I have prepared some complexes of boron trifluoride with several organic ligands. When we want to analyze the compounds by 11B or 19F nmr we are unable to observe the expected splitting pattern based on 2nI+1 rule, although we clearly see the presence of boron or fluorine. For instance a BF2 unit should show a triplet theoretically in the 11B NMR. Previous literature reports also generally didn't observe the splittings based on 2nI+1 rule. Can anyone provide some explanatin for this? Is it possible to observe the splittings by changing pulse sequence/relaxation time or other NMR parameters?
Due to the quadrupolar nature of boron, you typically don't see the couplings, except for very symmetrical molecules.
A more common example is 14N which is quite common, but you typically only see couplings to it if you are dealing with quaternary ammonium compounds.
Jani Rahkila Any ways to solve this issue? I came across this publication which says altering some of the NMR Parameters might help.
Magn. Reson. Chem. 2009, 47, 873–878.
Any further guidance on this will be highly appreciated.
Deepan Chowdhury This is not really my area of expertise, so I would contact the authors of that paper. They mention a modified pulse program, but I couldn't find any mention of what that pulse program or modification is.
It does, however, seem that even with that modification they were not able to obtain coupling constants for all compounds, and what they had were all fairly symmetrical around the boron atom, so you might struggle with your compounds if they are less symmetrical than that. It is probably worth a try though.
It is convenient to think about this problem in the time domain.
When the 11B signals’ decay rate - 1/T2* - is high compared to the inverse of the B-F coupling constant, the rapid FID decay rate reduces the data information content. The time-domain, signal modulation due to 19F J coupling is too slow to be manifested in the FID signals.
If the 11B signals’ decay rate is similar to the inverse of the B-F coupling constant some J-coupling modulation information will be present. This is the case for one-bond 11B-11B coupling in 11B 2D COSY spectra.[1] Here 11B-11B coupling information is available even though the 1D 11B frequency-domain signals are too broad to display J-coupling splittings.
In your your case, a 1D, 11B, J-modulated spin-echo experiment might shed light on the whether or not B-F coupling information is available. If 19F J-modulation occurs, then a heteronuclear 2D J-resolved spectrum would be useful. [2]
You did not mention the 19F 1D spectrum. I assume you can see the 1:1:1:1 F-B coupling.
1. Two-dimensional 11B-11B nuclear magnetic resonance spectroscopy as a probe of polyhedral structure: application to boron hydrides, carboranes, metallaboranes, and metallacarboranes.” Venable, T.L.; Hutton, W.C.; Grimes, R.N. J. Am. Chem. Soc. 1984, 106, 29-37.
2. “Two-dimensional correlated nuclear magnetic resonance of spin-coupled 11B-1H systems.” Finster, D.C.; Hutton, W.C.; Grimes, R.N. J. Am. Chem. Soc. 1980, 102, 400- 1.
Could you maybe see the 10B couplings in the 19F spectra? Integral spin nuclei tend to have longer a longer T2 than half-integral (gross simplification).
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