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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: Athos on February 26, 2015, 04:03:55 PM

Title: Same sample in two different NMR instruments
Post by: Athos on February 26, 2015, 04:03:55 PM

Hi guys,

Sorry for this topic, but I'm stucked on this question since several days, and I didn't find any help on the net.

You analyse a compound with two different NMR instruments, but the first instrument's magnetic field is twice as strong as that of the second.
A peak appears at 4ppm in the first spectrum.
What will be the chemical shift of this peak in the second spectrum ?

For me, chemical shift δ is controled by ΔE (spin-flip energy), which is determined by the magnitude of the magnetic field at the nucleus.
So low magnetic field, low δ and high magnetic field, high δ.
If the first magnetic is twice as strong, its δ is twice lower. And 4/2=2ppm.

But, I'm wrong, ist'n the right answer.
Can someone explain me please ?  ???

Title: Re: Same sample in two different NMR instruments
Post by: Babcock_Hall on February 26, 2015, 04:29:27 PM

Write out the definition of δ in ppm and the answer may come to you very quickly.

Title: Re: Same sample in two different NMR instruments
Post by: Athos on February 26, 2015, 04:34:35 PM

So in NMR spectroscopy, δ is a measure of how far the signal is from the signal for the reference compound.
In other words, it's the resonant frequency of a nucleus relative to a standart.

And δ=(Vsample - Vreference)/Vreference

So if δ=4 we have 1000-200/200

If the magnetic field is twice lower, the frequency is lower too. So the δ=1000-100/100=9 ?

Edit : Ok, I understand, magnetic field and frequency are inversely proportional, so for a same compound, if the magnetic field is lower, the δ in ppm is higher. But I'm not sure about my result.

Title: Re: Same sample in two different NMR instruments
Post by: Borek on February 26, 2015, 05:27:55 PM

Quote from: Athos on February 26, 2015, 04:34:35 PM

If the magnetic field is twice lower, the frequency is lower too.

Which frequency? Sample, reference, or both?

Title: Re: Same sample in two different NMR instruments
Post by: Babcock_Hall on February 26, 2015, 05:35:27 PM

@OP, As the magnetic field increases, how does this change ΔE?

Title: Re: Same sample in two different NMR instruments
Post by: Athos on February 26, 2015, 05:38:21 PM

@Borek : It's the resonance frequency, so I think the sample one ?

@Babcock_Hall : If the magnetic field increases, β-spin state decreases and α-spin state increases, so  ΔE increases. The difference in energy is dependent upon the external magnet strength.

And about my result ? Wrong or right ?

Title: Re: Same sample in two different NMR instruments
Post by: Borek on February 27, 2015, 03:10:20 AM

Quote from: Athos on February 26, 2015, 05:38:21 PM

If the magnetic field increases, β-spin state decreases and α-spin state increases, so  ΔE increases. The difference in energy is dependent upon the external magnet strength.

For sample, reference, or both?

Title: Re: Same sample in two different NMR instruments
Post by: Irlanur on February 27, 2015, 05:31:13 AM

So chemical shifts are tabulated in books. usually, no frequency is given in these tables. what might that mean;)

Title: Re: Same sample in two different NMR instruments
Post by: Athos on February 27, 2015, 08:23:55 AM

Quote from: Borek on February 26, 2015, 05:27:55 PM

Which frequency? Sample, reference, or both?

The frequency is a function of the magnetic field strength, so I think both ?

Title: Re: Same sample in two different NMR instruments
Post by: Borek on February 27, 2015, 09:06:28 AM

Quote from: Athos on February 27, 2015, 08:23:55 AM

I think both

Yes.

You have all necessary pieces of information. Just apply them.

Hint: actually the question is not "by how much?".