighbouring carbon atoms