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Distinguishing Impurities … Part 6

Although a 1H-13C HMBC experiment may take a long time to collect with adequate signal-to-noise, it can offer an idea as to which signals belong ‘together’. Generally, signals belonging to the same structure leave a trail of connectivity information via long range correlations. If the atoms belong to the same structure, then a typical HMBC pattern is as follows: proton A is correlated to carbon B, carbon B is correlated to proton C, proton C is correlated to carbon D, etc.

The 1H-13C HMBC experiment below shows a lack of long range connectivity information for the singlets at 1.68 and 2.14 ppm to any of the other signals. This leaves two possibilities: either the singlets do not pertain to the main unknown structure or the protons are separated far enough from any carbon atoms to show any correlations.



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