I was wrong in quite a few parameters (DE, O1P, P1) and no wonder nothing
was seen. The most important parameter is DE which needs to be long for
probe ringing to die down. I tested a few values and it seems the cutoff
value for DE is around 140 us when you get RG=8K by RGA. Increase DE
afterwards does not increase RG.
Detailed replies are attached below, with headers+signatures deleted.
Many Thanks,
Jiejun
------
From: "Mark O'Neil-Johnson" <Mark.Oneil-Johnson@nmr.bruker.com>
I have not been to W183 but I suspect that at 20 mHz you are getting
down into the acustic ringing from the probe material/body. This is the
natural phenomena of observations at low gamma nuclei. I do believe you
can tune there but the real cutoff should be at 109Ag (23 MHz). You may
hve to do some delayed acquisition with the DE to get rid of this
stuff. This is comming thru into the first set of data points. Maybe
you have to expand the fid to see it.
--- From: Rudi Nunlist <rnunlist@bloch.cchem.berkeley.edu>Set the RG manually, use 16k (or whatever you find useful for 13C). RG of 2 is not lilkely to woerk well. Alos, don't use 50 usec pulses, use a short pulse, and up the rep rate. With a weak flip you don't worry much about relaxation (once you are below 20 deg, or so).
--- From: pgs@zeap.spectrospin.ch ( Peter G. Scherer)I have tried this measurement too on a DRX500 using a BBO 5mm Probehead and it was successfull after some time (!!).
The key points are: 1. Due to strong ringing you have to increase the dead time DE to 250 us with digital filtering. 2. The resonance frequency is at 1315ppm (2M Na2WO4 in D2O) 3. The approximate 90 deg pulse is about 30us at pl1=0dB (B-LAX300). The relaxation time is relatively long as far as I remember (seconds). 4. Other parameters of interest: D1=2s SWH=3004Hz RG=16k P1=10us NS=512 SFO1=20.8374692 (i.e. O1P=1300ppm)
New E-mail: peter.scherer@spectrospin.ch ----------------------------------------------------------------
From: Oliver Zerbe <oliver.zerbe@pharma.ethz.ch>
it cannot figure out, what the problem is that causes the RG to be set automatically to a value of two. A 90 deg. pulse of 50 us on a BBO probe semms top be fairly long to me, have you calibrated the pulse, are you sure you are not using a 180? Make sure you have the appropriate filter connected to your probe.
The D1 is definetely too short, this will probably the primaray source of error. I would recommend to go to 20s or something like this. Relaxation is coupled to gamma, which is very low for 183W. Hence, T1 is long. 183W, of course, is also a very insensitive nucleus (for the same reason, low gamma), you need to have a very concentrated sample (in fact I would use a saturated solution) to get a decent direct-observed spectrum.
If you have a resolved proton coupling (on another sample of course), it is always much better to use inderect-detction (via 1H), This is very sensitive, and since183 W is 100%, very easy and quick.
--- From: detlef.moskau@spectrospin.chIt is probably the value of the prescan delay DE. What is the DE you have used? Try larger values, e.g. 10-20usec.
For any further assistance needed, please do not hesitate to contact me.
--- From: fgr@st-andrews.ac.uk (Frank Riddell)It looks to me like you will be operating at about 21 MHz giving about a 500 ppm spectral width with SW at 10,000 Hz. The chemical shift range of transition metals in this part of the periodic table is immense - thousands of ppm. The reference for W has been WF6 with Na2WO4 over 1000 ppm away from that. In addition the receptivity is about 4 x 10-3 that of 13C.
NMR here is very different from that of 13C etc!!!
My advice for you is to hunt by changing O1 in 10,000 Hz steps and alllow lots of time with each step. Beware of foldbacks!!
--- From: "Mariusz M Kozik" <kozik@canisius.edu>I have had a lot of experience with running 183 W NMR 10 years ago and never had any problem. Some suggestions:
1. I would set RG to much higher value myself, like 2000? 2. PW of 50 microsec seems very long to me, especially on 5 mm probe. Typically 183 W NMR is run on 10 mm probes with the pulse with around 25-30 microsec's. You could have been doing a 180 pulse! 3. With 2 M Na2WO4 you should get a reasonable S/N ratio after 32-64 scans (on 10 mm probe). 4. Also, W signals tend to be extremely narrow. If your file size is too small, and the resolution is poor, you could be missing completely a narrow signal somewhere. 5. It even is possible to find W on some (but not all) Bruker's 10 mm multinuclear probes that cover P thru silver. W is right below Ag and often possible to find on that probe.
--- From: "Peter Ziegler" <peter.ziegler@nmr.bruker.com>Select 183W in edasp (BF1) and then change O1 to 20.8369169 (SFO1). This frequency is based on 500.13MHz. The signal should be close to the center of the spectrum. The line width is only 2Hz and if you have a large SW and a small TD, you may not seeing the signal. On a 300 MHz instrument the 90 degree pulse is 30 usec at -6 dB.
Try to use the following parameters:
TD = 16K SW = 5000 Hz SFO1 = 20.8369169 NS = 2000 D1 = 2 s P1 = 40 usec PL1 = -3 dB LB = 2 Hz SI = 8K
After you find the signal you can use a couple of tricks such as linear prediction to straighten the base line out. Let me know if you need help on this.
--- From: "leonard charles dickinson" <charlie@telemann.pse.umass.edu>That RGA set RG to 2 means that you probably have some receiver ring down in the first points of the FID...increase DE till you don't see that, then with much higher RG you may have a chance of seeing something. Can you increase concentration until yo udo see something.....does probe tune with that high an ionic strength? Is it an indirect probe...I would start with a 10mm direct probe...after all it is 10x weaker than 13C, thus requiring 100x scans for equivalent s/n.
I would then set up for a PAROPT on O1 if the W chem shift scale is large ( who knows if the default O1 is OK?), then I would do a PAROPT on p1.
--- From: dave@kunmr.chem.ukans.edu (David Vander Velde)Jiejun, the T1 of the tungstate ion is really long. You may just be saturating your signal. The last time I did any 183-W was in the early 80s on a widebore 250, but I think the signal from a 1 M solution ought to appear in a very small number of scans (like <16). The 90 on these high ionic strength solutions also gets very long--I remember reading a paper from DuPont where it was stated the 90 was _4_ times longer on a ca. 2 M solution of some tungsten cluster in water than for a nonpolar solution. If I were you, I'd try dissolving even more tungstate in the solution, then looking for the signal with some small number of scans with a long delay. Personally, I have sworn off heavy metal NMR, good luck,
--- From: decatur@chem.columbia.edu (john decatur)Sounds like the probe ringdown is long which is normal for low frequency nuclei. rg is being set according to this ringdown. You could do an echo but an easier solution is just to set de to a large value, say 100us. To avoid phasing problems, set de = N/SW where N is an integer. Hope this helps.
--- From: Robert D Scott <scott@iastate.edu>Sometimes the low frequencies are hard because the pulse doesn't ring down fast enough, and overloads the receiver with a rolling baseline. I haven't done this on our drx, but I think that edscon lets you change DE, so you can delay the start of the acquisition to avoid digitizing the pulse. Be sure to write down the initial edscon values, because I think that it affects the entire instrument. Bruce Fulton suggests trying an echo, but I'm not sure that you have enough signal to do that.
---------- Jiejun Wu NMR Facility Manager Dept. of Chemistry (714)824-6010 (office) 516 Physical Sciences 1 (714)824-5649 (lab) Univ. of California, Irvine (714)824-8571 (fax) Irvine, CA 92697-2025 jwu@uci.edu (internet)