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Topic: Question in relation to spectrum of KBH4  (Read 5071 times)

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Offline earthnation112

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Question in relation to spectrum of KBH4
« on: April 12, 2018, 05:16:45 PM »
Question is:
1. Below is shown the 1H NMR spectrum of (KBH4). Explain the appearance of the spectrum. (spectra attached)

The answer given is:
Both 10B (I = 3, nat. ab. 20 %) & 11B (I = 3/2, nat. ab. 80 %) NMR active and couple of 1H. Hence 80 % of the sample contains 11BH4 which gives rise to a 1:1:1:1 1H NMR signal, whilst the remaining sample contains 10BH4 which gives rise to a 1:1:1:1:1:1:1 septet signal. Overall integration of the two overlapping signals should be 4:1

Can someone help me understand how the answer is obtained, I know in this forum you don't answer questions, rather you try directing the enquirer towards the answer but here I have the answer but don't understand it!

So I know its a 1H NMR spectrum, and I also know both 10B and 11B are NMR active and I also know they abundance being 20% and 80%, where does l = 3 and 3/2 come from? I also know that since 11B is more abundant the sample contains more of it than it does 10B. But why does this equate to 11B giving rise to a 1:1:1:1 signal and the other gives rise to septet signal? Also why is the integration of the two 4:1?   

Offline mjc123

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Re: Question in relation to spectrum of KBH4
« Reply #1 on: April 13, 2018, 04:46:23 AM »
I is the nuclear spin quantum number, you can find it in any textbook on the subject.
For a nucleus with spin 3/2, how many possible orientations are there of the spin vector relative to the magnetic field? What effect does this have on the nmr spectrum of a proton coupled to this nucleus?
Why is the integration 4:1? Oh come on...

Offline crawlingmcedge

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Re: Question in relation to spectrum of KBH4
« Reply #2 on: April 14, 2018, 08:18:15 AM »


DIfferent spin comes from the isotopic weight with even/odd combinations of protons and neutrons having different spin types.

Also just slowly reread your abundance values and your relative integrations. The answer really is right in front of you.

« Last Edit: April 14, 2018, 08:31:03 AM by crawlingmcedge »

Offline crawlingmcedge

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Re: Question in relation to spectrum of KBH4
« Reply #3 on: April 14, 2018, 08:18:52 AM »
Why is the integration 4:1? Oh come on...

 ;D ;D ;D


Offline earthnation112

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Re: Question in relation to spectrum of KBH4
« Reply #4 on: April 19, 2018, 07:52:30 AM »
I is the nuclear spin quantum number, you can find it in any textbook on the subject.
For a nucleus with spin 3/2, how many possible orientations are there of the spin vector relative to the magnetic field? What effect does this have on the nmr spectrum of a proton coupled to this nucleus?
Why is the integration 4:1? Oh come on...
Thanks for the advice went back to my 1st year analytical notes and realized that a 3rd year student like myself shouldn't be having trouble with such basic questions!!!
Think it's because we don't have a module dedicated to analytical chemistry this year I had forgot everything!! The problem with memorising information just for exams and then dumping later!
Thanks 

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