Why the relaxation times of different phosphorus species are vary?
Is it accurate to use the direct integration of 31P NMR spectrum for calculating the conversion percentage?
Can anyone help me with an article that demonstrates the relaxation time of 31P NMR, and how to determine the concentration of different phosphorus species using 31P NMR?
Thank you to you all in advance for any useful information.
Saddam
As with 1H or 13C, there is no reason why you couldn't use 31P integration for calculating conversion percentage, as long as you've made the appropriate considerations in your experiment:
- ensure interpulse delay is sufficient (typically 5x the longest T1 of the peaks you're using)
- ensure you collect enough data points to get a representative peak shape
- collect enough scans to get a sufficient signal/noise (at least 10:1, but more will definitely be better)
- be careful about NOE: ideally use inverse gated decoupling if necessary
I would say your best bet is to measure the T1 relaxation times of the peaks you're going to use. These considerations (particularly the T1) could make for a fairly long experiment, but there are advantages to using 31P for this:
- spectra are typically less crowded than a 1H spectrum and more intense than a 13C
- SOLVENT: you could even take an aliquot of your reaction mixture straight in the NMR tube and run it unlocked. With topshim, you can even shim on it - use the command topshim 1h lockoff o1p=x (where x is a 1H peak of the solvent). Make sure you turn the lock sweep off first!
Dear Steven,
dependent on the chemistry and the instrument your are using the difference in chemical shift of your educt and product 31P signal can be so big, that the excitation profile of typical 31P pulses may not be identical for all NMR signals you see. This can be overcompe by placing the carrier in the center between the educt and product related NMR signal you are intersted in.
But you still have to take care for T1 relaxation - we normally use a 30° excitation pulse to reduced the problem.
I hope this helps
Alfred
Dear All,
Thank you very much for all your kind comments, they sure make the case is more obvious to me now.
Saddam
http://www.sciencedirect.com/science/article/pii/S0165993612000702
http://www.sciencedirect.com/science/article/pii/S0021967315004264
http://nopr.niscair.res.in/handle/123456789/25324
https://link.springer.com/article/10.1007/s00216-011-5561-7
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