Hi all,
We have a similar structure as tartrazine and is trying to determine or confirm the carbon at position 7 on the attached image. I tried inadequate, hoesy and normal hmbc to locate the carbon marked red on the attached image. Basically we will need a 4 bond detection.
I also tried the standard parameters of the LR HSQMBC pulse sequence but might need to run it longer. However, to see further range bonding than 3 bond (like 4 or 5) is it of the best interest to change the jnxh to what value? If I set the offset and sweep only around the phenyl groups will this improve the situation or will there be some fold over?
Inadequate seems the way to go but the amount of NMR time is very long. Something like 18+ hours for a 30mg sample.
Regarding inadequate, stick to the inadequate_2d_pfg version (with such a long phase cycling it is difficult to obtain decent results without using gradients), and you need to see a 13C spectrum 1-4 scans if you want to see an spectrum in a reasonable amount of time. Also, have you measured the 13C T1s? With these difficult experiment you really need to optimize the relaxation delay. Here is a comparison with 30% menthol with the same threshold for countours (T1s around 3 s, relaxation delay set to 5s).
Regarding long range HMBC and HSQMBC you really need to do two things:
-Set the long-range coupling constant to something like 2 Hz (in the HMBC you may lose signals with such low coupling constant, hence why LR-HSQMBC exists)
-Set a lot of increments (otherwise small coupling do not evolve enough to be detected). 64 increments with NUS in a 2D where you expect several peaks in Y-dimension for each x point will lead to a loss of correlations.
I would use something like 25% or 50% of 1024 or 2048 increments.
A comparison between LR HSQMBC, HOESY, COLOC and HMBC to see the carbon 7 in tartrazine.