Muhammad,
Likely it’s related to the molecule. Eg. There’s a ‘slow rotor’
or restricted conformational change regime (e.g. ring flipping) in your molecule.
One way to tell is usually the 13C or 1H spectrum shows broadening but not both (different
time constants related to acquisition times: say, 400 MHz 1H vs. 100MHz 13C).
A ‘missing’ signal is just one so broadened it disappears into
the base line. Sometimes two signals broaden together and coalesce.
Improvements in line shape that occur going from CDCl3 to say DMSO-d6, are
often less about dielectric and more due to slower stumbling in a more viscous
solvent! Or possible breaking up intermolecular aggregations (slow tumblers).
You won’t get far reprocessing the FID. Remeasure at a higher (or lower)
temperature.
-Ted M.
> On Oct 1, 2025, at 8:04 AM, Karel Klika via groups.io wrote:
>
> Muhammad,
>
> These appear to be dynamic effects, so interconversion between conformers
> or whatever etc., and nothing to do with acquisition parameters (as evidenced
> by the DMSO sample of KB-P19-NO2 and ethyl benzene). To counteract the dynamic
> effects and move from the intermediate state between slow exchange (separate
> signals for the interconverting species) and fast exchange (an averaged spectrum
> of the interconverting species) one can increase the rate of interconversion
> by raising the temperature. Unfortunately CHCl3 is limited since its boiling
> point is only 60 C (you can use pressure tubes to go higher of course). Another
> approach is to reduce the frequency difference in the exchanging spins by going
> down in field if you have access to a lower field instrument.
>
> Regards,
> K. Klika
> ________________________________________
> From: Muhammad Adil Raees via groups.io
> Sent: 01 October 2025 17:38:00
> To: main_at_ammrl.groups.io
> Subject: [AMMRL] Help Needed: Broad/Missing ¹³C NMR Signals
>
> Hi all,
>
> I’m seeking advice on improving ¹³C NMR spectra that show
> broad or missing signals.
> Sample KB-P19-NO₂ in CDCl₃ had broad/missing signals, but
> in DMSO it became sharp with good intensity.
>
>
> Sample HY-VI-135-HH in CDCl₃ is also showing broad/missing signals
> and is insoluble in Acetone, DMSO, and MeOD. We specifically want to improve
> the ¹³C signals for HY-VI-135-HH.
>
>
> For these experiments, I used zgpg or zgpg30 pulse programs. NMR instrument
> is working fine — standard tests in CDCl₃ (ethyl benzene)
> show all signals clearly. The FID files and molecular structures are attached
> for reference.
>
> It seems sample-related, but I’m wondering:
>
> 1. Could the broad signals be due to acquisition parameters?
> 2. Are there ways to improve the signals for HY-VI-135-HH?
> 3. Could the broadening be inherent to the molecule’s chemistry?
>
>
> Any tips or shared experience would be greatly appreciated!
>
> Thanks,
> --
> Muhammad Adil Raees, Ph.D.
> Manager NMR Facility
> The Central Laboratory
> Syed Babar Ali School of Sciences and Engineering
> Lahore University of Management Sciences
> Opposite Sector-U, D.H.A. 54792, Lahore
> +92-330-2866755
> adil.raees_at_lums.edu.pk
>
>
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Received on Wed Oct 01 2025 - 11:51:23 MST