The past few blogs, Part 3 and Part 4, have examined impurity(ies) identification from short-range 2D NMR experiments without much success. If proton singlets, possibly attributed to impurities, are to be distinguished from the main unknown, then long-range 2D NMR experiments may help out by establishing long-range correlations to other atoms.
A 1H-1H TOCSY experiment with a mixing time of 30 ms is shown below. The correlations are colour-coded based on intensity, red for a high intensity and green for a low intensity. The black line spanning across the 2D NMR spectrum indicates the diagonal.
The alleged impurity singlets at 1.68 and 2.14 ppm do not show any long correlations to any of the remaining 1H signals as seen by a lack of any off-diagonal peaks. Although it is still possible that the singlets can pertain to the main unknown through a lack of any ‘neighbouring’ protons to couple to, more data is definitely needed to support/dismiss this claim.