Topic 19: Modern Analysis Techniques 2 2️⃣
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What is high resolution mass spectrometry (HMRS) used for ?
Provide an Mr value to four or more decimal places Values with this degree of precision can sometimes enable a compound to be identified from the Mr alone Useful if compounds have the same Mr when rounded
What is the theory behind NMR
Makes use of a strong magnetic field and radio frequency radiation Nucleus absorbs energy if both are present If odd number of nucleons present, the spin state of nucleons is flipped = resonance
Describe an NMR spectrum
Vertical axis sometimes labelled absorption (no units), often label is omitted however Horizontal axis is labelled chemical shift / δ symbol (units ppm)
What are the 2 types of NMR we need to be aware of ?
1H (proton) NMR Carbon 13 NMR
Name and explain the most common solvent for 13C and 1H NMR
CDCl3 - symbol D stands for deuterium, 2H isotope of hydrogen, containing a proton and a neutron whose spins cancel eachother out - produces no signals which can interfere with the signals from hydrogen atoms - produces only one signal that can interfere with signals from carbon atoms but can be easily removed from spectrum
What is the reference standard for a chemical shift (NMR)
(CH3)4Si
Why is (CH3)4Si used as the reference standard for a chemical shift
• highly shielded 1H nuclei • unreactive & non-toxic • low boiling point • produces a single strong signal which is easy to identify
What is a chemical shift of a proton (or group of protons)
A number (in units ppm) that indicates its behaviour in a magnetic field relative to (CH3)4Si. It can be used to identify the environment of the carbon atoms or hydrogen atoms attached to it
What does the number of peaks on an NMR spec tell us
The number of different chemical environments
What do the positions of each of the peaks on the NMR spectrum tell us?
Tell you the type of chemical shift
What do differences in height of peaks in 13C NMR spectrum tell us ?
Indicates greater number if carbon atoms causing that peak
What are the 2 types of 1H NMR spectrometry
Low resolution and high resolution.
What do the areas under the peaks in 1H NMR spectrometry correspond to
The number of hydrogen atoms in each environment
Give the framework of how to interpret a chemical shift
δ ~ 45-62ppm C -OH
If a peak is seen at 0ppm what does it correspond to (NMR)
(CH3)4Si - should be ignored
What is the difference between low and high resolution 1H NMR spectrometry?
Share same features bar one : - high res proton NMR has splitting patterns
What causes the splitting of peaks in high resolution 1H NMR spectrometry
The influence of any other protons on adjacent carbon atoms causes splitting (sometimes referred to as spin-spin coupling)
What are the types of splitting that can occur in 1H NMR spectrometry
If peak is not split = singlet If peak is split into 2 sub peaks = doublet If peak is split into 3 sub peaks = triplet If peak is split into 4 sub peaks = quartet
How is the n+1 rule used in splitting patterns of high res 1H NMR spectrometry
Splitting tells us number of protons on adjacent carbon(s) - count subpeaks minus 1 OH doesn’t get split
If unsure which peak is due to OH in high res 1H NMR what should be done ?
Add heavy water (D2O) and re-run, peak will disappear