Tuesday, September 9, 2008
Double Quantum Filtered COSY
COSY spectra are very useful in structure elucidation as they provide correlations between coupled spins. Often, NMR spectra have large singlet signals from uncoupled protons (such as t-butyl methyls, methoxy protons, excess water or a solvent signal) which provide no information in the COSY spectrum and perhaps even get in the way of looking for smaller coupled spins. In such cases one can use a double quantum filtered COSY sequence rather than a standard COSY 90 or COSY 45 sequence. Double quantum filtered COSY spectra filter out uncoupled singlets. A comparison of a standard COSY 90 and a double quantum filtered COSY sequence for ethyl acetate is shown below. One can see that the singlet is present in the COSY 90 spectrum but absent in the double quantum filtered COSY spectrum.
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9 comments:
Dear Glenn,
In the absence of comments w.r.t any drawbacks of this experiment in your short blog post, are you advocating the exclusive use of DQCOSY where the sample is known to exhibit singlet resonances?
Regards, John
John,
Thank you for your comment. DQF COSY's are usually run in phase sensitive mode and they are used with and without gradients. The phase sensitive DQF experiment requires twice as much time as a standard magnitude COSY for the same number of slices. The phase sensitive gradient DQF experiment has a bit less sensitivity than the standard phase sensitive COSY but has the advantage that the diagonals are in phase with the off-diagonals. In summary, I do not advocate the exclusive use of the DQF COSY if I can get the same information from a faster magnitude experiment. I use the DQF COSY every time I desire phase sensitivity (because both the diagonals and off diagonals are in phase) and when singlets get in the way of interpreting the coupling information.
Glenn
Dear Glenn,
i am doing Ph.D in NIPER(india). i want to incquire wheather we can get the information about coupling constant of proton which is in coupling with two different type of adjacent proton separately
Varun,
The simple answer to your question is "yes". I recommend the following review article:
Horst Kessler. Matthias Gehrke and Christian Griesinger, Angew. Chemie Int. Ed. Eng. vol 27(4) pp. 490-536 (1988).
in dqf cosy anti phase terms contribution in both diaagonal and offdiagonal.but i read antiphase contribution decreses the signal intensity.how the antiphas cancellation takes place?
Varma,
There is no "cancellation" of the antiphase terms. Both the diagonal and off-diagonal responses have the same antiphase characteristics and the spectrum can therefore be processed such that all responses are in double absorption mode.
Glenn
Dear Glenn,
I have a question related to achieving high resolved HH-Cosy Signals (similar in HSQC) in a smaller range (ppm) of interest. As far as I am aware the resolution is fairly proportionaly to the number of different sets of experiments per ppm on the F1 axis --> on a bruker machine that would be TD/SW, right? On instrumentation I used to work in the past, just choosing the appropriate SW did really do the trick. However, at the machine that I do currently have access to, this does not seem to work, in particular in the COSY - I get the smaller window of interest but by no means the improvement I used to see on other instruments in the past, in the HSQC it works better but artefacts at the border of the area of interest do occur (it is not signals folding back in).
Is it necessary that a specific type of COSY-experiment (HSQC) is used to allow for the attempted increase of resolution in the area of interest. Hope you can help me out here :-)! Sensitivity is usually not of concern - 1 scan is generally more than enough.
Christian
PS: I really very much appreciate the work you do for the global community!!!
Dear Christian,
In the absence of zero-filling, the digital resolution (in points/Hz) is determined by (1/2)(TD/SWH) which is equal the acquisition time. In the directly observed domain on Varian instruments, when the spectral width is reduced, the number of points is changed to maintain the same acquisition time. On Bruker instruments, when the spectral width is reduced, the acquisition time is changed to maintain the same number of points. Could this explain your observations? The resolution in the indirect domain is determined by the number of slices divided by the spectral width in the indirect domain. I believe that both Bruker and Varian instruments maintain the same number of slices when the spectral width in the indirect dimension is reduced.
I hope this helps.
Glenn
Glenn
Dear Glenn,
thanks for your information. Yes, also our Bruker instrument keeps the number of slices when I decrease the spectral width.
I will check the acquisition parameters more carefully. Maybe I can find out why we have these artefacts and not the desired increased resolution when reducing the spectral width and keeping the spectral with.
Anyhow, thank you for your rapid answer and keep on blogging :-).
Greets,
Christian
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