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Greetings, I want to ask for some advice in matter of 15N hard pulse calibration on Bruker machines. I'm working on Avance DRX 500 with 5mm TBI probe. This calibration is easy to made if 15N urea is avaible, but I don't have opportunity to use this compound. I have in my stable horses as follows: methylamine hydrochloride (99% 15N) ammonium chloride (99,9% 15N) aniline (15N >99%) sodium nitrite (99% 15N) I tried to use first two salts in DMSO-d6. Unfortunately I couldn't obtain proton spectrum with multiplet structure that is needed for pulse calibration (pulse sequence decp90), only broad peaks were visible ( protons attached to nitrogen directly). Aniline in Benzene also gave only broad signal. I tried to perform this calibration using 1D HSQC methodology but it also failed. (increasing pulse lenght in popt). What am I doing wrong ? How can I calibrate this pulse without urea ? Best regards, A. Leniak |
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Hello Arkadiusz, ... why do you don't use the easy "zg" or in this case "zgig" (decoupling inverse gated the Protons)! First you have to set the machine exactly on the nucleus, using wobb (both channels) or atm(m/a). Then you do a 15N experiment (rpar, ... something from Bruker parameters) and set your signal of your wish into the window and set this with "dpl" then you can do the "popt" experiment. You only have to get a strong single signal! Now, you can start "popt" ... this shows you quite nice the exact 90 degree/ 180 / 270 or 360 degree pulses. Please take care about a good number of transients/scans, a good recycle delay d1! You can do it in steps of 2 or 4 microseconds and do enough increments, to get it up to over 360 degrees running! You can do 15N on different compounds, ... glycine 15N enriched, NH3(neat), NH4-NO3, or pyridine. I have never done this with 1D-HSQC (this is too complicated) ... use a basic zg or onepulse instead! (15N has a negative gyromagnetic ratio, ... this is the reason not to use "zgdec"!!!! Use zgig instead, there is only a bad enhancement, if you use zgdec.) Have a closer look on Experiment 8.1 at "50 and more essential NMR experiments" from M. Findeisen and S. Berger. Yours, Uli |
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Hello Arkadiusz! I got the solution, ... you use the absolutely wrong probe! Change on your broadband or even better, if you have enough material, on your 10mm one this is also a broadband, a direct detection, one! You can not do this via the decoupling coil of your inverse detection triple coil/tuned one! There you can do only the inverse detection experiments very well, ... look for the proton directly attached to the nitrogen, ... this works there fine. But forget it the direct detection, do it never with this kind of probe!!!!! A short 15N measurement is ... start at Friday and stop it at Monday, ... 15N is not sensitive, has a negative gyromagnetic moment! This is the reason for a pulse program, which uses no NOE! There is no chance for a direct enhancement, like carbon. Sorry for this statement. Yours, Uli Thank you Ulrich for this answer. It's very helpful, but also sad. I hoped that changing probe is just distant menace. We have 10mm dedicated probe for 15N measurments, but TBI probe is more universal, and I'm not the only one who use DRX 500. But thank you, it saved a lot of my time. Arkadiusz - Arkadiusz Leniak (Aug 18 '14 at 07:49) Your idea is correct, if you are using a "Royal probe", OneNMR(TM) or a BBOFplus - these are new types, inverse and direct detection (and vice versa), but not the TBI! SORRY. DRX is quite old, and this is the reason for special probes for special purposes! Now you have a lonely probe for ALL! - Ulrich Haunz (Aug 19 '14 at 07:14) Yes, DRX is old granny but it is also behaving better than our new Avance III. Manual shimming and tunning could be little bothersome, but DRX is indestructible. I heard about these probes, but if something is too universal it can't be flawless, so where is the catch ? - Arkadiusz Leniak (Aug 20 '14 at 00:25) |
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Hello Arkadiusz Why not use your TBI for indirect detection of 15N, either HSQC for protonated nitrogen, or HMBC for non-protonated nitrogen? You can use 90% formamide, 10% DMSO-d6 and calibrate the 90 degree decoupler pulse for 15N with the pulse program decp90. Regards, Craig. |
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I've found some 15N urea and made this pulse calibration with decp90. TBI probe is enough for my measurments. 3 mg of compound, 4h, all peaks on spectrum. Thank you all for answers :) Best Regards Arkadiusz |
