What about the quadrupolar nucleus? 14N should show quite a difference between -NH2 and -NH3+. Whether this will work in high field with magic angle spinning I do not know. The pure 14N NQR signal in zero field would be quite different for these two entities.

Dear David,

thanks for your proposal - yes - I also was thinking of 14N and you are absolutely right, but: in the (pharmaceutical) molecules we are interested in we typically see 3-5 nitrogens (some protonated some not) - therefore the chemical shift information (absent in 14N) is of quite some help and the analysis of the superpostion of several quadrupolar spectra is - I guess without any experience - "tricky". But for a "simple" molecules 14N is definitely worth a try!

Thanks again

Alfred

Dear Dr. Ross,

This is indeed a difficult task. My college Gerhard Althoff-Ospelt proposes a 15N HETCORE. Here, the combination of 15N and 1H chemical shift could give rise to the protonation state. Which kind of equipment do you have? A 7mm probe would be advantageous with respect to sensitivity but a 4mm probe would also be suitable.

Another solution would off course be fast spinning and 1H detection.

Many Greetings

Kristof Grohe

Dear Kristof,

thanks for the idea! We have a 4mm probe, a 15N CP-MAS takes with a fully filled 100 ul Rotor about 1.5 days. Taking this into account - is there any hope for a HETCOR within one weekend? What CP "mixing" would you recommend....200 us would be my guess.

How indicative would you expect the 1H shift to be? This publication:

Article Observing in-phase single-quantum 15N multiplets for NH2 / N...

shows a difference of 1H shift between NH2 and NH3+ of about 1 ppm (in solution!). I would expect that such a small difference may also be induced by differences of crystal-environment without a change of charge?

One idea that came to my mind is the cppi (Wu and Zilm, JMR) sequence by which one can discriminate CH2 (negative signals) from CH3 (positive signals). This works for 13C nicely in my hands. But I do not have any ideas whether (and with which parameter settings) this might do for the 15N case. Maybe you know!

Again thanks and best wishes

Alfred

P.S. ...kind regards to Gerhard Althoff who may remember me as he trained my first steps towards ssNMR some years back!

Dear Alfred,

Regarding the shift differences, this is hard to tell for me. I have no real experience with NH4+. It could be a problem.

Regarding the HETCOR:

With respect to the 1D, 1/8 of the scans per row would be necessary. 32 Rows should be sufficient. In this case the HETCOR would be ca. 4 times as long as the 1D.

One could also try 1/16 of the scans.

A CPPI is a good idea but if you have already measured a cp buildup giving no site-specific behavior this might also not work, since it is based on differntial CP buildup rates. My hope would have been that the NH4+ would build up fastest.

Many Greetings,

Kristof

PS: I will forward the regards to Gerhard. Offcourse he knows you.

Dear Kristof,

...thanks for the clarification of NMR time needed!

• I think the problem with the not-too-different buildup curves between -NH3+ and NH2 comes from the fact that in many cases the hydrogen donor is in close spatial proximity to the acceptor. The only difference is whether the hydrogen was actually transferred (X-NH3+...-O-Y) - leading to charges or whether it was not transferred (e.g X-NH2....HO-Y).
• Again my question for a recommendation for CP in HETCOR....I proposed the 200 us....would you agree or vote for longer ones?
• Just an idea: Might there be any sequence exploiting/seeing J-couplings (90 Hz triplett versus quartett)? I guess this would need a decoupling which removes dipolar couplings but keeps J-couplings intact....not knowing whether something like this is possible from physics point at all...

Alfred

Dear Alfred,

Yes 200 us is a good figure. One could also use up to 500 but this would mean less selectivity.

Thank you for your explanation regarding the H-bond situation. I did not really thought of the fact that in principle the majority of 15N sees 3 H. Using J couplings is a good idea, but I do not know what could be done in a 4mm in this respect.

If you would like to try a 1H detected 15N HSQC or something similar, We could do a demo at Bruker with the sample.

The task is generally verry difficult. If you find a solution, please tell me.

Many Greetings

Kristof

Dear Kristof,

...thanks again - just two quick question:

• What rotors can you use in the NMR machine suited for 1H detected ssNMR? I guess smaller than 4mm - right?
• How compares NMR-time needed with this "high-spinning-speed equipment" when compared to 13C or 15N detection (having higher volumes for 4mm rotors, but obviously "less gamma" with X-nucleus detection)?

Thanks again

Alfred

dear colleague : hello

in HNMR : NH2 appeared at 6.2 ppm but NH3+ appeared at range of (8 to 9 ppm).

best regards

Dear Abdelkander,

in principle it might be correct that 1H NMR on solids offers a possiblity. Unfortunately we do not have access to the required equipment with ultra-high spinning speeds. We only can measure 13C and 15N spectra.

Thanks anyways and kind regards

Alfred

it's a shame my dear friend ????????? to confirm the structure of a chemical compound????? the most important is the HNMR or GPC or MALDITOF to confirm the molecular mass.

best regards

Dear Alfred,

Sorry for the late answer.

Regarding experimental time of a 2D (for instance hCH), if you compare a 4mm probe with a 1.3 mm probe, the experimental time for 1.3 is shorter. This is because the fill-factor of the coil gets better with smaller rotors, which acts in addition to the higher gamma.

Ofcourse everything depends also on sample properties like 1H t2....

In addition, it is easier and faster to measure 1H 1H correlation though space (1H 1H RFDR is comparable to 1H 1H Noesy) than 13C correlations.

There are lot more advantages about 1H detection (quantitative...).

However, for routine 1D work, the 13C detected CP using 4mm or 3.2mm probe is best because of the narrow 13C linewidth.

Did you had any success with your 14N measurements?

Many Greetings,

Kristof

Dear Kristof, thanks for your comments on sensitivities

...up to now I did not get a new sample with 15N lines looking promising enough for a HETCOR....already 15N 1D CP takes about one weekend.....but I will definitely keep this in mind!