I have a HMBC attached where we can see the correlations between the ethynyl proton and carbons 1, 2, 4, 34 and 35. No problems.
However, I need to find more information about the effect that leads to the splitting of the peaks observed for 34 and 35.
I believe in broad terms it relates to lability of the ethynyl proton. But even after a lot of reading I can't find the specific name/ details for this effect. Thanks.
The HMBC experiment is designed to record long range correlations using scalar coupling. Multiple bond scalar coupling is much weaker than single bond coupling so in the HMBC the problem is how to exclude the single bond peaks. One solution is not to bother excluding them but to make them easily distinguished from the long range correlations. Recording the HMBC without heteronuclear decoupling during acquisition causes the single bond correlations to be split by the large 1JCH coupling, typically ~140Hz. Of course the nJCH coupling (0-10 Hz) is present too, but because this is similar in magnitude to the peak width, splitting due to nJCH is generally not observed. Thus, pairs of peaks separated by ~140 Hz appearing either side of HSQC correlations are almost certainly single bond HMBC correlations. Modern versions of the HMBC experiment include J-filters to remove the single bond correlations. The three fold low pass J-filter in the sequence1 used by the Facility's standard HMBC parameter set works quite well and removes most single bond correlations, but not all. In crowded regions of your HMBC, particularly around strong methyl resonances, be aware that those weak peaks may be single bond correlations.
The HMBC experiment is designed to record long range correlations using scalar coupling. Multiple bond scalar coupling is much weaker than single bond coupling so in the HMBC the problem is how to exclude the single bond peaks. One solution is not to bother excluding them but to make them easily distinguished from the long range correlations. Recording the HMBC without heteronuclear decoupling during acquisition causes the single bond correlations to be split by the large 1JCH coupling, typically ~140Hz. Of course the nJCH coupling (0-10 Hz) is present too, but because this is similar in magnitude to the peak width, splitting due to nJCH is generally not observed. Thus, pairs of peaks separated by ~140 Hz appearing either side of HSQC correlations are almost certainly single bond HMBC correlations. Modern versions of the HMBC experiment include J-filters to remove the single bond correlations. The three fold low pass J-filter in the sequence1 used by the Facility's standard HMBC parameter set works quite well and removes most single bond correlations, but not all. In crowded regions of your HMBC, particularly around strong methyl resonances, be aware that those weak peaks may be single bond correlations.
Jürgen Wintner Thanks for the reply. I am not sure I understand - or perhaps I was not clear enough in the question.
The two *split* peaks at ~ 80ppm on the carbon scale do correspond to H35 with C35 and C34. I do understand that we are seeing direct correlation in the case of H35/C35. I just measured the distance between the peaks and found them to be 50Hz and 254Hz, for the correlations with C34 and C35 respectively. Is this within the range ~140Hz Or am I looking at a different effect?
I want to understand why they are 50Hz and 254Hz, as this pattern is somewhat repeated in other terminal ethynyl compounds.
And, yes, both of these correlations appear in the HSQC
Thanks again.
EDIT- are these correlations here *because* they were not suppressed; because they are not ~140Hz?
1J(CH) is sometimes observed in HMBC (esp. if one 'digs down' in the contour levels) but it's worth mentioning, here, that sometimes in HSQC experiments, the cross peaks disappear altogether for reasons related: the magnitude of 1J(CH). For example, when recording HSQC of terminal acetylenes, the expected CH correlation is missing because 1JCH ~ 250 Hz - far from the generalized 140 Hz that is nominally selected in the default experiment. Same thing goes for strained rings (e.g. cyclopropane, aziridine, even epoxides) where |1JCH| is much higher than 140 Hz
Very interesting; I was not aware of these J values in acetylenyls. I think that Juergen got it right. The 1J between 1H-35 and 13C-35 must be 254 Hz and the 2J between 1H-35 and 13C-34 must be 50 Hz and the used sequence does not suppress them efficiently (not that I find that too striking). Under these assumptions, the spectrum is fully comprehensible. The surprise (for me) are the large J values. I will ask a friend to run a DFT prediction on them.
They predict 262 and 52 Hz, respectively. Less then 5% off, which is amazing. I think this settles it.
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