Wednesday, November 14, 2007
HSQC and Edited HSQC Spectra
Many of you use a simple magnitude HMQC sequence to establish heteronuclear one-bond 1H -13C correlations. One can also use the phase sensitive HSQC sequences to obtain the same information. Although the data must be phased by the user, the phase sensitive sequences will provide data with higher resolution as absorption line shapes are much narrower than the magnitude signals obtained in non-phase-sensitive sequences. There is also an edited HSQC sequence available which provides multiplicity information similar to that of a 13C DEPT-135 sequence where CH and CH3 signals are phased up and CH2 signals are phased down. In the figure below is the HSQC and edited HSQC spectra of 3-heptanone. The red CH2 signals in the edited HSQC are negative with respect to the black CH3 signals. Using this sequence may save you time (and money) as there will be no need to run a 13C DEPT spectrum.
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4 comments:
If your spectrometer channel is equipped with "programmable pulse modulator" or "waveform generator (for Varian)", then use 'adiabatic' pulses on C13 channel and also for C13 decoupling during acquisition.
Hari
Hi,
I have recorded a spectrum of a known compound with Brukers hsqcedetgp pulse sequence. The CH and CH2 nicely behave and give the respective positive or negative signals - except two CH2 that have the same phase as the CHs. The corresponding peaks in the 1H spectra have an integral of roughly 2, supporting the CH2.
But NMR spectra never lie - is it theoretically possible for a CH2 to be "in the wrong phase"?
Anonymous,
I have never observed such behavior for CH2's. Do you have d21 set correctly? Have you tried manually phasing the spectrum starting with all phase correction constants set to zero?
Glenn
Hi there,
If that "weird" methylene belongs for example to a three-membered heterocycle ring (such as oxirane) then the corresponding one-bond coupling constant will be around 175 Hz, much higher than the standard 125 Hz the Bruker pulse sequence is optimized for. As a consequence, the corresponding HSQC cross peaks display opposite than expected (for standard methylenes) phase, having as you mentioned the same phase as methines and methyls.
I do not know what your compound is but having its structure would help to rationalyze the origin of the uncommon one-bond CH coupling which originates the unexpected signal phase.
Greetings from Spain
Ignacio
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