In order to overcome the problem of low sensitivity, one should use as much sample as possible either as a neat liquid (if the sample is a liquid) or in a very concentrated solution with a solvent free of oxygen (if the sample is a solid) - the more sample the better. There may also be problems if the quadrupolar dominated relaxation rate is too fast resulting in resonances that are very broad.
Thursday, April 24, 2008
17O NMR
17O is a spin I = 5/2 quadrupolar nucleus with a natural abundance of only 0.037%. Its receptivity with respect to 13C is 0.061. Unlike the complications of the NMR spectra of quadrupolar nuclei in the solid state, the spectra in solution are quite simple as all of the energy levels are equally spaced by the Zeeman interaction, the first and second order quadrupolar effects averaged by the rapid isotropic motion in solution. 17O has a chemical shift range of more than 1600 ppm, making it quite sensitive to the local chemical environment of the oxygen and therefore potentially useful to the chemist. The figure below is a 17O NMR spectrum of a mixture of water, acetone, ethanol, isopropyl alcohol and ethyl acetate acquired in less than 15 minutes on a 300 MHz NMR spectrometer using a standard broadband probe with a 5 mm NMR tube. One can see that every single oxygen site is resolved in the spectrum. In order to overcome the problem of low sensitivity, one should use as much sample as possible either as a neat liquid (if the sample is a liquid) or in a very concentrated solution with a solvent free of oxygen (if the sample is a solid) - the more sample the better. There may also be problems if the quadrupolar dominated relaxation rate is too fast resulting in resonances that are very broad.
In order to overcome the problem of low sensitivity, one should use as much sample as possible either as a neat liquid (if the sample is a liquid) or in a very concentrated solution with a solvent free of oxygen (if the sample is a solid) - the more sample the better. There may also be problems if the quadrupolar dominated relaxation rate is too fast resulting in resonances that are very broad.
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26 comments:
Glenn, although of course I believe you, I am surprised this only took 15 minutes. Perhaps you can comment on how many transients you were able to squeeze into 15 minutes (how short was D1)? Also, do you have a 10 mm probe for your 300 or is this 5 mm?
Mike
Mike,
Here are the specifics:
5 mm tube
Simple one pulse experiment
90 degree pulses.
NS = 6393
D1 = 0.1 sec
AQ = 0.04 sec
LB = 50 Hz
16 points affected by probe ringing were removed and predicted by linear prediction to get a flat baseline and proper phase (a future BLOG post)
Glenn
Hi Glenn,
always like reading your items; particularly, those about nuclei other than 13C and 1H. Agreeing with Mike, 15 minutes sounds really short, and makes me wonder why this was not commonly done at my former group in Nijmegen. We commonly used IR to identify the presence of esther functionalities, as opposed to free acid. What are the 17O shifts for free acids? That is, would 17O NMR be suitable for identification of free acid versus esters?
Oh, now I forgot to mention that a added the five spectra to the NMRShiftDB.org. The submissions still require approval by a reviewer, but should become public soon.
BTW, I had to guess the exact shift values by eye... might you include numerical shift values in addition to spectra next time? I would much appreciate that!
Egon,
I am certainly no expert in 17O NMR so I really can't provide the answer to you question regarding whether or not 17O NMR can be used to distinguish free acids from esters. My "feeling" is that it may be useful for this, but I would have to do some reading or a few experiments to find out. Keep in mind that you need alot of sample and there is the possibility that the lines will be broad. The spectrum I ran was a hastily made mixture of the solvents listed in the BLOG post. I assigned the spectrum by running each on its own.
Glenn
Egon,
Be careful with the shifts, I ran this spectrum unlocked without shimming the magnet. Initially I ran the spectra of D2O and H2O. I referenced to H2O at 0 ppm and then ran each compound on its own and then the mixture. I did notice that the lines shifted a bit from the spectrum of the neat compound compared to that of the mixture.
Glenn
Hi Glenn,
I was wondering, are there specific Oxygen 17 sequences or did you adapt a zgpg for example? If there are, what is the keyword by which I can find them in the Bruker library?
Anonymous,
I just used a standard one-pulse zg sequence without any decoupling. Here are the details:
5 mm tube
Simple one pulse experiment
90 degree pulses.
NS = 6393
D1 = 0.1 sec
AQ = 0.04 sec
LB = 50 Hz
16 points affected by probe ringing were removed and predicted by linear prediction to get a flat baseline and proper phase.
You could try the "aring" sequences from Bruker to help minimize acoustic ringing. See this post:
http://u-of-o-nmr-facility.blogspot.ca/2008/08/pulse-sequences-to-minimize-acoustic.html
Glenn
Hi Glenn, it's anonymous again,
Out of curiosity, what was your p1 ?
Anonymous,
I cannot recall the duration of the pulse. I measured the 17O 90 degree pulse on a sample of D2O. I suspect it was between 15 and 30 microseconds at -6dB using a 300 W amplifier.
Glenn
Hi Glenn,
I want to measure 17O on our Bruker 400MHz NMR instrument but don't have experience.
Could you please suggest some parameters for setting-up 17O measurement?
Yuji,
Start with a sample of D2O. Use the zg sequence. Calibrate the 90 deg pulse using high power.
the acquisition time should be about 0.04 sec. The recycle delay should be about 100 msec. You may have to increase the dead time (de) to a few tens of microseconds to accommodate probe ringing. Use about 50 Hz of exponential line broadening. Your samples should be neat liquids run unlocked or VERY concentrated samples in solvents free of oxygen.
Glenn
Hi Glenn,
I tried to run 17O NMR, but the instrument said
Cannot perform getprosol
The following informations are not found:
Observe Pulses for 17O
Any suggestions for this?
Thanks!
Yuji Liu
Yuji,
This just means that the 17O pulses have not been calibrated and entered into the 'prosol' table. Calibrate the 17O 90 degree pulse and enter the value in the 'prosol' table by usind the "edpeosol" utility. Once this is done "getprosol" will work.
Glenn
Hi Glenn,
I tried a 17O scan of neat D2O for about 90 mins, still no visible peaks near 0 ppm. What's the problem??
Here's my parameters
5mm tube with ~3mL D2O
Probe: PA BBO 400S1 BBF-H-D-05 Z SP
PULPROG zg
TD 400
AQ 0.04s
NS 30,000
DE 35us
D1 0.1s
LB 50Hz
Yuji
Yuji,
Open up your spectral width to 100,000 Hz. The signal may be outside of your window of observation. Once you find the signal, set the transmitter close to the signal and narrow down the spectral width. What is the pulse width, p1 and power, pl1?
Glenn
Dear Glenn, Good day.
I have done my O17-labelled experiments for a couple of samples. The results are good. I would like to cite the reference in my paper for "17O-NMR spectra for mixture of common laboratory solvents" which will support my experiments. But I don't find any scientific paper related to that. Do you have any suggestions for citing.
Look forward to your kind response.
Thank you very much
Sincerely
Srinivasa
Srinivasa,
This is a good review to reference.
https://www.sciencedirect.com/science/article/abs/pii/S0066410314000106
Glenn
Dear Glenn,
I hope you are doing well. I am facing some problems since the NMR I am using (Bruker 400MHz)doesn't have the getprosol for 17O saved (the values are zero). Until now I haven't been able to get the values, could you recommend me a place for look for them or do you know which values for pulse width and power should I use?
the solvent I am using is D2O, and the getprosol always fails.
Or should it be enough if I run atma for this nuclei?
Best
Dario
Dario,
You should first tune the probe to 17O using ‘atma’. If you have never run 17O on your instrument you may first have to run ‘atmm’ and save the values so ‘atma’ can be used in the future. Calibrate the 90 deg pulse using high power with the ‘zg’ pulse program. You will need a sample from which you can easily see a signal. Once you know the 90 deg pulse and appropriate power level you can use ‘edprosol’ to create prosol values that can be called up with ‘getprosol’ in the future.
Glenn
Dear Glenn,
thank you very much for your help. To start the calibration since I do not have any getprosol for this nuclei I can skip this step isnt it?
Or should I give some arbitrary values like 8uS and ~80W (-2dB) using "getrposol 17O 8 -2)?
I do not know which value for getprosol would help me to find a good signal faster.
I am using D2O to calibrate the instrument and I added 50uL of enriched 17O water (10%)
Best
Dario
Dario,
Set up your spectrometer for 17O.
Tune the probe to 17O.
Set pulse program to zg
Set power level to a safe high power level.
Set P1 to 10 usec or so.
Calibrate 90 deg pulse at your chosen power level.
Enter these values into the prosol table using edprosol.
Glenn
Hi Glenn,
I have a 17O spectrum for a carboxylic acid. I only saw one peak for the two acid oxygen. Would you kindly explain why?
Many thanks
Could it be that the acid is a deprotonated ion and the double bond is shared dynamically between the carbonyl and acid oxygen? This would make the two oxygens isochronous.
Glenn
Hi Glenn,
Yes it’s actually a benzoic acid in acetone. I’m not sure if it’s deprotonated. I was thinking that acid proton is exchangeable and makes the oxygen equivalent? However, in acetone we don’t have exchangeable proton.
Many thanks
It must be very soluble in acetone to allow you to find the 17O signal of the acid in the presence of the much more intense signal from the oxygen of acetone. Perhaps at such high concentration, you have inter-molecular proton exchange between solute molecules.
Glenn
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