[Compaq]

We had a test exam in spectroscopic methods a few weeks ago, and in my personal evaluation my professor mentioned that I had misinterpreted two singlets as a doublet. How do one exactly distinguish between these two? Is there a certain max value of the J-constant between the peaks that symbolise whether it's a s or d?

Any reply will be appreciated!

[Babcock_Hall]

Coupling constants generally fall into well defined ranges, depending on the nuclei and the number of atoms that separate the two nuclei, along with the hybridization of the bonds in question.  Another thing to remember is that if nucleus A is a doublet due to splitting from nucleus B, then B will also feel the coupling from nucleus A, with the same coupling constant.  Of course, that does not mean that B will necessarily be a doublet.

[discodermolide]

We had a test exam in spectroscopic methods a few weeks ago, and in my personal evaluation my professor mentioned that I had misinterpreted two singlets as a doublet. How do one exactly distinguish between these two? Is there a certain max value of the J-constant between the peaks that symbolise whether it's a s or d?

Any reply will be appreciated!

You could try and spin decouple the signal. If it is a doublet it will collapse to a singlet.
A ¹H-¹H 2D spectrum will also help as you will see the correlation(s).

[fledarmus]

There is a range - anything above 18 Hz is unlikely to be a coupling constant. Since the coupling constant is independent of power, if you have a very strong NMR (400-600MHz) you are much less likely to get overlap than if you have one of the very old NMRs (60MHz).

But the easiest way to tell is to find out what it is coupled to. If it is a doublet, then there is a single proton on an adjacent carbon somewhere that is coupled to it. If it is two singlets, then there are no protons on adjacent carbons capable of coupling to them. You will not have a doublet if everything else in your spectrum is singlets or easily identified isolated spin systems (ethyl groups, isopropyl groups, etc.).

[Babcock_Hall]

Another experimental technique is to take the spectrum at two different field strengths.  If you have a doublet, the J value is constant even when field strength changes.  Obviously, this does not apply to situations in which you just have a paper copy of a spectrum to interpret.

[Dan]

I had misinterpreted two singlets as a doublet. How do one exactly distinguish between these two?

As has been mentioned already, there is a range with which coupling constants should fall, and you need to consider the (possible) structure of the molecule in question. For your specific example from the exam, it would be easier to explain how to make the decision if you post the actual question.