Proton NMR - Spin-Spin Splitting and Multiplicity

In summary, the highlighted protons in the molecule (C6H5)-CH2-CH2-CH2-OH will have a multiplicity of either a quintet or a hextet, depending on whether or not first order coupling can be assumed. This can be determined by comparing the frequency difference from the centroid of the multiplets to the spin-spin coupling constant, with first order coupling being assumed for \Delta v\ /J > 20. The spin-spin coupling constants for methylene-methylene are usually about 5-7 Hz.
  • #1
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1. What will be the multiplicity due to spin-spin splitting of the highlighted protons in the molecule: (C6H5)-CH2-CH2-CH2-OH?



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The Attempt at a Solution


I know that the ones surrounding the highlighted CH2 group are not equivalent. If they were, the multiplicity would be 5. When I make a branching diagram, I get 6 to a multiplet. In the textbook, there was an example of a similar case where you could "assume" that they were equal, even though they were not. How do I know if I can "assume" in this case? My answer choices are a) doublet b) triplet c) quartet d) quintet e) hextet. I'm stuck between d) a quintet, if I assume that they are equal and e) the minimum, probably, but couldn't a multiplet of more than 6 form also? Any help? Thank you.
 
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  • #2
The assumption you are asked to make... is this first order coupling? Generally first order coupling is assumed for [tex]\Delta v\ /J > 20[/tex], where [tex]\Delta v\ [/tex] is the frequency difference from the centroid of the multiplets. Sometimes first order coupling is apparent where [tex]\Delta v\ /J =10[/tex]. The spin-spin coupling constants for methylene-methylene are usually about 5-7 Hz.

Remember that J coupling is independent of magnetic field strength whereas the absorption frequency is a function of field strength.
 
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  • #3
Thank you for your help. The problem never said that I could assume it was first-order coupling, but my professor also cleared up the situation. Thanks again.
 

What is Proton NMR?

Proton NMR is a spectroscopic technique used to study the structure and composition of molecules. It involves the use of a strong magnetic field to observe the behavior of hydrogen atoms (protons) in a molecule.

How does Spin-Spin Splitting occur in Proton NMR?

Spin-Spin Splitting occurs when two or more hydrogen atoms in a molecule have different chemical environments, resulting in slightly different energy levels. This causes the protons to have slightly different resonant frequencies, which appear as separate peaks in the NMR spectrum.

What is Multiplicity in Proton NMR?

Multiplicity refers to the number of peaks observed in a specific region of the NMR spectrum. It is determined by the number of neighboring protons and the type of coupling between them.

How is Spin-Spin Splitting used in structure determination?

Spin-Spin Splitting patterns can provide valuable information about the number and arrangement of protons in a molecule. By analyzing the multiplicity and relative intensities of the peaks, the structure of a molecule can be determined.

What factors affect the magnitude of Spin-Spin Splitting?

The magnitude of Spin-Spin Splitting is affected by several factors, including the strength of the magnetic field, the distance between the coupled protons, and the type of coupling (such as through single, double, or triple bonds).

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