In PMR generally amine (NH2) and hydroxy (OH) proton/s do not couple with nearby proton/s and give singlet signals. But in some cases they couple with neighbouring protons and produce peak multiplicity. So when they make coupling with neighbouring protons?
In general, hydroxy and amine group protons are very readily exchange with neighboring groups protons at room temperature. NMR can not recognize its position during its residing time. But for acquiring coupling of such protons, you need to perform the nmr experiment at low temperatures. Certainly, you can get coupled spectra at -40 oC temperature.
The reason is due to the higher rate of proton exchange on amino or hydroxyl groug as a result of higher electronegativity of nitrogen and sulfur atoms, this means that proton stays very short time on (N) or (S). Accordingly neighboring protons can not feel their spin orientations. This means that if we able to reduce the rate of proton exchange , then neighboring protons can overlap. This can be done only by run the spectrum in the presence of acid traces.
Earlier comments are wise..
As the OH and NH- protons undergo rapid exchange they will not couple with neighboring protons.. they appear as singlet/broad singlet
But if you reduce the rate of proton exchange, like by using the solvents like DMSO, the you may be able to see the coupling and so peak splitting.
Naturally present Nitrogen-14 possess integral value of spin. Hence it is capable of quadupolar interaction which broadens H2 of NH2. Also NH2 and OH protons can be observed in DMSO-d6/Acetone-d6/CDCl3 but not in D2O as D in D2O replaces Hydrogens of NH2 and OH. N-15 NMR however might show N-H couplings (but not with Carbon attached protons.
The coupling is constant in H─C─O─H for example. However (as referred to earlier) these lines are broadened by exchange, a phenomenon related to T2 relaxation. The coupling exists, but line-broading masks line-separation.
Amine (NH2) and hydroxy (OH) proton/s couple with nearby proton/s in PMR when the compounds are extra pure and splitting of the signal occurs by n+1 rule where n=number of neighboring protons.
Amine (NH2) and hydroxy (OH) proton/s will not couple .The long range the coupling will be very weak and disappears over more than tree bonds.
I synthesized 2-aminobenzoic acid in my lab but on my proton NMR is not showing the presence of NH2 and OH protons even though i used DMSO, please why?
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