NMR analysis

Just have to analyse some of our NMR spectra that were the result of one of our lab sessions yesterday, and include intergration, multiplicity, coupling constants.

Am I right in assuming that integration is the number of protons in that environment/peak and that it can be used to look at the ratio of protons in other environments.

That multiplicity is just singlet/doublet/triplet etc - the number of protons interacting with the one you are looking at.

Coupling constants (J), im not too sure, but from what I understand from looking it up its the difference between the peaks within a doublet/triplet..etc but how exactly would you work this out in J?

Just wanted to check this is correct :smile:

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Reply 1

EierVonSatan

cptbigt

Am I right in assuming that integration is the number of protons in that environment/peak and that it can be used to look at the ratio of protons in other environments.

yes

That multiplicity is just singlet/doublet/triplet etc - the number of protons interacting with the one you are looking at.

yes, the protons it couples to in non-identical environments

Coupling constants (J), im not too sure, but from what I understand from looking it up its the difference between the peaks within a doublet/triplet..etc but how exactly would you work this out in J?

You need to covert ppm into Hz (depends on the MHz of the machine you are using). ppm of peak x MHz(machine) = Hz (for J couplings)

Reply 2

cptbigt

Great, thank you! I will have to go find out the Mhz of the machine, being first year we didnt actually use the NMR, just gave in a sample and got given a spectra in return the next day :p:

Reply 3

EierVonSatan

cptbigt

Great, thank you! I will have to go find out the Mhz of the machine, being first year we didnt actually use the NMR, just gave in a sample and got given a spectra in return the next day :p:

It should be written somwhere on your spectra :smile:

Likely 250, 300 or 400 MHz

Reply 4

cptbigt

EierVonSatan

It should be written somwhere on your spectra :smile:

Likely 250, 300 or 400 MHz

Ah good point, thanks :smile:

Reply 5

cptbigt

I'm having trouble understand what coupling constants actually are and how to work them out. Any chance anyone could explain what they are and how to work them out? :smile:

Reply 6

EierVonSatan

cptbigt

I'm having trouble understand what coupling constants actually are and how to work them out. Any chance anyone could explain what they are and how to work them out? :smile:

I'm assuming you haven't had any courses on this yet? In brief terms the nuclei of the hydrogens act as small magnets and influence the magnetic properties of the protons next to them - this causes the splitting.

Singlets obviously don't have any coupling, doublets (1:1) have J equal to the distance between the two signals, triplets (1:2:1) have a J value equal to the distance between the middle signal and one of the other signals and quartets (1:3:3:1) have J values equal to the distance between any two adjacent peaks :smile:

Reply 7

cptbigt

EierVonSatan

Thanks!! Nah haven't done any courses on it yet :o:

So it's the distance between the two peaks that makes up the doublets?

Reply 8

EierVonSatan

cptbigt

Thanks!! Nah haven't done any courses on it yet :o:

So it's the distance between the two peaks that makes up the doublets?

Yeah the difference in hertz between the two peaks on a doublet will give you the coupling constant J :smile:

Reply 9

cptbigt

Did an experiment to convert dimethyl maleate into dimethyl fumarate (strangely familiar to the one in the other thread). Just got back the NMR spectra for it and we have to assign each peak and then work out the ratio of the cis to trans. Have the following peaks:

3.8ppm, singlet - CH3 groups attached to the esters.

6.8ppm, singlet - The CH in the cis isomer.

7.2ppm, singlet - The CH in the trans isomer.

Ratio: 4.125 : 1 : 0.1

And what is likely to be the solvent ppm? (CDCl3)

Do these look right? My experiment went rather wrong so god knows what I actually did make :rolleyes:

Reply 10

EierVonSatan

Heh, very similar experiment - I think I did trans and cis stibene in my first year :smile:

I think your values are in the right sort of area - having looked up the exact values you should have:

Cis: 3.7ppm and 6.2ppm
Trans: 3.8ppm and 6.9ppm

Reply 11

cptbigt

EierVonSatan

Heh, very similar experiment - I think I did trans and cis stibene in my first year :smile:

I think your values are in the right sort of area - having looked up the exact values you should have:

Cis: 3.7ppm and 6.2ppm
Trans: 3.8ppm and 6.9ppm

Hmm...inorganic labs were so much nicer :p:

Any reason why there are 3 smaller peaks beside the big main CH3 peak? and there any reason why my cis peak is at 6.8 and trans at 7.2 or is that just due to me, screwing up the experiment :o:

Also means there's like a 1:10 ratio of trans to cis, thought it was going to be a little more even then that :confused:

Reply 12

EierVonSatan

cptbigt

Hmm...inorganic labs were so much nicer :p:

Any reason why there are 3 smaller peaks beside the big main CH3 peak? and there any reason why my cis peak is at 6.8 and trans at 7.2 or is that just due to me, screwing up the experiment :o:

Also means there's like a 1:10 ratio of trans to cis, thought it was going to be a little more even then that :confused:

One of the smaller peaks is going to be the trans CH3, the others coule be solvent (have a look for a residual solvent peak table) e.g. acetone is 2.14ppm

And yeah, your ratio isn't great, but at least it's there :biggrin:

Reply 13

cptbigt

EierVonSatan

Thanks very much for your help! Yeah at least my peaks are roughly what they should be, ratio isn't great, but some people were getting quartets and over 10 peaks :p:

Reply 14

cptbigt

Hmm this isnt good... 7.24 (1) is the ppm for the solvent we used, this means the peak I thought was the trans peak....was the solvent haha. Damn...

EDIT: Now I just have a huge peak at 6.9-6.8 where trans *should* be and no cis peak.

Reply 15

cptbigt

Is it even possible to obtain just trans isomers?... :o:

Reply 16

EierVonSatan

cptbigt

lol yeah 7.26 is CDCl₃ - so that means you only have trans, so looks like it went well :biggrin:

edit: and yes it clearly is possible (as thermodynamics is on your side) :wink:

Reply 17

cptbigt

EierVonSatan

lol yeah 7.26 is CDCl₃ - so that means you only have trans, so looks like it went well :biggrin:

edit: and yes it clearly is possible (as thermodynamics is on your side) :wink:

Haha nice

Reply 18

cptbigt

So for the integration in the NMR analysis table, shall I just put the numbers that are underneath the peaks on the spectra? 18.45, 72.05 etc...

Reply 19

EierVonSatan

cptbigt

So for the integration in the NMR analysis table, shall I just put the numbers that are underneath the peaks on the spectra? 18.45, 72.05 etc...

No, assign a value to a number of protons - so if you have 18.45 for the alkene peaks then you know that there's two protons there - so the integration is 2H. To get the other divide 72.05 by 9.225 to get how many protons are there :smile: