April 15, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
A very useful bit of information on the proton count comes from integrating a 1H NMR spectrum. However, overlapping resonances can make integration less than straightforward and in some cases ambiguous.
Integration involves the following decisions to be made by the elucidator: which resonances to integrate, where the integral begins and ends, and what the reference integral is and what value to set it to.
The 1H NMR spectrum, illustrated below, is an example where multiplets overlap and affect the neighbouring integral values. The spectrum shows six labelled multiplets with integral information.
With the integrals in place, the elucidator needs to decide what reference integral to use. Multiplet E (or Multiplet A) is a good choice as the reference integral since it is isolated from other multiplets that would otherwise interfere with the integral. The next step is to decide what whole number to set the reference integral to; typically, it is a value of 1 or 2 or 3.
Multiplet B is a broad singlet and its integral overlaps with the integrals for Multiplets C and D. Although both Multiplets C and D appear to have an integral value of 1, one can check this by applying peak fitting (or deconvolution) to the broad singlet and subtracting out that contribution.
Multiplet F poses a different challenge. The integral shows a value of 6.71, which can be taken as 6 or 7 protons (I’d be more inclined to round up and take it as 7 protons). Peak fitting may help here but it may be more practical to resort to 2D NMR data for further analysis.
Multiplet A = CH
Multiplet B = NHn or OH
Multiplet C = CH
Multiplet D = CH
Multiplet E = CH
Multiplet F = 7 CHs (or any combination of CH, CH2, CH3 totalling 7 protons)