March 13, 2008
Depending on the experimental conditions, signals from exchangeable protons such as NH and OH can be present on a 1H NMR spectrum. A key to interpreting a 1H NMR is distinguishing between CHn protons (where n = 1, 2 or 3) and exchangeable protons. Acquiring an 1H-13C 2D NMR experiment can assist in this process, specifically, an HMQC, HSQC or HETCOR.
A partial 1H NMR spectrum for the carbamate compound is shown below. The aromatic region shows 5 multiplets with a relative integral of 2:2:1:2:2 ratio. The multiplets with integrals of 2 correspond to the aromatic CHs from the 3-ring system. The doublet (d) at 7.78 ppm corresponds to a CH or NH group. The chemical shift and coupling pattern (d) provide clues to the assignment, however, the coupling constant is very close to more than 1 multiplet. Supporting data is needed to rule out a CH assignment.
The HSQC, shown below, correlates H-C groups separated by a single bond. Since a carbon correlation to the doublet at 7.78 ppm is absent from the spectrum, one can say with a high degree of certainity that the doublet at 7.78 ppm does not belong to a CH and therefore by the process of elimination the doublet belongs to the NH group.
Tip: There may be additional value to use a database or library to compare 1H NMR spectra of similar compounds.
Tip: Start with the easy stuff to simplify data analysis.