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Author Topic: Confusion about conversion  (Read 1050 times)

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MOF

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Confusion about conversion
« on: December 19, 2018, 03:35:01 PM »

Hello guys, The question I'd like to ask you might be stupid but I found opposing informations about it. But here it is:

When you calculate the conversion of monomer to polymer by nmr, do you always need to use an internal standard as reference to set the 1H integration?
I found this paper in which they employed Benzophenone as reference. Does it mean is it used instead of TMS? Or they use is as internal standard to set the integration at 1 and then calculate the conversions?
Polym. Chem., 2017, 8, 592

Also, when they calculate the conversion of monomer in the polymerization, they don't mention using any reference and the converison is calculated on the integration of the urethane protons divided by the sum of the urethane and cyclic carbonate protons. Did they use any internal standard for calibration?

I'm just confused about it, what i knew is that the ratios of the two peaks would provide the conversion.

Thanks for any *delete me*
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zarhym

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Re: Confusion about conversion
« Reply #1 on: December 19, 2018, 08:10:56 PM »

TMS is a D-solvent. The peak of TMS is set at 0ppm. It is used to calibrate you chemical shift.

I am thinking that the Benzophenone used here is for quantitative NMR.

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kriggy

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Re: Confusion about conversion
« Reply #2 on: December 21, 2018, 12:19:34 AM »

a) you need somehow to reference the spectra to 0 ppm,this is done by adding TMS (or other reference compounds) or by referencing to residual solvent peak

b) to measure the conversion A  :rarrow: B you dont need anything except the spectra, you just compare the integration of some signals of your starting material and product (but you need to know which signals are belonging to which functional groups and you should compare the same functional groups at least CH3 to CH3 etc..)

c) you need the internal compound to get correct values to estimate chemical yields by NMR

Babcock_Hall

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Re: Confusion about conversion
« Reply #3 on: December 21, 2018, 04:00:15 AM »

I am not familiar with polymer NMR other than proteins.  However, there might be a problem in comparing the integrals between a reactant A and a product B if the proportionality constants between the integrals and concentrations were different.  That might happen if their respective T1 values were different, as they well might be, owing to different correlation times.  I also seem to recall reading something a long time ago about the utility of an integration standard when doing kinetics by NMR, but I do not have a reference handy.
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MOF

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Re: Confusion about conversion
« Reply #4 on: December 21, 2018, 07:23:26 PM »

Thank you very much for your replies.

I think this is also a problem of accuracy, since not adding an internal standard may lead to wrong integrations and therefore wrong values of conversion and reaction kinetics costants.

Anyway, (correct me if i'm wrong) concerning the paper i mentioned my idea is also that an internal standard is needed for the calculation of theconcentration in the model reaction (in the reference i cited above) which is necessary to plot a kinetic law, while for the kinetics of the polymerization the degree of advancement in the reaction can be calculated by the ratio of the peaks and thus dont need the addition of the internal standard.
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