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In a normal fully decoupled carbon experiment, can you integrate equivalent carbons? In other words , do all CH3's get the same signal enhancement so that you could integrate CH3's comparatively? I imaginable that the issue is still that T1 relaxation would make this still impossible. How about with Cr(acac)3 added? Could you then integrate equivalent carbons? I understand that you can set up an experiment specifically for integrating carbon spectra, but I'm trying to see if you can get any integration data from a typical decoupled carbon NMR. |
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Hi, The relaxation times for carbons can be quite different, even for supposedly "equivalent" groups (just sharing the fact that it is a CH3 group does not make the relaxation times equal, but probably comparable). But even if you choose a sufficiently high recovery delay to cover 5*T1 for the longest expected T1, you will have differing NOE enhancements depending on the local environment of the groups. This strongly depends on the geometry of the molecule and it is difficult to estimate if the NOE contributions are comparable or not. Cr(acac)3 will help you avoid exceedingly large recovery delays, but you still have to worry about NOEs. So if you really want integrable 13C NMR spectra, you have to determine the T1 times for all signals you want to integrate (with or without Cr(acac)3), set the recycle delay to 5*(longest T1) and you have to use inverse-gated decoupling (decoupler on during acquisition only). Carbon spectra also sometimes suffer fron poor digitization (too few dat apoints per peak). Use a reasonably long acquisition time and some zero filling for accurate integration - Kirk Marat (Jun 29 '12 at 08:17) |
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I am just making the observation that if one wants to use a peak integration as a quantitative indicator, for comparison sake, it would be convenient for more than 1.1% of the population to be represented in that integral. I have used the magnitude of the J(13C-13C) coupling to estimate 13C yield after having labeled the product. While fid is right about not always knowing the yield, the spectra can tell you the yield for the sample in the tube, particularly in HR liquid spectra. 1H decoupling is such a constant in 13C NMR, that I had forgotten that decoupling is a signal enhancement. I was thinking about my typical CP-MAS experiment, where signal enhancement is achieved through a transfer mechanism, not just the suppression 1H-13C dipolar interaction. You are right fid, giving sufficient delay should work. I would be interested to learn of the results you get, javaslinger. |
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hi w101bdk just because he is using Cr(acac)3 it doesnt mean he is aiming signal enhancement. Probably he just wants to make sure his relax delay is enough for all nuclei. Labeling 13C is a bad idea because u not aways know how much yeld u have. Also its unnecessary work. Natural abundance u know, its 1%. Pascal is right. If u decouple during acquisition only (and short acq time) and give a long relax delay afterwards NOE is minimized. Good luck... |
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What are you doing to the spins to get signal enhancement in a fully decoupled experiment? I thought signal enhancement implied some type of magnetization transfer, which would not happen in a decoupled bloch decay type experiment. One of the reasons integration works so well for 1H spectra is the 99% natural abundance of the spin population. If you need integrated peak areas from the methyl groups in a 13C spectrum, try adding a 13C label to the methyl groups in your sample. T1 relaxation is only a problem if you are limited on spectrometer time. Think 13C T1 is bad?, try 29Si. =D Just to set the good tone here I suggest that whoever votes posts down should make an explaining comment, because people can take the "minus ones" personally. - Evgeny Fadeev (Jul 12 '12 at 15:30) |
