Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: earthnation112 on April 12, 2018, 05:16:45 PM

Title: Question in relation to spectrum of KBH4
Post by: earthnation112 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?   
Title: Re: Question in relation to spectrum of KBH4
Post by: mjc123 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...
Title: Re: Question in relation to spectrum of KBH4
Post by: crawlingmcedge 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.

Title: Re: Question in relation to spectrum of KBH4
Post by: crawlingmcedge on April 14, 2018, 08:18:52 AM
Quote from: mjc123 on April 13, 2018, 04:46:23 AM
Why is the integration 4:1? Oh come on...

 ;D ;D ;D

Title: Re: Question in relation to spectrum of KBH4
Post by: earthnation112 on April 19, 2018, 07:52:30 AM
Quote from: mjc123 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...
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