I think I should say something here.
Firstly, good work to Water and Pixon, Pixon especially. I mostly agree with what they have said about NMR.
It is crucial to understand that in NMR, two atoms are in the same environment ONLY IF their connectivity in the molecule is exactly the same. You need to consider the immediate neighbours, the second neighbours, the third neighbours, etc.
In another example, consider the nylon 6-6 dimer HOOC-CH2-CH2-CH2-CH2-CONH-CH2-CH2-CH2-CH2-CH2-CH2-NH2
In this example, each CH2 are slightly different to each other. In 13C NMR, you would expect 12 different peaks, in 1H NMR, you would expect 13 different peaks.
This blurb is beyond VCE level, but is useful if you want to get to the bottom of what is meant by 'multiplicity' and 'multiplet'.
On the topic of multiplicity, the term multiplicity (not to be confused with multiplet) refers to the peak-splitting effects of spin-spin coupling between neighbouring nuclei. Different neighbouring nuclei have different splitting frequencies (the separation between the split peaks, splitting frequency depends on the chemical environment). The n+1 rule is an approximation for calculating how many stable energy states there are in spin-spin coupling.
Consider 1-propanol: CH3-CH2-CH2-OH, the underlined H is split by two neighbouring groups at two slightly different frequencies. CH3 splits it into a 4-tet, and CH2 splits it into a 3-tet. Thus the underlined H is split into a 12-tet peak. However, the two splitting frequencies are very similar, and if you draw out the splitting diagram with very similar splitting frequencies, you will get a 6-tet. This is the n+1 approximation (which is valid most of the time).
Multiplet arise from overlap of peaks in similar chemical environments. This is the assignment we give to peak patterns that are too complex to assign. Going back to the nylon 6-6 dimer example, we won't actually see 13 distinct H peaks, because many of the CH2 groups are very similar to each other and they will overlap. This overlap creates a complex peak pattern that is usually unresolved, and cannot be assigned to simple n-tet peaks.
Multiplets also arise from complex splitting patterns that involve isotopes, 2J, 3J and 4J splitting, and various other complex interactions that makes it different to the simple n-tet peaks (n+1 approximation). This is seen a lot in aromatic compounds, since 4J splitting is significant (4J splitting is very weak in aliphatic compounds).
In summary for VCE students:
-The only time when two atoms are in the same environment is if they are exactly the same (i.e. have the same connectivity no matter how far you look). This means there must be some symmetry element. If you can't find symmetry, they are most probably in different environments.
-Use the n+1 rule, but understand that the actual physics of NMR is a lot more complicated, and you see many more complex patterns. The n+1 rule is an useful approximation, but an approximation nonetheless.
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