Friday, January 23, 2009
Weak One-bond or Multiple Bond Correlations in 1H / 13C HMQC / HSQC Spectra
Many people are quite surprised to see either unusually weak one-bond correlations or weak multiple bond correlations in their 1H / 13C HMQC / HSQC spectra. These people must be reminded that there is nothing "magic" about these experiments - the responses are based solely on an assigned delay proportional a reciprocal coupling constant. The large scale success of the 1H / 13C HMQC / HSQC techniques can be attributed to the fact that most one-bond 1H - 13C coupling constants are very similar ( ~ 145 Hz). The pulse sequences are therefore run with a delay based on a 145 Hz coupling constant. When one-bond coupling constants are significantly different than 145 Hz then the correlation will be either very weak or absent in the spectrum. Also, if multiple bond couplings are unusually large then those multiple bond correlations may be present in the spectrum. The figure below is an example. In the 500 MHz HMQC spectrum of an alkyne (optimized for 145 Hz coupling), one can see an unusually small one-bond correlation between the terminal alkyne proton and its attached carbon. There is also a weak two-bond correlation between the terminal alkyne proton and the other alkyne carbon.
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4 comments:
Hello Glen F.
My name is Elizabeth and I have a little problem, I work whit a Bruker Avance 300 MHz but, change J it isn´t easy for me, How may I do it?, How may change the delays for change J=145 Hz, J=250 etc.
I am beginning, could you helpme?
thanks..
Elizabeth,
Most Bruker HMQC or HSQC pulse programs calculate the delay, d2 in the program based on a constant, CNST2, where CNST2 is typically set to a value of 145 (the average 1 bond proton carbon coupling constant). If you wanted to optimize your HMQC or HSQC sequence of 250 Hz coupling, simply type "CNST2 250" before you start collecting data.
Glenn
Hi Glenn,
I was searching for some info about this and found your blog. This particular case happened to me as well. I just measured the HMQC of my compound which has two terminal alkyne groups. I noticed the unusually weak one-bond correlation between the terminal proton and the terminal carbon, also the two-bond correlation of the proton with the other alkyne carbon. Do you know or do you have any paper or any theory that can explain why this happens?
Also, I measured the DEPT 90 and 135 for this compound, interestingly both alkyne carbons (terminal and the other) shows up in DEPT 90. In DEPT 135, both carbons also shows up in the positive signal region (CH). And the peaks for both carbons in both spectra are quite strong! Again, do you have any paper or theory as why this can happen?
Cheers,
Ray
PhD student
Dear Ray,
The reason behind the observations you make in your HMQC spectrum is that there is a delay in the HMQC sequence which is set based on the reciprocal of an "average" one-bond carbon-proton coupling constant. Your sample gives very weak correlations for the terminal alkynes because the one-bond carbon-proton coupling constants are much different than the average and the delay in the pulse sequence is not appropriate for your sample. Adjusting the delay in the HMQC will give you much stronger signals.
I have aslso made similar observations in DEPT spectra (see: http://u-of-o-nmr-facility.blogspot.ca/2007/11/why-does-my-quaternary-alkyne-carbon.html ). The explanation is much the same. There is a delay in the sequence based on the reciprocal of the "average" one bond coupling constant.
Glenn
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