6.3.2 - Spectomery Flashcards
Quick background check of everything you remember about NMR
What does NMR measure ?
The amount of energy absorbed by the nuclei.
What is the amount of energy absorbed by the nuclei dependent on ?
The environment it is in.
Understanding shielding.
What is a splitting peak ?
The number of in equivalent H’s on neighbouring C atoms + 1
Using the splitting peak theory explain the different peaks that would show up
Make sure to remember that Nuclei in identical chemical
environments do not show coupling amongst themselves!
When carrying out NMR we need to dissolve our sample to be able to carry out our experiment. What kind of solvents do we use to dissolve our sample and why ?
- Samples are dissolved in solvents without any 1H atoms, e.g. CCl4, CDCl3.
- This means that in the H NMR the solvent will not give any peaks
- The same solvent is used in C13 NMR and in this case there will be one peak due to the solvent
that will appear on the spectrum. However, it is known where this peak is so it can be ignored.
In the exam it is likely this peak will not occur on the spectra.
What is the use of TMS ?
Tetramethysaline is used as the standard for chemical shifts meat greens
How do we identify O-H and N-H protons ?
- Via proton exchange using D2O.
- If there is an OH or NH proton in the sample then deuterium will take its place.
- Addition of D2O to the sample of ethanol would make the peak in the above spectrum disappear. This is because D2O has an even number of nucleons so there would be no nuclear spin and so it wouldn’t be detected in the NMR spectrum.
What does elemental analysis help us to determine ?
The empirical and molecular formula of a compound. With this information we can work out the structure of a compound.
Exceptions to the proton nmr rule list
- hydrogen which are connected to the same carbon, but that carbon is double bonded to another carbon those hydrogens may not be in the same environment. This is due to the restricted rotation of the carbon carbon double bond as a result if the carbon under the other side is attached to different groups then the routes for those hydrogen will be different and they will be in different environments.
How do we determine the number of hydrogen and an environment?
So when you do it in the exam, you would notice that there will be a small number at the top of each peak which indicates to you the number of hydrogen in the environment how to walk this out is by calculating the area on the each under each peak.
And then sometimes after they have calculated the area under each peak, they can compare it with other proton peaks so the area under those peaks then they can form a ratio of area to area so like ratio of the area of one peak to the area of another peak and then you can calculate from that the number of hydrogen in another peak if that makes sense if not watch the upload video on proton NR area on each peak
