solve part c 20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded v
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solve part c
20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded views shift area 0.93 3.00 1.56 2.00 3.17 1.00 3.58 2.00 (b) TMS CH3 CH2CH2OH (d) (c) (a) 0 ppm Chemical shift (6) a) Match the protons in the molecule to the corresponding peaks in the spectrum. Explain how you come to your conclusions. (base your explanation on the spin-spin splitting and your understanding of chemical shifts). (6 pts) b) Explain why the peak at 3.58 6 is shifted farthest downfield? (4 pts) c) What is the purpose of the TMS peak? (4 pts) d) In NMR why, in terms of magnet strength, is bigger better? (4 pts)Explanation / Answer
c) TMS is tetramethyl silane. In TMS, there are 4 carboons attached to Si group and all 4 carbons have 3 hydrogens each. So, all the hydrogens are equivalent. They are in same atmosphere. Thus they all give a single peak at one position in NMR. TMS is used as a reference standard in NMR. TMS peak is considered at 0 ppm shift. All other peaks are in relative to TMS peak and their shifts are downfield to TMS. It is easy to give chemical shifts positions if we have a standard. The reasons why TMS is chosen as standard are:
It is unreactive and is volatile. So, if you want to recover the product from the sample, you can after TMS evaporates.
It is not affected by temperature and concentration.
The peak is outside the spectral region, so it does not interfere with the other peaks of interest.
All the protons are equivalent, so there is only on peak due to TMS.
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