Question

How do we decide how many signals we are likely to see in the `""^1H` or `""^13C{""^1H}" NMR spectrum"`...?

Answer 1

An old problem...and you have to use symmetry.....

Explanation:

In any form of spectroscopy the primary aim is to assess and to rationalize the experimental and numerical data. For `""^1H` `"NMR spectroscopy"`, you thus integrate ALL the signals you see in the spectrum, and while you may not know which protons these signals represent, you can often use the numerical ratios to assign a signal.

Any sets of protons (or other groups) that can be exchanged by a proper axis of rotation are said to be equivalent (`"homotopic"`) in the NMR spectrum, and give rise to the same observed chemical shift.

Any sets of protons (or other groups) that can be exchanged by a improper axis of rotation are said to be `"enantiotopic"` in the NMR spectrum, and should give rise to the same observed chemical shift in the standard NMR experiment.

And I have said much the same thing here.

Protons or other groups that are constitutionally equivalent, but CANNOT be interchanged by a symmetry operation or a fast moving process, are said to be `"diastereotopic"`, and will give rise to TWO separate absorptions in the NMR spectrum, and, if coupled together, will probably give rise to a messy multiplet.

So how to identify equivalent and diastereotopic protons? Well, first draw the molecule as symmetrically as possible, and identify the probable symmetry. And don't despise the utility of models to inform your reasoning. You will always find sets of molecular models on the desks of distinguished professors or organic and inorganic chemistry to help them vizualize their symmetry.