Junfeng,
Could you give us a 'zoom' of the bit(s) of the spectrum you are worried
about? I agree with Charlie - this doesn't look too bad in terms of
"antiphase" (peaks with both positive and negative components), but the
most obvious problem is a second negative peak near to the irradiated one.
There are a few possible causes which leap to mind:
*The large negative peak*
(1) Exchange (see Charlie's answer above) - although eyeballing that
spectrum I suspect that isn't the cause here, as the peaks look very
different in multiplicity.
(2) It's hard to tell from that spectrum, but that looks like the
irradiated signal is part of a relatively strongly coupled AB pair that you
are trying to irradiate (and the other 'half' of the AB pair is the signal
which thus goes negative when you irradiate). I'm not genius at NMR theory,
but I think you might be stuck with this negative peak unless you can shift
those signals further apart in frequency (so higher field spectrometer, or
change their chemical shift.... you might get the latter by adding about
10% benzene to the sample - this can cause quite big changes in chemical
shift that might do the job for you - although it could easily move them
closer together!).
(3) The selective pulse has a slight side-band which accidentally touches a
nearby signal. This is unlikely in your case, as you've changed the width
of the irradiation band. You can also test this by moving the selective
pulse slightly (so use a 100Hz wide band, and if this still gives the
problem, then run the experiment with the pulse centred on the band, then a
second experiment with it about 20Hz off-centre). We saw this exact problem
last week, and shifting the pulse fixed it completely.
(4) You might be near the negative NOE regime due to correlation time for
that pair of protons (this is likely if you're looking at a large molecule
which tumbles slowly in this solution). You're using CDCl3, and it doesn't
look like a huge molecule, so I suspect this isn't the problem, but we've
seen this happen with remarkably small molecules (Camphor is our favourite
for this - it gives SHOCKING NOE spectra. Try a ROE experiment instead (you
can test this whole theory by just running a 2D NOESY and a 2D ROESY and
comparing the results. ROE experiments will always give positive ROE
signals.
*Anti-phase** peaks*
(6) THe selective pulse isn't wide enough and you are getting evolution
during the pulse or not quite irradiating the whole multiplet. The
5Hz/10Hz/20Hz selective pulses are pretty narrow (and thus relatively
long?). You should always try to use a pulse which is as wide (and thus
short) as possible - we typically aim for 100Hz+. This minimises any
unwanted evolution during the pulse (which can cause antiphase peak shapes
in all sorts of selective experiments).
(5) Scalar coupling co-evolution i.e. zero-quantum interference. This can
be addressed by both increasing the mixing time (don't use mixing times of
<300ms for small molecules in most cases, as the coupling artifacts tend to
get worse in these cases) and also using the zero-quantum filters
(Thrippleton/Keeler version works very nicely out-of-the-box in our
experience with Agilent systems, although calibrating and optimising it on
a Bruker might be non-trivial?). But that usually gives anti-phase, rather
than simple negative, peaks (although not always).
Craig
Hi Junfeng,
If you are trying to get the NOE of a multiplet that is very close in
chemical shift (1 --> 2), you are probably irradiating the NOE peak ('2')
with the selective pulse. Try doing a difference experiment in which the
off-resonance RF is the same distance from the peak of interest, but on the
opposite side. The off-resonance effects will cancel.
Walt
On Feb 5, 2015, at 1:09 AM, jfxiang_at_iccas.ac.cn wrote:
Dear Alvicler
Thank you very much for your comments. I have attached the related one
dimensional NOE spectra. I hope somebody could help me out.
Considered the questions, I would like to describe them one by one
(1) How about the selectivity of the pulse? For this question, I simply
follow Bruker's manu. First calculate the 90 degree hard pulse of proton,
then fill this value in the prosol table. I have choosen the 180 degree
selective pulse with Gaussion1.180r.1000. I have teset several values of
pulse, for example, 5, 10, 15, 20, 25 and 30 Hz. However, none of them give
me a reasonable results.
(2) I have attached the proton spectrum, the position irradiated is located
at position 1. This proton is coupled with another proton at position 2. I
tried to evaluate the NOE enhancement of proton at position 2.
(3) I am a little innocent about the shaped pulses. I am also tried other
selective pulse instead gauss1.180r.1000, for example, Q3.1000, Rsnob.1000,
Reburp.1000 and bip720,50,20.1. They also offered me the same results.
I don't know if the information provided is clear enough. If there were any
idea, I could attached more data next time.
Best regards
Junfeng Xiang
NMR Lab, ICCAS， China
> -----原始邮件-----
> 发件人: "Alvicler Magalhães" <alvicler_at_uol.com.br>
> 发送时间: 2015年2月5日 星期四
> 收件人: "Tatiana Agback" <tatiana.agback_at_medivir.com>, jfxiang_at_iccas.ac.cn
> 抄送:
> 主题: Re: AMMRL: about one dimension selective NOE experiment to suppress selective population transfer
>
> Dear Fellows,
> I have some questions and this could provide the final answer for the problem, but please let me know more to help on it.
> How selective the pulse needs to be? We are talking about 10Hz, 100Hz, more then it?
> How you calibrate it?
> What kind of shaped pulse are you using for it?
> Could you send me more infos like 1D spectrum and the regions to make selective pulse?
> thanks in advance
> Regards
> Alvicler
>
>
>
> > On Feb 4, 2015, at 7:09 AM, Tatiana Agback <tatiana.agback_at_medivir.com
> wrote:
> >
> > Hi
> > I have observed this effect too. Can not get good bruker pulse sequence.
> > I would appreciate if those who have an answer for Junfeng Xiang put me as CC in e.mail
> >
> > Regards
> >
> > Tatiana
> >
> > Tatiana Agback, Ph.D.
> > Associate Director NMR
> > <image001.jpg>
> > Direct: +46 8 54683172
> > Mobile: +46 70 2235993
> >
> > From: jfxiang_at_iccas.ac.cn [mailto:jfxiang_at_iccas.ac.cn]
> > Sent: den 3 februari 2015 04:37
> > To: ammrl_at_ammrl.org
> > Subject: AMMRL: about one dimension selective NOE experiment to suppress selective population transfer
> >
> > Hello, everyone
> >
> > Recently, I have encountered a problem with selective NOE experiment. I want to determine the NOE enhancement of coupled protons by irradiating its coupled partners. However, the NOE spectrum has shown large anti-phase contribution from selective population transfer (SPT effect). Therefore, I couldn't obtain a reliable data about NOE enhancement. I have tried several methods in the Bruker pulse library, for example, selnogp, selnogpzs, selcssfnozs.2. All of them gave me the identical results. I have consulted with the literature, and found there were some possible methods in eliminating this unwanted antiphase cntribution (Fig 8 in J Magn Reson 125, 302-324, 1997). However, this pulse program is not available in Bruker or Joel pulse library. I wonder if anyone have met such problem before and could offer me some hints or solution? My NMR data is processed with TopSpin 3.2pl6.
> >
> > Thank you very much!
> >
> > Junfeng Xiang
> > NMR Lab, ICCAS, Beijing China
>
<one dimensional NOE spectra obtained by different methods.PDF>
Received on Fri Feb 06 2015 - 10:35:57 MST