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Dear all: I just synthesized a polymer. The integrations in the high-chemical-shift regime are consistent with theoretical values, with error less than 5%. The integrations in the low-chemical-shift regime corresponding to alkyl chains (delta=0.8-1.8 ppm), however, are greater than theoretical value by 30%. I'm pretty sure this is the polymer I want because elemental analysis is also good, but just can't figure out why integrations are off by so much. The possible solvent peaks there are hexanes and water, but I've had it vacuum dried for a week at 80 C and no such peaks were observed. I heard that the ratio of alkyl NMR integrations are usually higher than expected compared to the aromatic region due to longer relaxation times for aromatics. If so, how do I circumvent this issue? Increase scan time or delay time? I also notice there is something like "shoulder" for this alkyl region as in the spec http://imageshack.us/photo/my-images/863/nmr.png/. Does it look like I didn't a good job on baseline? Thank you very much for your kind help. |
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If that's a relaxation effect (significant difference in stationary magnetization), try to increase recovery delay to ensure that the difference of stationary magnetization is negligible... Have you try to integrate the whole alkyl area (because there are some overlap, and the signal is a bit broad) to see if the result is more consistent with what you expect ? Thanks, Yoan. Yes I integrated a relatively broad alkyl area (0.8-1.8 ppm) which corresponds to 10 protons per polymer molecule, but the integration is 13. Does increasing scan time help? If relaxation is an issue, does decreasing solute concentration help? - Davis Chen (May 15 '11 at 13:25) It will also help to decrease flip angle of the pulse in the case the pulse is close to 90 degrees. Shorter pulse will improve integrals. - Evgeny Fadeev (May 15 '11 at 20:09) If you talk about d1 (recovery delay), yes, that's help (like I say in my answer). I don't think that decrease concentration can help (in the case there are no oligomerization or aggregation). - Yoan Monneau (May 16 '11 at 05:40) I tried d1=10 sec, which is fairly sufficient for most cases according to a textbook. But I still got integral ~30% than theoretical value. I noticed that there is no sharp water peak in the region in question (0.8~1.8 ppm). - Davis Chen (May 25 '11 at 21:13) (cont'd) I also checked the d-chloroform as theNMR solvent and found the solvent itself has water peak. It seems that the water peak becomes too broad to be seen in this NMR spec, resulting in a higher integral. - Davis Chen (May 25 '11 at 21:14) |
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Use of a short value of the relaxation delay can lead to errors in relative areas (as Yoan Monneau has stated earlier). The rule of thumb is that the relaxation delay should be AT LEAST five times the LONGEST relaxation time (T1). If the longest relaxation time in your molecule is <2 seconds then the relaxation delay of 10s used in your experiment would be adequate. Large rigid molecules often have relaxation times less than 2s. However, longer relaxation times are possible if you are dealing with a flexible polymer. You should determine the range of T1 relaxation times in your polymer to ensure that your spectrum can be analyzed quantitatively. If a proton is sampling more than one environment on the time scale of NMR, then you may expect exchange broadening. Exchange broadening may lead to unobservable signals (and lower areas). However, you have stated that the aliphatics have higher integrated area than expected. The aliphatics are broad, which is an indicator of exchange broadening. However, exchange broadening of aliphatics should lead to LOWER areas, not HIGHER. Another possibility is that the increased signal area in the aliphatic region is due to bound (not free) solvent hexane. This would explain the broadening of aliphatics (broadening may, of course, also be due to chemical heterogeneity or shorter T2). You claim to have removed all solvents by heating to 80C in vacuum. However, a HUGE peak at 4 ppm, probably due to solvent water is still there (it may, of course, also be from water in D2O, if your spectrum was acquired in D2O). It is possible that the solvent hexane is NOT completely removed and is contributing to your higher signal in the aliphatic region. If your polymer is hydrophobic, it may be more difficult to remove the hexane than to remove the water, even though the bp of water is higher than that of hexane. Thanks, Sekhar, 4 pm peak is expected for my polymer, not the water peak. My hypothesis is water peak (which is supposed to occur at ~1.6 ppm in d-CHCl3) becomes broad for some reason and cannot be distinguished. The integration is higher than theoretical as it includes water's peak. Is it possible? - Davis Chen (May 26 '11 at 21:48) Water molecules can form clusters of varying size that differ in the extent of H-bonding giving rise to broadened lines. However, H-bonding should also increase the chemical shift a little bit. Polymer may be hydrated and this water may give a broad signal. - sekhar Talluri (May 27 '11 at 08:45) However, it would be advisable to use the Inversion recovery experiment to determine the range of T1 relaxation times in your polymer. - sekhar Talluri (May 27 '11 at 08:46) Suppose the higher integration originates from the water peak, does adding MeOH-d4 or any other deuterium species which can exchange proton with water suppress the water peak? - Davis Chen (Jun 01 '11 at 11:41) It may. You could try addition of WATER with a trace of acid. The chemical shift of water in CHCl3 is known to vary from ~1.5 ppm at very low concentration to ~4.5 ppm at concentrations adequate to form droplets. If your extra area is due to water it may start shifting to higher values of cs. - sekhar Talluri (Jun 02 '11 at 13:24) |
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Hi! In the answers to the question above I find the water clusters and the dynamics of protons in the cluster resulting in, broadening, narrowing etc., are all a matter of details based on the possible conjuctures for the\ size and structure of cluster. Since this happened to be coming up in certain other contexts also, I tried to optimize water clusters of variuos sizes and find the optimal structure/ or close to the optimmal and at those structures calculated NMR chemical shift of protons theoretically by QM methods. Find some of these outputs in the form of saved images in the link: http://www.ugc-inno-nehu.com/NMRS2011/Sheet0708.pdf I have a few of such related cluster matters reported at my webpage: http://www.ugc-inno-nehu.com/crsi13nscnmrs2011.html Jump in this page to the events 13nsc CRSI and NMRs2011 and poster presentation display sheets. |
