I'm currently taking VCE (Victorian Certificate of Education) Chemistry classes, and we're currently studying the interpretation of spectra produced by Hydrogen NMR (Nuclear Magnetic Resonance) spectroscopy.
When studying the spectra of High Resolution 1H NMR, the peaks representing the different Hydrogen environments are split into multiplets based on the protons surrounding these environments. There has been a considerable amount of confusion in my classes over the actual principles / rules of thumb on how to calculate the multiplets for a particular environment of a known chemical (i.e.; known structue), based off the 'n + 1' rule.
(i.e.; an environment with n neighbours will be split into n+1 multiplets).
We are absolute on the principles that;
- Peaks of a particular Hydrogen environment are not split by neighbouring protons in equivalent environments.
- OH does not, and is not split by, it's neighbouring environments.
However, immense confusion arised over whether the following principle was correct.
- Peaks of a particular Hydrogen environment will only be split by the protons in neighbouring environments once for each type of neighbouring environment.
e.g. The middle "CH2" environment in "CH3–CH2–CH3" will only have 4 peaks;
Although it has 6 neighbouring protons, they are two lots of the same environment (CH3).
As a class, we found numerous examples from different text books and sources that provide examples of ¹H NMR spectra which did not clarify the matter; Some considered all neighbouring protons as neighbours, others discriminated on the repeated neighbouring environments. For example, the CH2 in CH3–CH2–CH3 was sometimes split into 4 peaks or 7 peaks, depending on the source.
Many Chemistry teachers contradicted each other on the matter.
There was repeated self corrections made by the teachers, such that now nobody really knows whether this principle is correct or not.
So, is there anybody that has the correct information on the matter?
Is there a reasonable explanation behind this strange lack of correlation,
or is there a common misconception about multiplet splitting?
Ultimately; How many multiplets should the CH2 in CH3–CH2–CH3 have?
(Note that if there is a complicated explanation that I am currently only at Year 12 VCE level, so links to resources I can pursue would be extremely helpful! It's been established that VCAA (an authority for the education system in Australia) ensures that the chemicals featured in the exams for NMR analysis will not be of a structure so as to allow the ambiguity above.)