Understanding splitting patterns in boron NMR.

Hi,

I'm just attempting this question and I'm struggling to understand what it means by the strutural element which is common to both signals.

The 11B NMR spectrum of the collidine-boron adduct (6) (solution in deuterated pyridine) exhibits two quartets, with only the signal centred at  = -15.38 ppm belonging to (6). Which structural element is common to both signals (according to the coupling pattern)? Thus suggest the structure of the second borane adduct (7) causing the quartet at = -9.41 ppm.
Any help would be really appreciated. Thanks

Can you show me what the adduct (6) looks like?

Hi,

This is the reaction that we were doing, showing the boron adduct (6) as the product.

Thanks

The questions tells you that you have 2 quartets, it should hopefully be obvious from the structure of your adduct that the quartet is a result of coupling to three equivalent Hydrogen nuclei. Hopefully you can see from the NMR spectrum that the coupling constants (ie the splitting) is of the order of 100Hz (ie, big (for reference, 3 bond couplings in hydrocarbons are of the order of 10Hz)). This is reassuring, because Boron is directly bonded to the Hydrogen with which it is coupling, so you would expect a large coupling constant. So no problems so far hopefully seeing why adduct (6) gives a quartet with a large coupling constant.

The question then wants you to point out that the feature not caused by (6) has the same quartet structure, with a similarly large coupling constant, indicating that the three B-H bonds remain intact.

Hopefully you might be able to work out what is causing this other feature?