One of the most valuable properties that allows the 1H-NMR spectrum to give structural information is spin-spin coupling, which is caused by spin coupling between hydrogen nuclei that are not chemically identical. Different spin states interact through chemical bonds in a molecule to give rise to this coupling, which occurs when a nucleus being examined is disturbed or influenced by a nearby nuclear spin. In 1H-NMR spectra, this effect is shown through peak splitting that can give direct information concerning the connectivity of atoms in a molecule.
My question is, in which case would this coupling not occur, despite the symmetry?
Magnetically equivalent nuclei possess the same resonance frequency and only one characteristic spin-spin interaction with the nuclei of a neighbouring group.
The spin-spin coupling between magnetically equivalent nuclei does not appear in the spectrum.
Dear prof.,
Please take a look for the reference below.
Abragam, The Principles of Nuclear Magnetism, Oxford University Press2006.
Regards
Dear Ammar A Y Almulathanon and Mustafa M. Kadhim
Thank you so much for their participation and valuable answers.
Regards,
When a spins' transverse relaxation time becomes short compared to the J coupling constant separation, the resulting frequency domain broad line width can obscure the J-coupling. A 10 Hz coupling constant will not be observed when the resonance line width is 100 Hz.
This happens all the time for peaks with normal line widths (~1 to 2 Hz). Long-range coupling constants (J coupling through 4 or more bonds) can be as small as 0.1 Hz).
Some situations where this can be relevant for large J-coupling values are:
- chemical exchange
- dynamic equilibrium between structures and, or molecular aggregate sizes
- interactions with paramagnetic metals
- spins with I > 1/2 (quadrupole relaxation)
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